A critical appraisal and evaluation of modern PDFs
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Abstract
We review the present status of the determination of parton distribution functions (PDFs) in the light of the precision requirements for the LHC in Run 2 and other future hadron colliders. We provide brief reviews of all currently available PDF sets and use them to compute cross sections for a number of benchmark processes, including Higgs boson production in gluon–gluon fusion at the LHC. We show that the differences in the predictions obtained with the various PDFs are due to particular theory assumptions made in the fits of those PDFs. We discuss PDF uncertainties in the kinematic region covered by the LHC and on averaging procedures for PDFs, such as advocated by the PDF4LHC15 sets, and provide recommendations for the usage of PDF sets for theory predictions at the LHC.
1 Introduction
Summary of major hard processes used in the various PDF analyses and the confidence level criteria employed. Detailed references to the different specific data sets used by the various groups are given in Refs. [1, 2, 3, 4, 5, 6, 7] and also the specific statistical analysis applied is described in these papers. Note that different analyses use partly different data sets for some processes
PDF sets  \(\Delta \chi ^2\) criterion  Data sets used in analysis 

ABM12 [2]  1  Incl. DIS, DIS charm, DY 
CJ15 [1]\(^\mathrm{a}\)  1  Incl. DIS, DY (incl. \(p{\bar{p}}\rightarrow W^\pm X\)), \(p\bar{p}\) jets, \(\gamma \)+jet 
CT14 [3]\(^\mathrm{b}\)  100  Incl. DIS, DIS charm, DY, \(p\bar{p}\) jets, pp jets 
HERAPDF2.0 [4]  1  Incl. DIS, DIS charm, DIS jets (only HERA data) 
JR14 [5]  1  Incl. DIS, DIS charm, DY, \(p\bar{p}\) jets, DIS jets 
MMHT14 [6]  2.3 ...42.3 (dynamical)  Incl. DIS, DIS charm, DY, \(p\bar{p}\) jets, pp jets, \(t\bar{t}\) 
NNPDF3.0 [7]\(^\mathrm{c}\)  n.a.  Incl. DIS, DIS charm, DY, \(p\bar{p}\) jets, pp jets, \(t\bar{t}\), \(W+\text{ charm }\) 
To further test the SM and to identify signals for new physics, measurements need to be compared to precise theoretical predictions, which need to incorporate higher order radiative corrections in quantum chromodynamics (QCD) and, possibly, the electroweak sector of the SM. In order to reach the benchmark precision set by the accuracy of the experimental data, nexttonexttoleading order (NNLO) corrections in QCD are often required. At nexttoleading order (NLO) in QCD, the residual theoretical uncertainty from truncating the perturbative expansion commonly estimated by variations of the renormalization and factorization scales \(\mu _r\) and \(\mu _f\) are often too large compared to the experimental accuracy. Nonetheless, for observables with complex final states, and indeed for many BSM signals, one must still contend with NLO calculations, which will continue to require corresponding NLO fits.
Parton distribution functions (PDFs) in the proton serve as an essential input for any cross section prediction at hadron colliders and have been measured with increasing precision over the last three decades. Likewise, the strong coupling constant \(\alpha _s(M_Z)\) at the Z boson mass scale \(M_Z\) and the masses \(m_h\) of the heavy quarks \(h=c, b, t\) are well constrained by existing data and their determination is accurate at least to NNLO. However, despite steady improvements in the accuracy of PDF determinations over the years, the uncertainties associated with PDFs, the strong coupling \(\alpha _s(M_Z)\), and quark masses still dominate many calculations of cross sections for SM processes at the LHC. A particularly prominent example is the cross section for the production of a SM Higgs boson in the gluon–gluon fusion channel.
The currently available PDF sets are CJ15 [1], accurate to NLO in QCD, as well as ABM12 [2], CT14 [3], HERAPDF2.0 [4], JR14 [5], MMHT14 [6], and NNPDF3.0 [7] to NNLO in QCD. These provide a detailed description of the parton content of the proton, which depends on the chosen sets of experimental data as well as on the theory assumptions and the underlying physics models used in the analyses. Both theoretical and experimental inputs have direct impact on the obtained nonperturbative parameters, namely, the fitted PDFs, the value of \(\alpha _s(M_Z)\) and the quark masses. Moreover, they can lead to large systematic shifts compared to the uncertainties of the experimental data used in the fit. For precision predictions in Run 2 of the LHC it is therefore very important to quantify those effects in detailed validations of the individual PDF sets in order to reduce the uncertainties in those nonperturbative input parameters. Moreover, this will allow one to pinpoint problems with the determination of certain PDFs. Any approach to determine the parton luminosities at the LHC which implies mixing or averaging of various PDFs or of their respective uncertainties, such as that advocated in the recent PDF4LHC recommendations [8], is therefore potentially dangerous in the context of precision measurements, in particular, or when studying processes at kinematic edges such as at large values of Bjorken x or small scales \(Q^2\). The precision measurements of the LHC experiments themselves help to constrain the different sets of PDFs and may even indicate deviations from SM, cf. [9] for an example. It is thus of central importance that comparisons for all available PDF sets are performed in a quantitative manner and with the best available accuracy.
In this paper we briefly discuss the available world data used to constrain PDFs in Sect. 2 and stress the need to include only compatible data sets in any analysis. The data analysis relies on comparison with precise theoretical predictions, with many of these implemented in software tools. In this respect, we underline in Sect. 3 the importance of opensource code to provide benchmarks and to facilitate theory improvements through indication and reduction of possible errors. In addition, Sect. 3 is devoted to a discussion of a number of crucial theory aspects in PDF fits. These include the treatment of heavy quarks and their masses, QCD corrections for \(W^\pm \) and Zboson production applied in the fit of lightflavor PDFs, and the importance of nuclear corrections in scattering data off nuclei. The strong coupling constant is correlated with the PDFs and is therefore an important parameter to be determined simultaneously with the PDFs. The state of the art is reviewed in Sect. 4. The need to address PDF uncertainties for cross section predictions is illustrated in Sect. 5, with the Higgs boson cross section in the gluon–gluon fusion channel being the most prominent case. Other examples include the production of heavy quarks at the LHC in different kinematic regimes. Our observations illustrate important shortcomings of the recent PDF4LHC recommendations [8] which are addressed in Sect. 6, where alternative recommendations for the usage of sets of PDFs for theory predictions at the LHC are provided. Finally, we conclude in Sect. 7.
2 Data sets and results for PDF fits
We begin with an overview of the currently available data which can be used to determine PDFs and present the fit results of the various groups.
2.1 Data sets used in PDF fits
The data used in the various PDF fits overlap to a large extent, as indicated in Table 1. However, there are also substantial differences which are related to the accuracy required in the analysis, the feasibility of efficiently implementing the corresponding theoretical computations, or the subjective evaluation of the data quality, to name a few.
The core of all PDF fits comprises the deepinelastic scattering (DIS) data obtained at the HERA electron–proton (ep) collider and in fixedtarget experiments. While the former has used only a proton target, the latter have collected large amounts of data for the deuteron and heavier targets as well. The analysis of nucleartarget data requires an accurate account of nuclear effects. This is challenging already in the case of the looselybound deuteron (cf. Sect. 3), and even more so for heavier targets. Therefore, in general, data sets for DIS on targets heavier than deuteron are not used. Nonetheless, different combinations of data sets for the neutrinoinduced DIS off iron and lead targets obtained by the CCFR/NuTeV, CDHSW, and CHORUS experiments are included in the CT14, MMHT14, and NNPDF3.0 analyses, but are not used by other groups to avoid any influence of nuclear correction uncertainties. One can also point out the abnormal dependence of the DIS structure functions on the beam energy in the NuTeV experiment [11] and the poor agreement of the CDHSW data with the QCD predictions on the \(Q^2\) slope of structure functions [12, 13, 14] as an additional motivation to exclude these data sets.
The kinematic cuts applied to the commonly used DIS data also differ in various analyses in order to minimize the influence of higher twist contributions. Another important feature of the DIS data analyses in PDF fits concerns the use of data for the DIS structure function \(F_2\) instead of the data for the measured cross sections. These aspects will be discussed in Sect. 3.
Compilation of precise data on W and Zboson production in pp and \(p\bar{p}\) collisions and the \(\chi ^2\) values per number of data points obtained for these data sets in different PDF analyses using their individual definitions of \(\chi ^2\). The NNLO fit results are quoted as a default, while the NLO values are given for the CJ15 [1] and HERAFitter [31] PDFs. Missing table entries indicate that the respective data sets have not been used in the analysis. Other data sets of lower accuracy, which have become obsolete and data sets superseded are listed in Table 3
Experiment  ATLAS  CMS  DØ  LHCb  

\(\sqrt{s}\) (TeV)  7  7  8  1.96  7  8  
Final states  \(W^+\rightarrow l^{+}\nu \)  \(Z\rightarrow e^{+}e^{}\)  \(W^{+}\rightarrow \mu ^{+}\nu \)  \(Z\rightarrow l^{+}l^{}\)  \(W^{+}\rightarrow \mu ^{+}\nu \)  \(Z\rightarrow l^{+}l^{}\)  \(W^{+}\rightarrow \mu ^{+}\nu \)  \(W^{+}\rightarrow e^{+}\nu \)  \(W^{+}\rightarrow \mu ^{+}\nu \)  \(Z\rightarrow e^{+}e^{}\)  \(W^{+}\rightarrow \mu ^{+}\nu \)  
\(W^\rightarrow l^{}\nu \)  \(\gamma ^{*}\rightarrow e^{+}e^{}\)  \(W^{}\rightarrow \mu ^{}\nu \)  \(\gamma ^{*}\rightarrow l^{+}l^{}\)  \(W^{}\rightarrow \mu ^{}\nu \)  \(\gamma ^{*}\rightarrow l^{+}l^{}\)  \(W^{}\rightarrow \mu ^{}\nu \)  \(W^{}\rightarrow e^{}\nu \)  \(W^{}\rightarrow \mu ^{}\nu \)  \(W^{}\rightarrow \mu ^{}\nu \)  
\(Z\rightarrow l^{+}l^{}\)  \(Z\rightarrow \mu ^{+}\mu ^{}\)  \(Z\rightarrow \mu ^{+}\mu ^{}\)  
Cut on the lepton \(P_T\)  \(P_T^l>20\,\mathrm{GeV}\)  \(P_T^e>25\,\mathrm{GeV}\)  \(P_T^{\mu }>25\,\mathrm{GeV}\)  –  \(P_T^{\mu }>25\,\mathrm{GeV}\)  –  \(P_T^{\mu }>25\,\mathrm{GeV}\)  \(P_T^{e}>25\,\mathrm{GeV}\)  \(P_T^{\mu }>20\,\mathrm{GeV}\)  \(P_T^{e}>20\,\mathrm{GeV}\)  \(P_T^{\mu }>20\,\mathrm{GeV}\)  
Luminosity (1/fb)  0.035  4.9  4.7  4.5 (4.8)  18.8  19.7  7.3  9.7  1  2  2.9  
Reference  [19]  [20]  [21]  [22]  [23]  [24]  [25]  [26]  [27]  [28]  [29]  
NDP  30  13  11  132  22  132  10  13  31  17  34  
ABMP15 [30]\(^\mathrm{a}\)  29.8  –  22.5  –  –  –  16.9  18.0  44.1  18.2  –  
CJ15 [1]  –  –  –  –  –  –  20  29  –  –  –  
CT14 [3]  42  –  12.1\(^\mathrm{b}\)  –  –  –  –  34.7  –  –  –  
\(\chi ^2\)  JR14 [5]  –  –  –  –  –  –  –  –  –  –  – 
HERAFitter [31]  –  –  –  –  –  –  13  19  –  –  –  
MMHT14 [6]  39  17  –  149  –  –  21  –  –  –  –  
NNPDF3.0 [7]  35.4  7.3\(^\mathrm{c}\)  18.9  149.6\(^\mathrm{d}\)  –  –  –  –  –  –  – 
Same as Table 2 for the Tevatron and LHC data sets of lower accuracy and those, which have become superseded but are still used in various PDF analyses
Experiment  CDF  CMS  DØ  LHCb  

\(\sqrt{s}\) (TeV)  1.8  1.96  7  1.96  7  
Final states  \(W^+\rightarrow l^+\nu \)  \(W^+\rightarrow e^+\nu \)  Z  \(W^+\)  \(W^+\rightarrow l^+\nu \)  \(Z\rightarrow e^+e^\)  \(W^+\rightarrow e^+\nu \)  \(W^+\rightarrow \mu ^+\nu \)  Z  \(W^+\rightarrow e^+\nu \)  \(W^+\)  \(W^+\rightarrow \mu ^+\nu \)  \(Z\rightarrow e^+e^\)  
\(W^\rightarrow l^\nu \)  \(W^\rightarrow e^\nu \)  \(W^\)  \(W^\rightarrow l^\nu \)  \(W^\rightarrow e^\nu \)  \(W^\rightarrow \mu ^\nu \)  \(W^\rightarrow e^\nu \)  \(W^\)  \(W^\rightarrow \mu ^\nu \)  
Cut on the lepton \(P_T\)  –  \(P_T^{e}>25~\mathrm{GeV}\)  –  –  \(P_T^{l}>25~\mathrm{GeV}\)  –  \(P_T^{e}>35~\mathrm{GeV}\)  \(P_T^{\mu }>20~\mathrm{GeV}\)  –  \(P_T^{e}>25~\mathrm{GeV}\)  –  \(P_T^{\mu }>20~\mathrm{GeV}\)  \(P_T^{e}>20~\mathrm{GeV}\)  
Luminosity (1/fb)  0.11  0.17  2.1  1.  0.036  0.036  0.84  0.3  0.4  0.75  9.7  0.037  0.94  
Reference  [32]  [33]  [34]  [35]  [36]  [37]  [38]  [39]  [40]  [41]  [42]  [43]  [44]  
NDP  11  11  28  13  12  35  11  10  28  12  14  10  9  
ABMP15 [30]  –  –  –  –  –  –  –  –  –  –  –  –  –  
CJ15 [1]  12  –  27  –  –  –  –  –  16  –  14  –  –  
CT14 [3]\(^\mathrm{a}\)  8.9  1  48  –  –  –  10.1  8.3\(^\mathrm{b}\)  17  –  –  9.9  –  
\(\chi ^2\)  JR14 [5]  –  –    –  –  –  –  –  –  –  –  –  – 
HERAFitter [31]\(^\mathrm{c}\)  –  –  32  15  –  –  –  –  23  –  16  –  –  
MMHT14 [6]  –  –  40  30  10 \(^\mathrm{d}\)  22  9  –  16  27  –  16  20  
NNPDF3.0 [7]  –  –  44.4  –  –  –  8.0  –  17.1  –  –  7.2  14.3 
Often, jet production in pp and \(p\bar{p}\) collisions is used as an additional process to constrain the largex gluon PDF. Here, the QCD corrections are known to NLO and the calculation of the NNLO ones is in progress [15]. The incomplete knowledge of the latter is problematic in view of a consistent PDF analysis at NNLO when including those jet data. This will be discussed in Sect. 4 in connection with the determination of the value of the strong coupling constant \(\alpha _s\).
In addition to these major categories of data commonly used to constrain PDFs, some complementary processes are also employed in some cases, as indicated in Table 1. These comprise the hadroproduction of topquark pairs from pp and \(p\bar{p}\) collisions and the associated production of W bosons with charm quarks in pp collisions. Sometimes, also jet production in ep collisions and prompt photon (\(\gamma \)+jet) production from pp and \(p\bar{p}\) collisions is considered. Except for \(t\bar{t}\) production, the necessary QCD corrections are known to NLO only, so that the same arguments as in the case of jet hadroproduction data apply, if those data are included in a fit at NNLO accuracy. For \(t\bar{t}\) production, only the inclusive cross section is considered at the moment in the available PDFs and there is a significant correlation with PDFs, especially of the gluon PDF with the topquark mass.
Taken together, the set of these data has a number of data points (NDP) of the order of few thousand, and provides sufficient information to describe the PDFs with an ansatz of about \(\mathcal{O}(30)\) free parameters. The parameters can include the strong coupling constant \(\alpha _s(M_Z)\) and the heavyquark masses \(m_c\), \(m_b\) and \(m_t\), which are correlated with the PDFs, as will be discussed in Sects. 3 and 4. This provides sufficient flexibility for all PDF groups and it is routinely checked that no additional terms are required to improve the quality of the fit. The exception is the NNPDF group, which typically uses \(\mathcal{O}(250)\) free parameters in the neural network.
Apart from those considerations there is the general problem of the quality of the experimental data, that is to say whether or not the PDFs are extracted from a consistent data set. The various groups have different approaches, which roughly fall into two classes according to the different confidence level (c.l.) criteria for the value of \(\chi ^2\) in the goodnessoffit test. One approach is to fit to a very wide (or even the widest possible) set of data, while the other one rejects inconsistent data sets. In the former case, a tolerance criterion for \(\Delta \chi ^2\) is introduced (e.g. \(\Delta \chi ^2=100\)), while the latter approach maintains that \(\Delta \chi ^2=1\). For the various PDF groups this information is listed in Table 1.
It is important to note that the \(\chi ^2\) values obtained with Eq. (1) will not necessarily correspond to numbers quoted by PDF groups due to different \(\chi ^2\) definitions, data treatment and other parameters, see also Table 1.
2.2 Results for PDFs
Before we start a detailed discussion of the theoretical aspects of the PDF determinations we would like to illustrate the present status of PDF sets at NNLO in QCD and discuss briefly some differences, which are clearly visible. The currently available sets at NNLO in QCD are shown in Figs. 1, 2, 3, 4, 5 and 6. The lightquark (u, d) valence PDFs together with the gluon and the quark sea distributions (\(x\Sigma = 2x(\bar{u}+\bar{c}+\bar{d}+\bar{s})\) for four active flavors) with the respective uncertainty bands are displayed in Figs. 1, 3 and 5 at the scales \(Q^2=4~\,\mathrm {GeV}^2\), \(100~\,\mathrm {GeV}^2\) and \(M_Z^2\) in the range \(10^{4} \le x \le 1\) for the sets ABM12 [2], HERAPDF2.0 [4] and JR14 [5]. Likewise, Figs. 2, 4 and 6 show the sets CT14 [3], MMHT14 [6] and NNPDF3.0 [7].
The main features of the present NNLO PDFs in Figs. 1, 2, 3, 4, 5 and 6 in the main kinematic region of x and \(Q^2\) relevant for hard scattering events at Tevatron and the LHC can be characterized as follows. The agreement in the distributions \(xu_v\), and to a slightly lesser extent \(\Sigma \), is very good for ABM12, JR14 and HERAPDF2.0, as shown in Fig. 1. For the valence PDF \(xd_v\) there is also an overall reasonable agreement, but the distribution deviates by more than \(1 \sigma \) at \(x \gtrsim 0.1\) in the case of HERAPDF2.0. One should note that \(xd_v\) is more difficult to measure in \(e^{\pm }p\) DIS at HERA than \(xu_v\) and additional constraints from deuteron data are important to fix the details of this PDF, as discussed in Sect. 3 below.
The results on the gluon momentum distribution xg are clearly different at low values of x. Here, JR14 obtains the largest values, followed by ABM12 and HERAPDF2.0, with the latter displaying a valencelike shape below \(x = 10^{3}\). For CT14, MMHT14 and NNPDF3.0 there is very good agreement for \(xu_v\), cf. Fig. 2. Some differences are visible in case of \(xd_v\), where CT14 reports larger values than NNPDF3.0 at \(x \gtrsim 5 \cdot 10^{3}\) and vice versa for smaller x. The spread in \(\Sigma \) for the sets in Fig. 2 is much greater than those by ABM12, JR14 and HERAPDF2.0. This is true as well for the gluon PDF xg with the CT14 uncertainty band for the gluon PDF also covering the predictions for the distributions by ABM12, and HERAPDF2.0. Note that the error bands for CT14 in Figs. 2, 4 and 6 correspond to the c.l. of 68 %.
The disagreement in \(xd_v\) between HERAPDF2.0 and ABM12 or JR14 persists through the evolution from \(Q^2 = 4~\,\mathrm {GeV}^2\) to \(Q^2 = M_Z^2\), cf. Fig. 3 and 5. Likewise, the spread in \(xd_v\) between CT14, MMHT14 and NNPDF3.0 becomes more pronounced, as shown in Fig. 4 and 6. On the other hand, differences in the singlet PDFs \(\Sigma \) and xg, while still somewhat visible at \(Q^2 = 100~\,\mathrm {GeV}^2\), largely wash out at scales \(Q^2 = M_Z^2\) which govern the physics of central rapidity events at the LHC. Those remaining differences persist at large scales (as in the case of the gluon PDFs at large \(x > 0.1\)) and will have a significant impact. The crucial test for all PDF sets comes through a detailed comparison of cross section predictions to data. This will be discussed in the remainder of the paper, in particular in Sects. 3 and 5.
3 Theory for PDF fits
In the following we describe the basic theoretical issues for a consistent determination of the twisttwo PDFs from DIS and other hard scattering data, on the basis of perturbative QCD at NNLO using the \(\overline{\mathrm {MS}}\, \) scheme for renormalization and factorization.
3.1 Theory for analyses of DIS data
The world DIS data are provided in terms of reduced cross sections by the different experiments. QED and electroweak radiative corrections [45, 46] are applied, which requires careful study of different kinematic variables [46, 47, 48, 49]. In this way also the contributions from the exchange of more than one gauge boson to the partonic twist2 terms are taken care of. In part, also the very small QED corrections to the hadronic tensor are already accounted for. These have a flat kinematic behavior and amount to \(\mathcal{O}(1\,\%)\) or less [50, 51, 52, 53].
 (i)
One is fitting the higher twist terms in \(F_i\). A rigorous approach requires the knowledge of their scaling violations (term by term) and of the various Wilson coefficients to higher orders in \(\alpha _s\), see e.g. Sect. 16 in Ref. [54]. Since at present this is practically out of reach, such fits remain rather phenomenological. Moreover, the size of the (nonsinglet) higher twist contributions to the structure function \(F_2\) vary strongly with the correction applied to the leadingtwist term up to nexttonexttonexttoleading order (N\(^3\)LO), as shown in Ref. [58, 62]. Also, the nonsinglet and singlet higher twist contributions are different [63, 64].
 (ii)
One has to find appropriate cuts to sufficiently reduce the higher twist terms. For instance, in the flavor nonsinglet analysis of Ref. [58] the cuts are taken to be \(Q^2 \ge 4~\,\mathrm {GeV}^2\), \(W^2 = M^2 + Q^2(1x)/x \ge 12.5~\,\mathrm {GeV}^2\). In the combined singlet and nonsinglet analysis of Ref. [64], \(Q^2 \ge 10~\,\mathrm {GeV}^2\), \(W^2 \ge 12.5~\,\mathrm {GeV}^2\) have been used. These bounds are found empirically by cutting on \(W^2\) and/or \(Q^2\) starting from larger values. Applying these cuts severely limits the amount of largex DIS data to be fitted, and usually leads to an increase of the errors of \(\alpha _s(M_Z)\) and other fitted fundamental parameters and distributions.
3.1.1 Massless PDFs
For all QCD calculations we use perturbation theory. The factorized representation in terms of Wilson coefficients and PDFs is obtained using the lightcone expansion [69, 70, 71, 72]. For a proper definition of the Wilson coefficients and the PDFs one has to use the LSZ formalism and refer to asymptotic states at large times \(t \rightarrow \pm \infty \), given by massless partons. We first describe the massless contributions in Eqs. (7) and (8), and then discuss the contribution of heavy quarks. The Wilson coefficients in Eq. (8) have a perturbative expansion in the strong coupling constant. At one [73], two [74, 75, 76, 77, 78, 79, 80, 81], and threeloop order [56, 57, 82, 83, 84] they have been calculated for the neutralcurrent structure functions \(F_{i}\), with \(i=1,2,3\), except for the \(\gamma Z\) mixing contribution at three loops.
The evolution equations (10), (11) can be either solved in x or Mellin (or moment) Nspace. In Mellinspace, defined by the transform Eq. (14), an analytic solution is possible [117, 118, 119, 120] by arranging the solution systematically in powers of the coupling constants \(a_s(\mu ^2)\) and \(a_s(\mu _0^2)\), and even forming factorizationscheme invariant expressions. In case of the xspace solutions this is usually not done due to the necessary iterative solution. In the smallx region the iterative solution usually leads to a pileup of a few per cent [121]. This can be corrected for in xspace solutions by applying the method given in [122]. Likewise, the iterated solution can be obtained in Mellin Nspace and is a standard option of the evolution program QCDPegasus [123].
3.1.2 Heavyquark structure functions
Disregarding contributions from charm at the input scale (“intrinsic charm”), cf. [124, 125, 126], the heavyflavor corrections to the DIS functions are described by Wilson coefficients. The leading order results are of \(\mathcal{O}(a_s)\). Higher order corrections in the perturbative expansions are, therefore, of \(\mathcal{O}(a_s^2)\) at NLO, and of \(\mathcal{O}(a_s^3)\) at NNLO, similar to the case of the longitudinal structure function [56, 57]. The corrections in the neutral and chargedcurrent cases are available in one [127, 128, 129, 130, 131, 132, 133, 134] and twoloop order [135, 136, 137, 138], where the latter corrections were given in semianalytic form.
For \(Q^2 \gg m_i^2\) the asymptotic corrections to \(F_L\) are available at threeloop order [139, 143]. For \(F_2\), four out of the five massive Wilson coefficients, \(L_{2,q}^{ns}\), \(L_{2,q}^{ps}\), \(L_{2,g}^{s}\) and \(H_{2,q}^{ps}\) are known as well [105, 139, 144, 145, 146] at large scales \(Q^2\). For the remaining coefficient, \(H_{2,g}^{s}\), an estimate has been made in Ref. [147] based on the anticipated smallx behavior [148], a series of moments calculated in [140], and twoloop operator matrix elements from Refs. [149, 150]. This provides a good approximation of the NNLO corrections.
3.2 Heavyflavor PDFs
An important issue in PDF fits concerns the number of active quark flavors and the theoretical description of heavy quarks such as charm and bottom. Due to the large range of hard scales Q for the scattering processes considered, different effective theories may be applied. At low scales, when \(Q \simeq \mathcal{O}(\mathrm{few})~\,\mathrm {GeV}\), one typically works with \(n_f = 3\) massless quark flavors, setting \(n_f = 3\) in the hard scattering cross section, the evolution kernels and the anomalous dimensions. In this case, only the lightquark PDFs for up, down and strange are taken into account. At higher scales, e.g., for hadroproduction of jets at high transverse momentum \(p_t\) or top quarks, additional dynamical degrees of freedom lead to theories with \(n_f \,{>}\, 3\). By means of the renormalization group and matching these are related to the case with \(n_f = 3\) massless quarks. Technically, one has to apply decoupling relations [151] at some matching scale \(\mu \), for instance in the transition of \(\alpha _s^{(n_f)} \rightarrow \alpha _s^{(n_f+1)}\). This introduces some logarithmic dependence on the masses of the heavy quarks \(m_c\), \(m_b\) and \(m_t\) for charm, bottom and top. One should also note that the matching of the effective theories for \(n_f \rightarrow n_f+1\) does not need to be smooth. In fact, it introduces discontinuities, such as for the running coupling as a solution of the QCD \(\beta \)function at higher order in the perturbative expansion, where \(\alpha _s^{(n_f)}(\mu ) \ne \alpha _s^{(n_f+1)}(\mu )\) in the \(\overline{\mathrm {MS}}\, \)scheme at the matching scale \(\mu \), see e.g., [152].
The matching conditions are typically imposed at the scale \(\mu = m_h\), and \(f_{h + {\bar{h}}} = 0\) is assumed for scales \(\mu \le m_h\). The necessary heavyquark OMEs \(A_{ij}\) depend logarithmically on the heavyquark masses as \(\alpha _s^l \ln ^k(\mu ^2/m_h^2)\) with \(0 \le k \le l\) in the perturbative expansion. As discussed above, the OMEs are known to NLO analytically [149, 154] and at NNLO either exactly or to a good approximation [105, 140, 144, 147, 155]. Thus, charm and bottom PDFs can be consistently extracted in QCD with a fixed number \(n_f =3,4\) or 5.
It should be stressed, however, that the decoupling relations for PDFs in Eqs. (17 18)–(20) assume the presence of one heavy quark at a time upon moving from lower scales to higher ones. Beginning at threeloop order, however, there are graphs containing both charm and bottomquark lines, and charm quarks cannot be treated as massless at the scale of the bottomquark due to \((m_c/m_b)^2 \approx 1/10\). Such terms cannot be attributed to either the charm or bottomquark PDFs, but rather one has to decouple charm and bottom quarks together at some large scale. The simultaneous decoupling of bottom and charm quarks in the strong coupling constant \(\alpha _s\) is discussed, for instance, in Ref. [156].
3.3 Heavyquarks schemes
3.3.1 Variableflavor number schemes
The hard scattering cross sections also depend on the number of flavors \(n_f\) and additional parton channels may open up, which have to be included as well. In addition, processes involving massive quarks depend logarithmically on the ratio \(Q^2/m_h^2\), where Q is some hard scale associated with the scattering. For the heavyflavor Wilson coefficients in Eq. (16) these logarithms are of the type \(\alpha _s^l \ln ^k(Q^2/m_h^2)\) with \(1 \le k \le l\) in perturbation theory. These originate from collinear singularities screened by the heavyquark mass due to the constrained phase space for gluon emission from massive quark lines, and as a prefactor of these logarithms one has the standard splitting functions. In addition to logarithmic terms, there are also power corrections \((m_h^2/Q^2)^l\) in the heavyflavor Wilson coefficients, usually appearing in form of higher transcendental functions. In the asymptotic regime of \(Q^2 \gg m_h^2\) the logarithms dominate and the kinematic dependence is measured experimentally, for instance in the tagged flavor case for charmquark pairs in the structure function \(F_2^{c\bar{c}}\). Logarithms of a similar kind are also experimentally observed in differential distributions, e.g. due to the QED corrections proportional to \(\ln ^k(Q^2/m_l^2)\) with \(m_l\) being the charged lepton mass, cf. [45, 46].
The resummation of the logarithms \(\alpha _s^l \ln ^k(Q^2/m_h^2)\) to all orders in perturbation theory is effectively carried out by the transition \(n_f \rightarrow n_f+1\) along with the introduction of new heavyquark PDFs as described in Eqs. (17 18)–(20). Whether such a transition is appropriate or not depends, of course, on the detailed kinematics. If the hard scale is closer to threshold, \(Q^2 \simeq m_h^2\), a description with \(n_f\) light flavors is more suitable, while for \(Q^2 \gg m_h^2\) one switches to a theory with \(n_f+1\) massless flavors. In order to achieve a unified description for hard scattering cross sections both at low scales \(Q^2 \simeq m_h^2\) and asymptotically for \(Q^2 \gg m_h^2\), socalled variableflavor number schemes (VFNS) have been constructed. Effectively, these aim at an interpolation between the asymptotic limits of the quarks being very light or very heavy relative to the other hard scales of the process. At the LHC such considerations apply to processes with bottom quarks in the initial state such as single topquark production as well as bottomquark initiated Higgs boson production (see Ref. [157] for more recent studies and Ref. [158] for the socalled Santander matching scheme for Higgs boson production in \(b\bar{b}\) annihilation).
Of particular interest for PDF fits is the reduced cross section for the pairproduction of heavy quarks in DIS, which is parametrized in terms of the DIS heavyquark structure functions \(F_i^h\) for \(i=2,L\) in Eq. (16) and with heavyflavor Wilson coefficients which are known exactly at NLO [135], and to a good approximation at NNLO [147] in QCD. For the interpolation \(n_f \rightarrow n_f+1\) of the heavyquark structure functions \(F_i^h\) a number of socalled generalmass VFNS (GMVFNS) have been discussed in the literature, such as ACOT [159, 160, 161], BMSN [153], FONLL [162] or RT [163]. These keep \(m_h \ne 0\) and are to be distinguished from the zeromass VFNS (ZMVFNS), which describes essentially the massless case. Note that presently the GMVFNS are applied only to one single heavy flavor at the time. That is the sequential transition \(n_f \rightarrow n_f+1\), so that the charm or bottom quarks are not considered simultaneously and charmquark mass effects in the bottomquark structure function \(F_i^b\) are neglected as discussed above.
The various GMVFNS contain a number of additional assumptions, and some come in more than one variety. The GMVFNS differ, for instance, in the way the low\(Q^2\) region is modeled. This modeling is a necessary undertaking to provide a reasonable behavior of the VFNS in the kinematical regime of present DIS data. Additional assumptions in the GMVFNS are related to the matching scale \(\mu \) for the transition \(n_f \rightarrow n_f+1\) as the adopted choice \(\mu = m_h\) is not unique, see [164] for an indepth discussion.
Finally, the logarithmic accuracy of the resummation for large scales \(Q^2 \gg m_h^2\), or the order of perturbation theory in current implementations of GMVFNS, is often not consistent. For example, NNLO evolution [103, 104] of the massless PDFs is sometimes combined with the heavyquark OMEs at NLO [149, 154], omitting NNLO results [105, 140, 144, 147].
Values of the charmquark mass and renormalization scheme used in the PDF fits together with a summary of schemes chosen for the description of the charmquark structure function \(F_2^c\) and the theoretical accuracy for the massive quark DIS Wilson coefficients. The values of \(\chi ^2\)/NDP for the DIS charm production cross section data [165] and HERA inclusive cross section data [4] are given in two columns with the account of PDF uncertainties (with unc., where CT14 PDF errors scaled from 90 % c.l. to 68 % c.l., i.e., reduced by a factor 1.645) and for the central prediction of each PDF set (nominal). In xFitter [166, 167], the values of electroweak parameters like the Fermi constant and Wboson mass are taken from Ref. [55]. The values for CT14 and for PDF4LHC with the SACOT(\(\chi \)) scheme have been determined with a cut on \(Q^2 \ge 5 \,\mathrm {GeV}^2\) on the HERA data [165]
PDF sets  \(m_c\) (GeV)  \(m_c\) renorm. scheme  Theory method (\(F_2^c\) scheme)  Theory accuracy for heavy quark DIS Wilson coeff.  \(\chi ^2\)/NDP for HERA charm data [165] with xFitter [166, 167]  \(\chi ^2\)/NDP for inclusive HERA data [4] (\(Q^2_\mathrm{min}= 3.5\) GeV\(^2\)) with xFitter [166, 167]  

With unc.  Nominal  With unc.  Nominal  
ABM12 [2]\(^\mathrm{a}\)  \(1.24~^{+~0.05}_{~0.03}\)  \(\overline{\mathrm {MS}}\, \) \(m_c(m_c)\)  FFNS \((n_f=3)\)  \(\mathcal{O}(\alpha _s^3)_\mathrm{approx}\)  65/52  66/52  1450/1145  1478/1145 
CJ15 [1]  1.3  \(m_c^\mathrm{pole}\)  SACOT [160]  \(\mathcal{O}(\alpha _s^2)\)  117/52  117/52  1458/1145  1465/1145 
CT14 [3]\(^\mathrm{b}\)  
NLO  1.3  \(m_c^\mathrm{pole}\)  SACOT(\(\chi \)) [161]  \(\mathcal{O}(\alpha _s^2)\)  51/47  70/47  1397/1145  1455/1145 
NNLO  1.3  \(m_c^\mathrm{pole}\)  SACOT(\(\chi \)) [161]  \(\mathcal{O}(\alpha _s^2)\)  64/47  130/47  1445/1145  1685/1145 
HERAPDF2.0 [4]  
NLO  1.47  \(m_c^\mathrm{pole}\)  RT optimal [163]  \(\mathcal{O}(\alpha _s^2)\)  67/52  67/52  1340/1145  1355/1145 
NNLO  1.43  \(m_c^\mathrm{pole}\)  RT optimal [163]  \(\mathcal{O}(\alpha _s^2)\)  62/52  62/52  1346/1145  1361/1145 
JR14 [5]\(^\mathrm{c}\)  1.3  \(\overline{\mathrm {MS}}\, \) \(m_c(m_c)\)  FFNS \((n_f=3)\)  \(\mathcal{O}(\alpha _s^3)_\mathrm{approx}\)  62/52  62/52  1561/1145  1651/1145 
MMHT14 [6]  
NLO  1.4  \(m_c^\mathrm{pole}\)  RT optimal [163]  \(\mathcal{O}(\alpha _s^2)\)  72/52  78/52  1411/1145  1526/1145 
NNLO  1.4  \(m_c^\mathrm{pole}\)  RT optimal [163]  \(\mathcal{O}(\alpha _s^2)\) \(^\mathrm{d}\)  71/52  83/52  1366/1145  1424/1145 
NNPDF3.0 [7]  
NLO  1.275  \(m_c^\mathrm{pole}\)  FONLLB [162]  \(\mathcal{O}(\alpha _s^2)\)  58/52  60/52  1402/1145  1453/1145 
NNLO  1.275  \(m_c^\mathrm{pole}\)  FONLLC [162]  \(\mathcal{O}(\alpha _s^2)\)  67/52  69/52  1393/1145  1496/1145 
PDF4LHC15 [8]\(^\mathrm{e}\)  −  −  FONLLB [162]  −  58/52  64/52  1369/1145  1481/1145 
−  −  RT optimal [163]  −  71/52  75/52  1396/1145  1496/1145  
−  −  SACOT(\(\chi \)) [161]  −  51/47  76/47  1378/1145  1497/1145 
3.3.2 Validation with DIS charmquark production
The H1 and ZEUS combined data for the DIS charm production cross section are unique for tests of GMVFNS and span the region of \(2.5 \le Q^2 \le 2000~\,\mathrm {GeV}^2\) and \(3\times 10^{5} \le x \le 0.05\). Values for the charmquark mass and \(\chi ^2\)/NDP for the individual PDF sets ABM12, CJ15, CT14, HERAPDF2.0, JR14, MMHT14, NNPDF3.0 as well as the averaged set PDF4LHC15 are given in Table 4, along with the information on the scheme choice for the heavyquark structure functions and on the theoretical accuracy for the massive quark DIS Wilson coefficients. For reference, Table 4 also list the \(\chi ^2\)/NDP values for the HERA inclusive cross section data [4]. Comparisons to data for the DIS charm production cross section are shown in Figs. 7, 8, 9 and 10. Note that Table 4 adopt the standard definition of perturbative orders for the heavyquark structure functions. This is not shared by CT14, MMHT14 and NNPDF3.0 in their GMVFNS. There the Born contribution to the heavyquark Wilson coefficients for \(ep \rightarrow c{\bar{c}}\), which is proportional to \(\mathcal{O}(\alpha _s)\), is referred to as being “NLO”. Analogously, the oneloop corrections of order \(\mathcal{O}(\alpha _s^2)\) are denoted by “NNLO”.
The remarkable fact in Table 4 and Fig. 9 is, however, that the GMVFNS SACOT(\(\chi \)) [161] of CT14 [3] and RT optimal [163] of MMHT14 [6] have difficulties in describing the DIS charm production data. Note that MMHT14 models the heavyquark Wilson coefficient functions at \(\mathcal{O}(\alpha _s^3)\) for low \(Q^2\) as described in [163] using known leading threshold logarithms [168] and \(\ln (1/x)\) terms [148], which have been shown not to be leading. This is indicated as \(\mathcal{O}(\alpha _s^2)\) in Table 4. Note that CT14 has applied a universal cut of \(Q^2 \ge 4~\,\mathrm {GeV}^2\) on all DIS data, excluding the bin at \(Q^2 = 2.5~\,\mathrm {GeV}^2\) in the HERA data [165] from the fit (cf. the upper left plot in Fig. 9). We have checked that including the low \(Q^2\) bin leads to a dramatic deterioration of the fit quality.
In addition, the schemes SACOT(\(\chi \)) and RT optimal as well as FONLLC [162] of NNPDF3.0 [7] do not improve the fit quality when comparing NLO and NNLO fits. We note in this context that those fits do not include the exact asymptotic [105, 140, 144, 146] and approximate [147] \(O(\alpha _s^3)\) results for the heavyquark Wilson coefficients in their theory predictions. The averaged set PDF4LHC15 [8], shown in Fig. 10, mixes PDFs derived with different mass schemes (ACOT, FONNL and RT) and does not describe the data very well for virtualities up to \(Q^2 \lesssim 20~\,\mathrm {GeV}^2\).
3.3.3 Charmquark mass
Dedicated studies of the charmquark mass dependence have been performed by several groups. In the \(\overline{\mathrm {MS}}\, \)scheme, the value of \(m_c(m_c)=1.24~^{+~0.04}_{~0.08}~\)GeV has been obtained in [169] together with \(\chi ^2/\)NDP=61/52 for the description of the HERA data [165] as a variant of the ABM11 fit [64]. Other groups, which keep a fixed value of \(m_c\) in the analyses, cf. Table 4, have studied the effects of varying \(m_c\) in predefined ranges. This has been done, for example, in the older NNPDF2.1 [170] and MSTW analyses [171] as well as for the MMHT PDFs [172]. The latter yields a pole mass of \(m_c^\mathrm{pole}=1.25~\,\mathrm {GeV}\) as the best fit with \(\chi ^2\)/NDP = 75/52, while the nominal fit uses \(m_c^\mathrm{pole}=1.4~\,\mathrm {GeV}\) at the price of a deterioration in the value of \(\chi ^2\)/NDP = 82/52. HERAPDF2.0 [4] has performed a scan of the values of \(\chi ^2\)/NDP leading to \(m_c^\mathrm{pole} = 1.43~\,\mathrm {GeV}\) at NNLO quoted in Table 4 as the best fit. NNPDF3.0 computes heavyquark structure functions with expressions for the pole mass definition, but adopts numerical values for the charmquark pole mass, \(m_c^\mathrm{pole} = 1.275~\,\mathrm {GeV}\), which corresponds to the current PDG value for the \(\overline{\mathrm {MS}}\, \)mass. This value is different from the one used in NNPDF2.3, namely \(m_c^\mathrm{pole} = \sqrt{2}~\,\mathrm {GeV}\). Within the framework of the CT10 PDFs [173] the charmquark mass in the \(\overline{\mathrm {MS}}\, \)scheme has been determined in Ref. [174] using the SACOT(\(\chi \)) scheme at order \(\mathcal{O}(\alpha _s^2)\), although with a significant spread in the central values reported (\(m_c(m_c)=1.12  1.24~\)GeV) depending on assumption in the fit.
In this context, it is worth to point out that the running mass \(m_c(\mu )\) in the \(\overline{\mathrm {MS}}\, \)scheme is free from renormalon ambiguities and can therefore be determined with high precision. The PDG [55] quotes \(m_c(m_c) = 1.275 \pm 0.025~\,\mathrm {GeV}\) based on the averaging different mass determination in various kinematics. DIS charmquark production analyzed in the FFNS \((n_f=3)\) leads to \(m_c(m_c) = 1.24 \pm 0.03~^{+~0.03}_{~0.03}\) GeV at NNLO [169], while measurements of the \(\overline{\mathrm {MS}}\, \)mass in \(e^+e^\) annihilation give, for instance, \(m_c(m_c) = 1.279 \pm 0.013\) GeV [175] and \(m_c(m_c) = 1.288 \pm 0.020\) GeV [176]. The determination from quarkonium 1S energy levels yields \(m_c(m_c) = 1.246 \pm 0.023\) GeV [177]. All these values are consistent with each other within the uncertainties.
In contrast, the accuracy of the pole mass \(m_c^\mathrm{pole}\) is limited to be of the order of the QCD scale \(\Lambda _\mathrm{QCD}\) and, moreover, the conversion from the \(\overline{\mathrm {MS}}\, \)mass \(m_c(m_c)\) at low scales to the pole mass \(m_c^\mathrm{pole}\) does not converge. Using \(\alpha _s(M_Z)=0.1184\), for example, the conversion yields for the central value of the PDG \(m_c^\mathrm{pole}=1.47~\,\mathrm {GeV}\) at one loop, \(m_c^\mathrm{pole}=1.67~\,\mathrm {GeV}\) at two loops, \(m_c^\mathrm{pole}=1.93~\,\mathrm {GeV}\) at three loops, and \(m_c^\mathrm{pole}=2.39~\,\mathrm {GeV}\) at fourloops [142]. The PDG quotes \(m_c^\mathrm{pole}=1.67 \pm 0.07~\,\mathrm {GeV}\) for conversion at two loops.
The low values for the pole mass of the charm quark assumed or obtained in some PDF fits as shown in Table 4 are thus not compatible with other determinations and with the world average. The rigorous determination of the charmquark mass discussed, for instance, in [169] provides a more controlled way of determining \(m_c\) from the world DIS data, taking also into account its correlation with \(\alpha _s(M_Z)\).
3.4 Lightflavor PDFs
3.4.1 Up and downquark distributions
The total quark contribution to nucleon matrix elements is known fairly well due to constraints from the available DIS data obtained in the fixedtarget and collider experiments in the xrange \(10^{4} \lesssim x \lesssim 0.8\). However, a thorough disentangling of the quark flavor structure is still a challenging task in any PDF analysis. At moderate and large x values this has been routinely achieved by using a combination of the DIS data obtained on proton and deuteron targets. However, uncertainties in the modeling of nuclear corrections in the deuteron introduce a controllable source of theoretical uncertainty on the dquark PDF obtained in this way, especially at large x, as discussed below.
An alternative way to resolve the u and dquark contributions is to use data on W and Zboson production obtained in pp and \(p\bar{p}\) collisions at the LHC and Tevatron, respectively. Those experiments probe the W and Z rapidity distributions up to rapidities of \(y=34\), depending on details of the experiments, with an integrated luminosity of \(O(1)~\mathrm{fb}^{1}\) achieved in each run. Such data samples are quite competitive in accuracy with the ones obtained in fixedtarget DIS experiments, and provide simultaneously constraints on the quark and antiquark PDFs at large and small x. Furthermore, the dquark PDF extracted from a combination of the existing data on DIS off protons and W / Zboson production in \(pp (p\bar{p})\) collisions is not sensitive to nuclear corrections. Moreover, if DIS data with small hadronic invariant masses \(W^2\) are not used in the analyses in order to reduce the sensitivity to higher twist contributions, the statistical potential of the deuteron data is reduced and they become less competitive as compared to the collider data, cf. Fig. 11.
As mentioned above, one can further constrain the u and dquark flavor separated distributions by utilizing fixedtarget deuteron DIS data. However, nuclear effects need to be accounted for in cross sections and structure functions in order to access the underlying PDFs. The theoretical uncertainty inherent in this nuclear correction procedure should be added to the statistical PDF uncertainties. Nonetheless, the reduction of the uncertainties due to the increased number of fitted data points is even greater, leading to an overall smaller dquark PDF uncertainty than in fits performed without deuterium data [30, 191, 192]. Furthermore, as shown in [1], and discussed in more detail below, it is possible to significantly reduce the nuclear correction uncertainty by exploiting the interplay of the deuteron DIS data and the recent highstatistics DØ data on the reconstructed \(W^\pm \) boson charge asymmetry at large rapidity, which is equally sensitive to the d / u ratio but is not affected by nuclear corrections.
The W / Zboson collider data also provide a valuable constraint on the smallx quark PDFs. In particular, the charge asymmetry of leptons originating from the W decays is sensitive to the SU(2) flavor asymmetry of the nonstrange sea, also referred to as the “isospin” asymmetry \(I(x)=[\bar{d}(x)\bar{u}(x)]\) at small x. This asymmetry is constrained by the DY data from fixedtargets with protons and deuterons collected by the Fermilab experiment E866 [193]. However, the E866 data are not sensitive to the value of I(x) at small x (\(x \lesssim 0.2\)). Therefore, I(x) is sometimes parametrized in a Reggelike form as \(I(x) \sim x^{0.5}\) such that it vanishes at \(x=0\) (cf. the MMHT results in Fig. 12).
The largerapidity tail of the W / Zboson production data allows for a modelindependent check of I(x) at small x. The asymmetry preferred by the combination of the currently available LHC and Tevatron data turns out to be negative at \(x<0.01\), while the Reggelike limit with a vanishing I(x) can still be recovered at \(x \lesssim 10^{5}\), cf. the ABMP15 results in Fig. 12. The CT14 analysis only includes the Tevatron forward DY data, but also confirms the negative trend in I(x) at small x, with errors in I(x) being substantially larger than those from ABMP15.
Finally, an important issue is the theoretical accuracy which is employed in the description of the DY data. There are significant differences as shown in Table 5 and these cause an additional spread in the fit quality and the results for the PDFs when comparing different NNLO PDF sets.
3.4.2 Strangequark distribution
The main information on the strange sea distribution comes from charmquark production in neutrinoinduced chargedcurrent DIS experiments. The publication of data from CHORUS and NOMAD has recently enlarged the statistics available for those experiments. As a net result, the uncertainty in the strange PDF is now reduced down to a few percent at \(x\sim 0.1\) (cf. Fig. 13). However, at small x the strange sea distribution is still poorly known in view of the restricted kinematics of the production of charm quarks from fixed targets. Furthermore, since neutrino DIS experiments usually involve nuclear targets, care needs to be taken when extracting freenucleon PDFs from the nuclear cross sections. Nuclear effects in neutrino DIS and possible differences between those in chargedlepton DIS have been discussed recently in Refs. [192, 194, 195], for instance. Supplementary information on the strange sea at small x, independent of nuclear effects, can be obtained from the associated production of charm quarks and W bosons in the pp collisions at the LHC. A constraint from collider data on \(W+c\) is potentially less sensitive to the cquark fragmentation model compared to the one from semileptonic decays of charm, which plays major role in the existing fixedtarget DIS experiments. The \(W+c\) data collected by ATLAS and CMS prefer a somewhat enhanced strange sea as compared to the fixedtarget determination, cf. Fig. 13. However, the NNLO QCD corrections to this process are still unknown. They are not taken into account in the analysis of \(W+c\) data so far and may have a substantial influence on the fit. The strange sea extracted by ATLAS from an analysis of the combined inclusive data on the W and Zboson production is even further enhanced, which suggests a restoration of SU(3) flavor symmetry in the sea distributions. However, the accuracy of this determination is poor due to a limited potential of the inclusive data in disentangling the quark flavors. Therefore, the ATLAS result is in fact comparable with other determinations within uncertainties.
In general, the existing experimental constraints on the strange PDF are relatively poor. Therefore, the results of various determinations demonstrate a significant spread, which is mainly driven by the data selection. An additional spread between results of earlier PDF analyses appears due to implementation issues. In particular, the strong strangesea suppression observed in the NNPDF2.1 analysis [170] was related to an error in the DIS charmquark production cross section being off by a factor of two for low scales due to an additional factor of \((1+m_c^2/Q^2)\) in Eq. (34) of Ref. [170]. This is now correct in NNPDF3.0 [7]. The CT10 analysis [66], which reported an enhanced strange sea, may be flawed due to a wrong sign of the photonZ interference for massive quarks in the structure function \(xF_3\) [3]. This error also concerns the earlier results on the strange–antistrange asymmetry [196] and has now been corrected in CT14 [3]. Finally, the MSTW [197] analysis suffered from an error in the NLO QCD correction for the chargedcurrent DIS charmquark production as it had omitted a part of the gluon Wilson coefficient at NLO, which was corrected in MMHT14 [6].
3.5 Nuclear corrections
Many global PDF analyses make use of data with deuterium targets, such as leptondeuteron DIS and protondeuteron DY, as a way of obtaining stronger constraints on the flavor dependence of PDFs that are not possible with proton data alone. The use of deuterium data requires that one takes into account differences between PDFs in the deuteron and those in the free nucleon, which arise from effects such as nuclear Fermi motion and binding of the nucleons in the nucleus, as well as nucleon offshell corrections and nuclear shadowing. While some analyses assume that nuclear corrections in the deuteron are negligible, a number of recent global PDF studies have incorporated nuclear effects into their analyses [1, 5, 64, 191, 198, 199, 200].
Description of the ATLAS data at \(\sqrt{s} = 7\) TeV for \(W^\pm \rightarrow l^\pm \nu \), \(Z\rightarrow l^+l^\) (Ref. [19]) used in the PDF fits. The columns indicate the QCD accuracy of the theoretical predictions along with the tools used to obtain them
PDF sets  \(\chi ^2\)/NDP (ATLAS data [19])  Theory accuracy  Theory method 

ABM12 [2]  34.5/30  NNLO  
ABMP15 [30]  32.3/30  NNLO  FEWZ3.1 [178] 
CT14 [3]  42/30  NNLL  ResBos [180] 
MMHT14 [172]  39/30  NNLO  APPLGrid [181], Cfactors [182] (kinematic dependence with FEWZ3.1 [178]) 
NNPDF3.0 [7]  35.4/30  NNLO  APPLGrid [181], Cfactors [183] (kinematic dependence with FEWZ3.1 [178]) 
The nucleon offshell corrections, on the other hand, are somewhat more model dependent, and several model studies have been performed to estimate their effect on nuclear PDFs [202, 203, 204, 205, 206]. Some earlier PDF analyses [191, 199, 200] used specific physicsmotivated models for the offshell corrections, while more recent approaches have fitted the offshell parameters directly to data [1, 205]. Other analyses [6, 207] have attempted to parametrize the entire nuclear correction in terms of a universal, \(Q^2\)independent function, without appealing to physical constraints. In this approach, to account for the effects of Fermi smearing a functional form must be used that produces the steep rise in the \(F_2^d/F_2^N\) ratio at high x, such as with a logarithm raised to a high power [207].
The effects of the nuclear corrections are most directly visible in the extraction of the dquark PDF at large x, see [1]. Figure 14 shows that omitting nuclear smearing effects in the deuteron leads to an overestimated d / u ratio at \(x \gtrsim 0.6\). In fact, omitting nuclear corrections induces a strong tension between the SLAC deuteron DIS data (see, e.g., [208]) and the recent high precision Wboson asymmetry data from the DØ collaboration at the Tevatron [26], which are sensitive to the dquark PDF in a similar largex range as the SLAC data, but are not affected by nuclear corrections. It also causes an artificial deformation of the dquark distribution, leading to essentially uncontrolled systematic errors when quark distributions are needed beyond the x range constrained by the data. This illustrates not only the theoretical but also the phenomenological need for such corrections when considering data at large x. Of course, one can choose to avoid nuclear effects altogether by using only proton data; however, doing so increases the uncertainty on the dquark PDF at both small and large values of x, as Fig. 14 illustrates. Additional details concerning the role of nuclear corrections when using deuterium target data in global fits can be found in Ref. [1]. The extrapolation of nuclear effects from the deuteron to heavy nuclei is unclear, especially in view of the differences between the offshell quark deformation fitted using deuteron targets [1] and using the ratio of heavy nuclei to deuteron structure functions [205]. As mentioned in the previous section, in general care should also be exercised when using neutrinonucleus scattering data to obtain, for example, constraints on strangequark PDFs, due to the currently poor understanding of the interaction dynamics of the final state heavy quark propagating through the target nucleus [192].
3.6 Software and tools
Data used in the PDF fits cover a wide range of kinematics and stem from a large number of different scattering processes. In order to achieve an accurate theoretical description of both the PDF evolution and the hard scattering cross sections, welltested software and tools are necessary. Benchmark numbers for the PDF evolution have long been established, see e.g., the Les Houches report [209], and opensource evolution codes such as QCDNUM [210, 211] and Hoppet [212] are available in Bjorken xspace and QCDPegasus [123] in Mellin Nspace. This is an important development as it allows to expose the software used in the PDF fits to systematic validation, the need of which can be illustrated with recent theory improvements published by various groups. For example, MSTW [197] has tested its NNLO evolution code against QCDPegasus [123] and corrected the implementation of one of the heavyquark OMEs.
For the hard scattering cross sections of the various processes, fast fitting methods like fastNLO [213, 214] and APPLGrid [181] have been developed. In addition, some groups have also published opensource code for the theory predictions of all physical cross sections employed in their analyses. The ABM11 and ABM12 fits [2, 64] use OPENQCDRAD [215] code, which is publicly available. The HERAPDF2.0 fit [4] relies on the QCD fit platform xFitter (formerly known as HERAFitter) [166, 167], which is an opensource package that provides a framework for the determination of PDFs and enables the choice of theoretical options for obtaining PDFdependent cross section predictions. In particular, xFitter allows for a choice of different available schemes for treatment of heavy quarks in DIS. In Mellin Nspace, an efficient method exists [216, 217] which improves on that by [218] and which has been widely used in analyses, e.g. [217]. However, no code has been made publicly available so far.
Given the increasing precision of PDF analyses, which is driven by the accuracy of the experimental data, there is ongoing demand to provide theoretical predictions that are as precise as possible. This has stimulated recent checks of the analysis software used by various groups and has resulted in a number of documented improvements. The list includes, for example, the corrections to the different parts of the DIS cross section calculations in the NNPDF2.1, MSTW and CT10 PDF analyses as mentioned in the discussion of the PDFs for strange sea above.
This illustrates that there is a continued need for benchmarking the hard scattering cross sections of relevance for PDF determinations in order to consolidate the accuracy of theory predictions for those observables. In this respect, opensource software may facilitate future theory improvements and may help to establish standards for precision theory predictions.
4 Strong coupling constant
The value of the strong coupling constant \(\alpha _s(M_Z)\) has a direct impact on the size of a number of cross sections at the LHC, such as Higgs boson production, see Sect. 5, and is therefore an important parameter. Due to QCD factorization, \(\alpha _s\) exhibits a significant correlation with the gluon PDF and also with the charmquark mass, as documented in the published correlation matrices, see for instance [64]. Therefore, the strong coupling constant has come to require particular attention in the context of global PDF analyses.
Values of \(\alpha _s(M_Z)\) obtained or used in the nominal PDF sets of the various groups
PDF sets  \(\alpha _s(M_Z)\)  Method of determination 

ABM12 [2]  \(0.1132 \pm 0.0011\)  Fit at NNLO 
CJ15 [1]  \(0.1183 \pm 0.0002\)  Fit at NLO 
CT14 [3]  0.118  Assumed at NNLO 
HERAPDF2.0Jets [4]\(^\mathrm{a}\)  \(0.1183~^{~+0.0040}_{~0.0034}\)  Fit at NLO 
JR14 [5]  \(0.1136 \pm 0.0004\)  Dynamical fit at NNLO 
\(0.1162 \pm 0.0006\)  Standard fit at NNLO  
MMHT14 [172]\(^\mathrm{b}\)  0.118  Assumed at NNLO 
NNPDF3.0 [7]  \(0.1150.121\)  Assumed at NNLO; preferred value 0.118 
PDF4LHC15 [8]  0.118  Assumed at NNLO 
Year  \(\alpha _s({M_Z})\)  Method/data sets/reference  

SY  2001  \(0.1166 \pm 0.0013\)  \(F_2^{ep}\) [223] 
2001  \(0.1153 \pm 0.0063\)  \(xF_3^{\nu N}\) (DIS off heavy nuclei) [223]  
A02  2002  \(0.1143 \pm 0.0020\)  [224] 
MRST03  2003  \(0.1153\pm 0.0020\)  [225] 
BBG  2004 (06, 12)  \(0.1134~_{~0.0021}^{+~0.0019}\)  
GRS  2006  0.112  Valence analysis, NNLO [228] 
AMP06  2006  \(0.1128 \pm 0.0015\)  [229] 
JR  2008  \(0.1128 \pm 0.0010\)  Dynamical approach [230] 
2008  \(0.1162 \pm 0.0006\)  With jet hadroproduction at NLO [230]  
ABKM  2009  \(0.1135 \pm 0.0014\)  \(n_f=3\) FFNS heavy quark scheme [198] 
2009  \(0.1129 \pm 0.0014\)  BMSN heavy quark scheme [198]  
MSTW  2009  \(0.1171 \pm 0.0014\)  [231] 
Thorne  2013  0.1175  DIS, Drell–Yan data; 
(MSTW)  Incl. higher twist, GMVFNS [232]  
ABM11\(_J\)  2010  \(0.1134  0.1149 \pm 0.0012\)  With jet production at Tevatron (NLO) [233] 
NNPDF2.1  2011  \(0.1173 \pm 0.0007 \pm 0.0009\)  
ABM11  2012  \(0.1134\pm 0.0011\)  [64] 
ABM12  2013  \(0.1132\pm 0.0011\)  [2] 
Thorne  2013  0.1136  DIS, Drell–Yan data 
(MSTW)  Incl. higher twist, FFNS [232]  
CT10  2013  0.1140  With jet hadroproduction data [66] 
JR  2014  \(0.1136 \pm 0.0004\)  Dynamical approach [5] 
2014  \(0.1162 \pm 0.0006\)  Standard fit [5]  
CT14  2015  \(0.1150~_{~0.0040}^{+~0.0060} \)  \(\Delta \chi ^2 > 1 \) and with data for DIS off heavy nuclei [3] 
MMHT  2015  \({0.1172} \pm 0.0013\)  With data for DIS off heavy nuclei [236] 
Of course, measurements of \(\alpha _s(M_Z)\) are not limited to global fits of PDFs, but stem from a large number of different processes and methods at different scales, see, e.g., [219, 220, 221] for discussions and comparisons. Here we restrict ourselves to issues of \(\alpha _s\) arising in PDF fits. In Table 6 we give an overview of the \(\alpha _s\) values currently used in the PDF analyses. There, two aspects are important. Firstly, some PDF analyses leave \(\alpha _s\) as a free parameter in their fits, which obviously allows one to control its correlation with other PDF parameters and avoids potential biases. Secondly, among the NNLO values of \(\alpha _s(M_Z)\) used there exists a large spread of \(\alpha _s\) values, ranging from \(\alpha _s(M_Z) = 0.1132\) to 0.1183. Some of those fitted values of \(\alpha _s(M_Z)\) are significantly smaller than, for example, an average provided by the PDG [55] in 2014, which gives \(\alpha _s(M_Z) = 0.1185 \pm 0.0006\) at NNLO, and is often quoted as a motivation for fixing \(\alpha _s(M_Z) = 0.118\) as in some entries in Table 6. In the recent 2015 update, the PDG [222] reports the value \(\alpha _s(M_Z) = 0.1181 \pm 0.0013\) with the uncertainty increased by a factor of two.
The jet data sets and the theory approximations used in the NNLO PDF fits. The threshold corrections of Ref. [237] neglect the dependence on the jet radius R. Ref. [238] has determined the regime of validity (“safety cuts”) of the threshold approximation of Ref. [240] by comparing to the exact NNLO result for the gg channel [15]
PDF set  Theory accuracy  Data sets used 

CT14 [3]  NLO with the scale set to the individual \(p_T\) of jet  Tevatron + LHC 
MMHT14 [172]  NLO + \(\mathcal{O}(\alpha _s^4)_\mathrm{approx}\) threshold corrections [237]  Tevatron 
NNPDF3.0 [7]  NLO + \(\mathcal{O}(\alpha _s^4)_\mathrm{approx}\) threshold corrections [238]  Tevatron + LHC (“safety cuts”) 
For instance, the inclusion of data for the hadroproduction of jets, e.g., from the LHC, does have an impact on the value of \(\alpha _s(M_Z)\) and can therefore provide valuable constraints. However, it is important to note that the perturbative QCD corrections to the hard scattering cross section are only known completely to NLO, while the exact NNLO result for the gg channel [15] and approximations based on soft gluon enhancement [237, 238, 239, 240] indicate corrections as large as 15–20 %. Those corrections and their magnitude depend, of course, on the details of the kinematics, the choice of the scale and on the jet parameters (e.g., jet radius R). For high \(p_T\) they are dominated by threshold logarithms \(\ln (p_T)\) accompanied by logarithms \(\ln (R)\) for small jet radii [240].
The \(\alpha _s(M_Z)\) values in PDF analyses currently determined with the help of jet data (cf. Table 7) are, strictly speaking, valid to NLO accuracy only and therefore subject to significantly larger theory uncertainties due to the variation of the renormalization scale. The various groups employ different approaches in their NNLO analyses to cope with this inconsistency, such as using dynamical scales or applying some variant of threshold corrections, as detailed in Table 8. As a result of these efforts, the gluon PDF and \(\alpha _s\) obtained, for example, in the MMHT14 and NNPDF3.0 analyses are in a good agreement.
Different modeling of important theory aspects, such as whether or not to include target mass corrections, higher twist contributions and nuclear corrections in the description of DIS data, or whether or not to use a VFNS in the description of DIS heavyquark data, can account for the range of \(\alpha _s(M_Z)\) values in Table 7. With largely similar model assumptions, NNPDF2.1 [234, 235], MSTW [231] and MMHT [236] obtained the range \(\alpha _s(M_Z) = 0.1171  0.1174\). All these choices can lead to systematic shifts of the value of \(\alpha _s(M_Z)\). Let us briefly mention some of the issues in detail.
The Higgs cross section at NNLO in QCD (computed in the effective theory) at \(\sqrt{s}=13\) TeV for \(m_H=125.0\) GeV at the nominal scale \(\mu _r=\mu _f=m_H\) with the PDF (and, if available, also \(\alpha _s\)) uncertainties. The columns correspond to different choices for the central value of \(\alpha _s(M_Z)\) using the nominal PDF set. The numbers in parenthesis are obtained using the PDF sets CT14nnlo_as_0115, HERAPDF20_NNLO_ALPHAS_115, MMHT2014nnlo_asmzlargerange and NNPDF30_nnlo_as _0115
PDF sets  \(\sigma (H)^\mathrm{NNLO}\) (pb) nominal \(\alpha _s(M_Z)\)  \(\sigma (H)^\mathrm{NNLO}\) (pb) \(\alpha _s(M_Z)=0.115\)  \(\sigma (H)^\mathrm{NNLO}\) (pb) \(\alpha _s(M_Z)=0.118\) 

ABM12 [2]  \( 39.80 \pm 0.84 \)  \( 41.62 \pm 0.46 \)  \( 44.70 \pm 0.50 \) 
CJ15 [1]\(^\mathrm{a}\)  \( 42.45_{0.18}^{+0.43}\)  \( 39.48_{0.17}^{+0.40}\)  \( 42.45_{0.18}^{+0.43}\) 
CT14 [3]\(^\mathrm{b}\)  \( 42.33_{1.68}^{+1.43}\)  \( 39.41_{1.56}^{+1.33}\) ( 40.10)  \( 42.33_{1.68}^{+1.43}\) 
HERAPDF2.0 [4]\(^\mathrm{c}\)  \( 42.62_{0.43}^{+0.35}\)  \( 39.68_{0.40}^{+0.32}\) ( 40.88)  \( 42.62_{0.43}^{+0.35}\) 
JR14 (dyn) [5]  \( 38.01 \pm 0.34 \)  \( 39.34 \pm 0.22 \)  \( 42.25 \pm 0.24 \) 
MMHT14 [6]  \( 42.36_{0.78}^{+0.56}\)  \( 39.43_{0.73}^{+0.53}\) ( 40.48)  \( 42.36_{0.78}^{+0.56}\) 
NNPDF3.0 [7]  \( 42.59 \pm 0.80 \)  \( 39.65 \pm 0.74 \) (\( 40.74 \pm 0.88 \))  \( 42.59 \pm 0.80 \) 
PDF4LHC15 [8]  \( 42.42 \pm 0.78 \)  \( 39.49 \pm 0.73 \)  \( 42.42 \pm 0.78 \) 
Higher order constraints from fixedtarget DIS data can also lead to shifts in \(\alpha _s(M_Z)\) [59]. For instance, NMC has measured the DIS differential cross sections and extracted the DIS structure functions \(F_2^\mathrm{NMC}\) [242]. At the time of the NMC analysis, however, the relevant DIS corrections to \(\mathcal{O}(\alpha _s^3)\) [57] were not available (see discussion after Eq. (5) above). This information is, however, important and has to be taken into account now. In case of fitting \(F_2^\mathrm{NMC}\) and not describing \(F_L(x,Q^2)\) at NNLO, much larger values of \(\alpha _s(M_Z^2)\) are obtained [67]. It is therefore strongly recommended to fit the published differential scattering cross sections using \(F_L(x,Q^2)\) at \(\mathcal{O}(\alpha _s^3)\). Presently, the MMHT [236] analysis uses \(F_L(x,Q^2)\) only at NLO. One should note, however, that the values of \(F_L(x,Q^2)\) at NNLO are significantly different in the smallx region (see [67]).
Finally, great care needs to be exercised when DIS data off nuclei are included in global fits, see Sect. 3. Details of modeling of nuclear corrections can in fact also cause systematic shifts in the value of \(\alpha _s(M_Z)\). Therefore, Table 7 indicates if scattering data on heavy nuclei have been included in the determination. For example, MMHT [236] has reported a comparatively high value of \(\alpha _s(M_Z)\) as a consequence of fitting the NuTeV \(\nu \)Fe DIS data [185]. In general, determinations of \(\alpha _s(M_Z)\) should be based upon, or at least crosschecked with, fits using proton and deuteron DIS data only.
5 Cross section predictions for the LHC
5.1 Higgs boson production
The dominant production mechanism for the SM Higgs boson at the LHC is the gluon–gluon fusion process. The large size of the QCD radiative corrections to the inclusive cross section at NLO, see, e.g. Ref. [244], together with the sizable scale uncertainty have motivated systematic theory improvements. In the effective theory based on the limit of a large topquark mass (\(m_t \rightarrow \infty \), integrating out the topquark loop, but using the full \(m_t\) dependence in the Born cross section), this has led to the computation of the corresponding corrections at NNLO [245, 246, 247] and even to N\(^3\)LO in QCD [106, 248]. This shows an apparent, if slow, convergence of the perturbative expansion, along with greatly reduced sensitivity to the choice for the renormalization and factorization scales \(\mu _r\) and \(\mu _f\). At N\(^3\)LO the total scale variation amounts to 3 % and estimates of the fourloop corrections support these findings [249].
This leaves, as the largest remaining source of uncertainties in the predictions of the physical cross section, the input for the strong coupling constant \(\alpha _s\) and the PDFs. Despite the impressive progress in theory and experiment, the situation resembles that after the completion of the NLO QCD corrections, when it was pointed out in Ref. [244] that one of the main residual uncertainties in the predictions was due to the gluon PDF.
The values of the charmquark mass (onshell scheme \(m^\mathrm{pole}\)) and the strong coupling \(\alpha _s(M_Z)\) in the MSTW analysis [171] using the set MSTW2008nnlo_mcrange together with the value for \(\chi ^2\)/NDP for the HERA data [165] and the Higgs cross section at NNLO in QCD (computed in the effective theory) at \(\sqrt{s}=13\) TeV for \(m_H=125.0\) GeV at the nominal scale \(\mu _r=\mu _f=m_H\). The numbers in parentheses are obtained using the PDF set MSTW2008nnlo_mcrange_fixasmz with the value of \(\alpha _s(M_Z)\) fixed to \(\alpha _s(M_Z)=0.1171\)
\(m_c^\mathrm{pole}\) (GeV)  \(\alpha _s(M_Z)\) (best fit)  \(\chi ^2\)/NDP (HERA data [165])  \(\sigma (H)^\mathrm{NNLO}\) (pb) best fit \(\alpha _s(M_Z)\)  \(\sigma (H)^\mathrm{NNLO}\) (pb) \(\alpha _s(M_Z)=0.1171\) 

1.05  0.1157  73/52  40.65  (41.63) 
1.1  0.1159  69/52  40.85  (41.70) 
1.15  0.1160  66/52  41.04  (41.78) 
1.2  0.1162  64/52  41.25  (41.85) 
1.25  0.1164  64/52  41.47  (41.93) 
1.3  0.1166  63/52  41.69  (42.00) 
1.35  0.1168  63/52  41.93  (42.09) 
1.4  0.1171  65/52  42.16  (42.16) 
1.45  0.1173  68/52  42.42  (42.24) 
1.5  0.1175  73/52  42.64  (42.31) 
1.55  0.1177  80/52  42.88  (42.38) 
1.6  0.1180  88/52  43.16  (42.46) 
1.65  0.1182  99/52  43.34  (42.51) 
1.7  0.1184  112/52  43.59  (42.58) 
1.75  0.1186  127/52  43.81  (42.63) 
Same as Table 10 for the MMHT14 analysis [172] using the set MMHT2014nnlo_mcrange_nf5 and setting \(\alpha _s(M_Z)\) to the best fit value. The numbers of Ref. [182] keep full account of the correlation between the PDFs and \(\alpha _s\). The values of \(\chi ^2\)/NDP for the HERA data [165] are those quoted in [172] for the best fit value of \(\alpha _s(M_Z)\). The numbers in parentheses are obtained with the value of \(\alpha _s(M_Z)\) fixed to \(\alpha _s(M_Z)=0.118\)
\(m_c^\mathrm{pole}\) (GeV)  \(\alpha _s(M_Z)\) (best fit)  \(\chi ^2\)/NDP (HERA data [165])  \(\sigma (H)^\mathrm{NNLO}\) (pb) best fit \(\alpha _s(M_Z)\)  \(\sigma (H)^\mathrm{NNLO}\) (pb) \(\alpha _s(M_Z)=0.118\)  

This work  Ref. [182]  
1.15  0.1164  78/52 (71/52)  40.48  41.01  (42.05) 
1.2  0.1166  76/52 (70/52)  40.74  41.18  (42.11) 
1.25  0.1167  75/52 (76/52)  40.89  41.33  (42.17) 
1.3  0.1169  76/52 (77/52)  41.16  41.48  (42.25) 
1.35  0.1171  78/52 (79/52)  41.41  41.68  (42.30) 
1.4  0.1172  82/52 (83/52)  41.56  41.83  (42.36) 
1.45  0.1173  88/52 (89/52)  41.75  42.00  (42.45) 
1.5  0.1173  96/52 (96/52)  41.81  42.14  (42.51) 
1.55  0.1175  105/52 (106/52)  42.08  42.29  (42.58) 
Of particular interest is the impact of additional parameters in the PDF fits, such as the charmquark mass, on the Higgs cross section. The differences in the treatment of heavy quarks and the consequences for the quality of the description of charmquark DIS data have already been discussed in Sect. 3. ABM12 [2] fits the value of \(m_c(m_c)\) in the \(\overline{\mathrm {MS}}\, \)scheme and the uncertainties in the charmquark mass are included in the uncertainties quoted in Table 9. Other groups keep a fixed value of the charmquark mass in the onshell scheme, cf. Table 4, and vary the value of \(m_c^\mathrm{pole}\) within some range. Such studies have been performed in the past by NNPDF2.1 [170] and MSTW [171] and more recently by MMHT [172].
PDF sets  \(m_c^\mathrm{pole}\) (GeV)  \(\alpha _s(M_Z)\) (fixed)  \(\chi ^2\)/NDP (HERA data [165])  \(\sigma (H)^\mathrm{NNLO}\) (pb) fixed \(\alpha _s(M_Z)\) 

NNPDF2.1 [170]  \(\sqrt{2}\)  0.119  65/52  \( 44.18 \pm 0.49 \) 
1.5  0.119  78/52  \( 44.54 \pm 0.51 \)  
1.6  0.119  92/52  \( 44.74 \pm 0.50 \)  
1.7  0.119  110/52  \( 44.95 \pm 0.51 \)  
NNPDF2.3 [250]  \(\sqrt{2}\)  0.118  71/52  \( 43.77 \pm 0.41 \) 
NNPDF3.0 [7]  1.275  0.118  67/52  \( 42.59 \pm 0.80 \) 
NNPDF has performed a study of the \(m_c\) dependence in [170], which shows the same trend as for MSTW and MMHT, i.e., the smaller the chosen value of \(m_c^\mathrm{pole}\), the better the goodnessoffit for the HERA data [165]. In addition, Table 12 displays the changes in the charmquark mass values from \(m_c^\mathrm{pole} =\sqrt{2}~\,\mathrm {GeV}\) to \(m_c^\mathrm{pole} = 1.275~\,\mathrm {GeV}\) in the evolution of the NNPDF fits from v2.1 [170] and v2.3 [250] to v3.0 [7], with the obvious correlation of smaller cross sections for Higgs boson production with smaller chosen values of \(m_c^\mathrm{pole}\).
As pointed out already in Sect. 3, onshell masses \(m_c^\mathrm{pole}=1.2  1.3~\,\mathrm {GeV}\), as preferred by the goodnessoffit analyses in Tables 10, 11 and 12 for the charmquark data from HERA [165], are not compatible with the world average of the PDG [55]. Thus, in some PDF fits, the numerical value of the charmquark mass effectively takes over the role of a “tuning” parameter for the Higgs cross section. Note that the three analyses are based on partly different data sets, theory and methodology.
5.2 Hadroproduction of heavy quarks
5.2.1 Topquark hadroproduction: inclusive cross section
The inclusive cross section for topquark pair production at NNLO in QCD at \(\sqrt{s}=13\) TeV for a pole mass of \(m_t^\mathrm{pole}=172.0\) GeV at the nominal scale \(\mu _r=\mu _f=m_t^\mathrm{pole}\) with the PDF (and, if available, also \(\alpha _s\)) uncertainties. The columns correspond to different choices for the central value of \(\alpha _s(M_Z)\) using the nominal PDF set. The numbers in parenthesis are obtained using PDF sets CT14nnlo_as_0115, HERAPDF20_NNLO_ALPHAS_115, MMHT2014nnlo_asmzlargerange and NNPDF30_nnlo_as _0115
PDF sets  \(\sigma (t{\bar{t}})^\mathrm{NNLO}\) (pb) nominal \(\alpha _s(M_Z)\)  \(\sigma (t{\bar{t}})^\mathrm{NNLO}\) (pb) \(\alpha _s(M_Z)=0.115\)  \(\sigma (t{\bar{t}})^\mathrm{NNLO}\) (pb) \(\alpha _s(M_Z)=0.118\) 

ABM12 [2]  \( 715.1 \pm 21.3 \)  \( 741.7 \pm 10.3 \)  \( 786.5 \pm 10.9 \) 
CJ15 [1]\(^\mathrm{a}\)  \( 786.7~_{~11.8}^{+~5.1} \)  \( 742.0~_{~11.1}^{+~4.8} \)  \( 786.7~_{~11.8}^{+~5.1} \) 
CT14 [3]\(^\mathrm{b}\)  \( 834.2~_{~36.5}^{+~36.0}\)  \( 786.7~_{~34.4}^{+~34.0}\)  \( 834.2~_{~36.5}^{+~36.0}\) 
( 791.4)  
HERAPDF2.0 [4]\(^\mathrm{c}\)  \( 804.1~_{~17.0}^{+~11.4}\)  \( 757.8~_{~16.0}^{+~10.8}\)  \( 804.1~_{~17.0}^{+~11.4}\) 
( 756.8)  
JR14 (dyn) [5]  \( 719.3 \pm 9.1 \)  \( 739.6 \pm 5.7 \)  \( 784.2 \pm 6.0 \) 
MMHT14 [6]  \( 831.8~_{~17.5}^{+~13.9} \)  \( 784.5~_{~16.5}^{+~13.1}\)  \( 831.8~_{~17.5}^{+~13.9} \) 
( 794.8)  
NNPDF3.0 [7]  \( 831.8 \pm 15.0 \)  \( 784.4 \pm 14.2 \)  \( 831.8 \pm 15.0 \) 
(\( 800.9 \pm 16.5 \))  
PDF4LHC15 [8]  \( 832.5 \pm 16.4 \)  \( 785.1 \pm 15.5 \)  \( 832.5 \pm 16.4 \) 
The values of the charmquark mass (onshell scheme \(m_c^\mathrm{pole}\)) and the strong coupling \(\alpha _s(M_Z)\) in the MMHT14 analysis [172] together the inclusive cross section for topquark pair production at NNLO in QCD computed with the set MMHT2014nnlo_mcrange_nf5 at \(\sqrt{s}=13\) TeV for a pole mass of \(m_t^\mathrm{pole}=172.0\) GeV at the nominal scale \(\mu _r=\mu _f=m_t^\mathrm{pole}\) and setting \(\alpha _s(M_Z)\) to the best fit value. The numbers of Ref. [182] keep full account of the correlation between the PDFs and \(\alpha _s\). The values of \(\chi ^2\)/NDP for the HERA data [165] are those quoted in [172] for the best fit value of \(\alpha _s(M_Z)\). The numbers in parentheses for the cross section and \(\chi ^2\)/NDP are obtained using the PDF set with the value of \(\alpha _s(M_Z)\) fixed to \(\alpha _s(M_Z)=0.118\)
\(m_c^\mathrm{pole}\) (GeV)  \(\alpha _s(M_Z)\) (best fit)  \(\chi ^2\)/NDP (HERA data [165])  \(\sigma (t{\bar{t}})^\mathrm{NNLO}\) (pb) best fit \(\alpha _s(M_Z)\)  \(\sigma (t{\bar{t}})^\mathrm{NNLO}\) (pb) \(\alpha _s(M_Z)=0.118\)  

This work  Ref. [182]  
1.15  0.1164  78/52 (71/52)  810.2  815.0  (835.8) 
1.2  0.1166  76/52 (70/52)  813.0  817.3  (835.4) 
1.25  0.1167  75/52 (76/52)  814.0  818.3  (834.8) 
1.3  0.1169  76/52 (77/52)  816.5  819.3  (834.2) 
1.35  0.1171  78/52 (79/52)  819.0  821.8  (833.4) 
1.4  0.1172  82/52 (83/52)  819.0  822.4  (831.8) 
1.45  0.1173  88/52 (89/52)  820.2  823.1  (831.5) 
1.5  0.1173  96/52 (96/52)  818.8  823.1  (830.0) 
1.55  0.1175  105/52 (106/52)  821.0  823.6  (829.0) 
In a similar study as for Higgs boson production in Table 9 we illustrate in Table 13 the PDF dependence of the inclusive cross section \(\sigma (t{\bar{t}})^\mathrm{NNLO}\) for various sets with uncertainties \(\Delta \sigma (\mathrm {PDF}+\alpha _s)\). The computation is performed in the theoretical framework as implemented in the HATHOR code [256]. In Table 13 we choose \(\sqrt{s}=13\) TeV and fix the pole mass \(m_t^\mathrm{pole}=172.0\) GeV and the scales at \(\mu _r = \mu _f = m_t^\mathrm{pole}\). For this fixed value of \(m_t^\mathrm{pole}\), we show the impact of different values for the strong coupling constant at NNLO. We choose \(\alpha _s(M_Z)\) either corresponding to the respective PDF set or fixed to the values 0.115 and 0.118. The results in Table 13 display a spread in a range \(\sigma (t{\bar{t}})^\mathrm{NNLO} = 715  834~\text{ pb }\) using the nominal value of \(\alpha _s(M_Z)\) for each PDF set, which amounts to a relative range of more than \(15\,\%\). This decreases to about \(6\,\%\), if the values of \(\alpha _s(M_Z)\) are fixed to 0.115 or 0.118.
The theoretical predictions at leading order depend parametrically on the strong coupling constant and the topquark mass to second power, as well as on the convolution of the gluon PDFs, \(\sigma (t{\bar{t}})^\mathrm{LO} \propto (\alpha _s^2/m_t^2) \left( g \otimes g \right) \). Therefore, it is necessary to fully account for the correlations between the topquark mass, the gluon PDF and the strong coupling when comparing to experimental data. A number of analyses have considered \(t{\bar{t}}\) hadroproduction data. ABM12 [2] has included data for topquark pairproduction in a variant of the fit to determine the \(\overline{\mathrm {MS}}\, \)mass \(m_t(m_t)\), keeping the full correlation with \(\alpha _s(M_Z)\) and the gluon PDF. On the other hand, CMS has determined the topquark pole mass as well as the strong coupling constant in a fit which kept all other parameters mutually fixed [258], while Ref. [259] has explored constraints on the gluon PDF from \(t{\bar{t}}\) hadroproduction data using fixed values for \(\alpha _s(M_Z)\) and the pole mass \(m_t^\mathrm{pole}\).
In the global analyses by MMHT14 [6] and NNPDF3.0 [7] those data were also used to fit \(\alpha _s(M_Z)\) and the gluon PDF. These analyses employ a fixed value for the pole mass \(m_t^\mathrm{pole}\), which is motivated by precisely measured topquark masses from kinematic reconstructions, i.e., Monte Carlo masses, but does not account for the above mentioned correlation with \(\alpha _s(M_Z)\) and the gluon PDF. Moreover, the Monte Carlo mass requires additional calibration [260].
PDF sets  \(m_c^\mathrm{pole}\) (GeV)  \(\alpha _s(M_Z)\) (fixed)  \(\chi ^2\)/NDP (HERA data [165])  \(\sigma (t{\bar{t}})^\mathrm{NNLO}\) (pb) fixed \(\alpha _s(M_Z)\) 

NNPDF2.1 [170]  \(\sqrt{2}\)  0.119  65/52  \( 847.1 \pm 16.3 \) 
1.5  0.119  78/52  \( 850.8 \pm 14.3 \)  
1.6  0.119  92/52  \( 842.9 \pm 13.6 \)  
1.7  0.119  110/52  \( 840.1 \pm 14.1 \)  
NNPDF2.3 [250]  \(\sqrt{2}\)  0.118  71/52  \( 835.7 \pm 14.9 \) 
NNPDF3.0 [7]  1.275  0.118  67/52  \( 831.8 \pm 15.0 \) 
5.2.2 Topquark hadroproduction: differential distributions
The differential cross section of the topquark pair production is also known to NNLO in QCD [261]. Publicly available codes such as Difftop [262] provide differential distributions to approximate NNLO accuracy based on softgluon threshold resummation results. We use Difftop to calculate the distribution in the topquark rapidity \(y_t\) for protonproton collisions at \(\sqrt{s}=13\) TeV at NNLO\(_\mathrm{approx}\) accuracy using the ABM12, CT14, MMHT14, NNPDF3.0, and the PDF4LHC15 PDF sets at NNLO with their respective \(\alpha _s\) values. Here, we take the topquark pole mass to be \(m_t^\mathrm{pole}=172.5\) GeV, following the preferences in the LHC analyses. The renormalization and factorization scales are set to \(m_t^\mathrm{pole}\) and the choice of a dynamical scale does not change the following discussions.
By using differential cross sections, not only the sensitivity of topquark pair production to the PDFs can be estimated, but also possible effects on the experimental acceptance by changing the PDF choice. In the experimental analysis, the PDF dependent acceptance corrections arise mostly from the PDF dependent normalization of the production cross section and originate from the phase space regions uncovered by the detector. Usually, the acceptances are determined by using Monte Carlo simulations as a ratio of the number of reconstructed events in the fiducial volume of the detector (visible phase space) to the number of events generated in the full phase space. In the case of topquark pair production, the visible (full) phase space would correspond to the topquark rapidity range of \(y_t<2.5\) \((y_t<3)\). Here, an acceptance estimator and a related extrapolation factor are calculated by using Difftop predictions for the respective cross section ratios \(\sigma _\mathrm{vis}/\sigma _\mathrm{tot}\) and \(\sigma _\mathrm{unmeasured}/\sigma _\mathrm{tot}\). Such estimators are not expected to describe the true experimental efficiency, but are helpful for drawing conclusions about PDF related effects.
The predictions of the topquark rapidity and the acceptance estimates obtained by using Difftop with different PDFs are shown in Fig. 15. The largest difference in the global normalization of the predicted cross sections is observed if the ABM12 PDFs are used instead of the CT14, NNPDF3.0 or MMHT14 sets. The origin of this effect is again the smaller nominal value of \(\alpha _s\) in ABM12 in combination with a smaller gluon PDF in the x range relevant to topquark pair production at \(\sqrt{s}=13~\,\mathrm {TeV}\). The corresponding acceptance estimators and their uncertainties, obtained from the error propagation of the corresponding PDF uncertainties at 68 % c.l., however, demonstrate significant differences also in the expected acceptance corrections, obtained by using ABM12 alternative to other PDFs.
The recent PDF4LHC recommendation [8] for calculation of the acceptance corrections for precision observables, such as the topquark pairproduction cross section in the LHC Run 2 data taking period, is to use the set PDF4LHC15_100, which is obtained by averaging the CT14, MMHT14 and NNPDF3.0 PDFs. While the central prediction obtained by using PDF4LHC15 is indeed very close to those obtained with the CT14, MMHT14 or NNPDF3.0 PDFs, the error on the corresponding acceptance estimator somewhat underestimates the acceptance spread of the individual PDFs with their uncertainties. Furthermore, it does not cover the difference in the acceptances to the one using the ABM12 PDF. Therefore, for the conservative estimate of the acceptance correction and its uncertainty, as demanded in the measurement of SM precision observables, the use of the PDF4LHC15_100 set would lead to a significant underestimation of the uncertainty on the resulting cross section measurement.
A further conclusion from Fig. 15 is that in the case of topquark pair production, once calculational speed is needed, it seems to be sufficient to consider a reduced choice of PDF sets. For instance, instead of using the averaged set PDF4LHC15_100 one can take just one of the three PDFs, CT14, MMHT14 or NNPDF3.0. Alternative PDF choices can then always be studied to some approximation with a reweighting method. In spite of the valiant effort in Ref. [8] to provide a uniform solution, the PDF choice for measurements of precision observables must be decided on a casebycase basis for each particular process.
5.2.3 Bottomquark hadroproduction
Bottomquark production in protonproton collisions at the LHC is also dominated by the gluon–gluon fusion process. Therefore, the LHCb measurements of Bmeson production in the forward region [263] with rapidities \(2.0< y < 4.5\) at \(\sqrt{s}=7\) TeV probe the gluon distributions simultaneously at small x up to \(x \sim 2 \times 10^{5}\) and at large \(x \simeq 1\). The smallx region is not accessible with HERA DIS data, for example. The potential improvements of PDFs near the edges of the currently covered kinematical region, namely, at small x and low scales, was first illustrated in [264, 265] using differential LHCb data on hadroproduction of \(c\bar{c}\) and \(b\bar{b}\) pairs.
In the present comparison in Table 16, the normalized cross sections, \((\mathrm{d}\sigma /\mathrm{d}y) / (\mathrm{d}\sigma /\mathrm{d}y_0)\), for bottomquark production are calculated from the absolute measurements published by LHCb, with \(\mathrm{d}\sigma /\mathrm{d}y_0\) being the cross section in the central bin, \(3< y_0 < 3.5\), of the measured rapidity range in each \(p_T\) bin [264]. In the absence of NNLO QCD corrections, the theoretical predictions are obtained at NLO in QCD [266, 267, 268] using a fixed number of flavors, \(n_f = 3\), for the hard scattering cross sections. Since data for the hadroproduction of heavy quarks other than top have not been considered for publicly available PDF fits thus far, issues of any model dependence such as in [158] due to the use of GMVFNS cannot be quantified. In the calculation of the normalized cross sections, the theoretical uncertainty is strongly reduced, since variations of the renormalization and factorization scales as well as of the fragmentation parameters do not significantly affect the shape of the y distributions for heavyflavor production, while this shape remains sensitive to PDFs.
The values of \(\chi ^2\)/NDP for the normalised bottomquark cross sections measured at LHCb [263] using the NLO PDFs of the individual groups. The left column accounts for the quoted PDF uncertainties (with the CJ15 and CT14 PDF uncertainties rescaled to 68 % c.l.), while the right column uses the central prediction of each PDF set
PDF sets  \(\chi ^2\)/NDP (with unc.)  \(\chi ^2\)/NDP (nominal) 

ABM11 [64]\(^\mathrm{a}\)  222 / 244  394 / 244 
CJ15 [1]  241 / 244  272 / 244 
CT14 [3]  166 / 244  241 / 244 
HERAPDF2.0 [4]  219 / 244  366 / 244 
JR14 [5]  205 / 244  217 / 244 
MMHT14 [6]  165 / 244  202 / 244 
NNPDF3.0 [7]  160 / 244  197 / 244 
PDF4LHC15 [8]  173 / 244  218 / 244 
The values for \(\chi ^2\)/NDP given in Table 16 are computed with the QCD fit platform xFitter for the individual PDF sets obtained at NLO, namely, ABM11 [64], CJ15 [1], CT14 [3], HERAPDF2.0 [4], JR14 [5], MMHT14 [6], NNPDF3.0 [7], as well as the averaged set PDF4LHC15 [8]. All PDFs provide a good description of the data, despite the fact that none of the groups use any data sensitive to the gluons at very low x, in the region directly probed by the LHCb Bmeson measurement. Remarkably, one finds that \(\chi ^2/\text{ NDP } < 1\) for the vast majority of the groups (left column in Table 16), suggesting that the derived PDF uncertainties at the edges of the so far measured regions might be inflated.
5.2.4 Charmquark hadroproduction
Charmquark hadroproduction offers another possibility to illustrate the consistency of the theory predictions for the various PDF sets. The exclusive production of charmed mesons in the forward region at LHCb probes the gluon distribution down to smallx values of \(x \sim 5 \times 10^{6}\) at \(\sqrt{s}=7\) TeV, and data can be confronted with QCD predictions at NLO accuracy, see, e.g., [269, 270].
For the inclusive cross section of the reaction pp \(\rightarrow \) \(c\bar{c}\) the QCD predictions are known up to NNLO in the \(\overline{\mathrm {MS}}\, \)scheme for the charmquark mass and display good convergence of the perturbative expansion and stability under variation of the renormalization and factorization scales [269]. In Figs. 16 and 17 we compare the theory predictions at NLO and NNLO with \(m_c(m_c) = 1.27~\,\mathrm {GeV}\) in the \(\overline{\mathrm {MS}}\, \)scheme, see Sect. 3, for the scale choice \(\mu _r=\mu _f=2m_c(m_c)\) as a function of the centerofmass energy \(\sqrt{s}\) to available experimental data. These data span a large range in \(\sqrt{s}\), which starts with fixed target experiments at energies up to \(\sqrt{s} = 50~\,\mathrm {GeV}\) summarized in [271] and HERAB data [272] (purple points in Figs. 16, 17). At higher energies RHIC data from PHENIX and STAR [273, 274] (black points in Figs. 16, 17) are available and the LHC contributes measurements at energies \(\sqrt{s}=2.76~\,\mathrm {TeV}\) from ALICE [275], at \(\sqrt{s}=7~\,\mathrm {TeV}\) from ALICE [275], ATLAS [276] and LHCb [277], and at the highest available energy \(\sqrt{s}=13~\,\mathrm {TeV}\) from LHCb [278] (blue points in Figs. 16, 17). The total cross sections of LHCb have been obtained from charmed hadron production measurements in a limited phase space region [277, 278] using extrapolations based on NLO QCD predictions matched with parton shower Monte Carlo generators.
The theory predictions for the PDF sets ABM12, CJ15, CT14 and JR14 at NLO and NNLO are shown in Fig. 16, together with the respective PDF uncertainties. For all these PDF sets the perturbative expansion is stable, the theory computations agree well with the data and predictions, e.g., for a future collider with \(\sqrt{s} \simeq 100~\,\mathrm {TeV}\), yield positive cross sections. The PDF uncertainties obtained for CT14, however, do increase significantly above energies of \(\sqrt{s} \simeq 1~\,\mathrm {TeV}\).
The same information for the sets HERAPDF2.0, MMHT14, NNPDF3.0 and PDF4LHC15 is displayed in Fig. 17. These predictions all agree with data at low energies but start to behave very differently for HERAPDF2.0, MMHT14 or NNPDF3.0 at energies above \(\sqrt{s} \simeq \mathcal{O}(10)~\,\mathrm {TeV}\) and for PDF4LHC15 above \(\sqrt{s} \simeq \mathcal{O}(100)~\,\mathrm {TeV}\). At the same time, the associated PDF uncertainties in this region of phase space become very large, thereby limiting the predictive power. Typically, the PDF uncertainties of the NNLO sets are even larger than at NLO. In the case of MMHT14 the consistency of the NNLO predictions with LHC data from ALICE [275], ATLAS [276] and LHCb [277, 278] at energies of \(\sqrt{s}=7~\,\mathrm {TeV}\) and \(13~\,\mathrm {TeV}\) deteriorates. For NNPDF3.0 the central prediction at NNLO displays a change in slope for energies above \(\sqrt{s} \simeq 3~\,\mathrm {TeV}\) leading to a steeply rising cross section. The most striking feature, however, are the negative cross sections for HERAPDF2.0, MMHT14 and PDF4LHC15 at energies above \(\sqrt{s} \simeq \mathcal{O}(30  100)~\,\mathrm {TeV}\), depending on the chosen set. This is an effect of the negative gluon PDF for those sets at values of x within the kinematic reach of current or future hadron colliders up to \(\sqrt{s} \simeq 100~\,\mathrm {TeV}\). This results in an instability of the perturbative expansion of the \(\sigma _{pp \rightarrow c{\bar{c}}}\) cross section at high energies when the contribution from the quark–gluon channel dominates. The reason for a negative gluon PDF in the NNLO set of PDF4LHC15 (being some average of the CT14, MMHT14 and NNPDF3.0 sets) is unclear. In contrast, other PDFs shown in Fig. 16 demonstrate stability of the perturbative expansion through NNLO up to very high energies and good consistency of the predictions with the experimental data.
5.3 \(W'/Z'\) production
Cross sections sensitive to largex parton distributions typically fall rapidly with increasing x values, leading to limitations in the quantity and precision of experimental data and the kinematic range over which they can be obtained. Consequently, the precision to which one can constrain largex PDFs decreases with x, and systematic uncertainties due to extrapolations into unmeasured regions of x (or those excluded by cuts) increase. Similarly, the theoretical uncertainties due to various approximations in the treatment of nuclear corrections for deuterium data, or target mass and higher twist effects, also become larger.
Note that as the rapidity or mass of the produced boson increases, so does the momentum fraction \(x_{1,2} = (M_{W'}/\sqrt{s})\, e^{\pm y_{W'}}\) of one or both partons, in which case the luminosity behaves as \(\mathcal{L}^ \sim \bar{u}(x_2) d(x_1)\). Except for the highest \(M_{W'}\) values, the PDF uncertainty typically remains small up to large values of \(y_{W'}\), corresponding to \(x_1 \approx 0.65\), beyond which it rises dramatically for all \(M_{W'}\). This is precisely the region where data constraining the dquark PDF are scarce, and theoretical assumptions play an important role [1]. This is particularly pronounced for fits that exclude DIS data at low invariant masses, such as the three fits included in the PDF4LHC combination [8]. For large \(W'\) masses, the \(\bar{u}\) PDF is evaluated at \(x_2 \sim 0.2  0.5\), where data are either nonexistent or have large errors, giving rise to the increased uncertainties in some of the PDF sets at \(y_{W'} \sim 0\).
The relative uncertainties in the luminosities in Fig. 18 have been scaled to a common 68 % c.l., as in the tables in the previous sections. One observes a very large range of uncertainties for the various PDF sets, which stems from different tolerance criteria used and different methodologies employed for the treatment of data at high values of x. The smallest uncertainty is obtained for the CJ15 PDF set, which makes use of low invariant mass data to constrain the highx region, and does not employ additional tolerance factors inflating the uncertainties. The MMHT and CT14 PDF sets have larger errors, due to stronger cuts on lowmass DIS data and larger tolerances, and consequently the averaged PDF4LHC15 set gives similarly large uncertainties.
This is also illustrated in Fig. 19, where the central values for the \(W'^\) luminosity for several PDF sets are compared relative to the luminosity computed from the central PDF4LHC15 distributions. The different theoretical assumptions utilized in the fits produce systematic differences in the largex PDFs, which give rise to ratios of central values that are of the same order as the overall PDF4LHC15 68 % c.l. uncertainty, and in the case of the NNPDF3.0 set are about twice as large.
The fact that the uncertainty bands of the individual sets overlap with that of the PDF4LHC15 set is not, however, an indication that the latter is a good estimate of the PDF uncertainties in this extrapolation region. Rather, the PDF4LHC15 band effectively represents a statistical envelope of the systematic theoretical differences between the sets included in the combination. A comparison with the luminosity computed using the CJ15 PDF set, which is not included in the PDF4LHC15 combination, is instructive in this respect. The two main theoretical assumptions affecting the \(W'^\) luminosity are the nuclear corrections in deuterium (applied or fitted in the CJ15 and MMHT14 analyses, as well as in JR14 and ABM12), and the parametrization of the dquark PDF.

The MMHT14 curve follows CJ15 closely until \(y_{W'} \approx 1 (x \approx 0.65)\), after which the dquark PDF turns upwards relative to CJ15, in the region not constrained by the largex and low\(W^2\) SLAC data utilized in CJ15.

The CT14 curve is lower than CJ15 at \(y_{W'} \lesssim 0.6\) (\(x \lesssim 0.45\)), and higher at larger \(y_{W'}\), because of the neglect of nuclear corrections. At \(y_{W'} > 1\) the dquark PDF is essentially unconstrained since neither the low\(W^2\) SLAC data nor the reconstructed Tevatron Wboson production data are included in the fit.

The NNPDF3.0 fit, which excludes low\(W^2\) DIS data and does not utilize nuclear or hadronic corrections, consistently deviates from all others. It is, however, compatible with those within its own uncertainties, which at large x are about four times larger than that of the other fits.
6 Recommendations for PDF usage
Recommendations for the usage of PDFs generally aim in providing guidance for estimates of the magnitude and the uncertainties of cross sections in a reliable but also efficient way. First recommendations have been provided by the PDF4LHC Working Group in the Interim Recommendations [280]. There, the MSTW [197] PDF was used as a central set for predictions at NNLO in QCD and the procedure for calculation of the PDF uncertainties, based on an envelope of several PDF sets, was proposed. This approach has been criticized for being impractical. The 2015 PDF4LHC recommendations [8] have evolved from related discussions and aim in improving the efficiency of cross section computations by averaging several PDFs along with their respective uncertainties. Here, we briefly recall these suggestions and put them into context of the findings of the previous sections. We comment on several shortcomings of the recommendations [8] and propose an alternative for the PDF usage at the LHC.
6.1 The 2015 PDF4LHC recommendations: A critical appraisal
The 2015 PDF4LHC recommendations [8] distinguish four cases: (i) Comparisons between data and theory for Standard Model measurements, (ii) Searches for Beyond the Standard Model phenomena, (iii) Calculation of PDF uncertainties in situations when computational speed is needed, or a more limited number of error PDFs may be desirable and (iv) Calculation of PDF uncertainties in precision observables.
For the case (i), the recommendation is to use the individual PDF sets ABM12 [2], CJ12 [191], CT14 [3], JR14 [5], HERAPDF2.0 [4], MMHT14 [6], and NNPDF3.0 [7]. It is not clear, why the full account of the PDF dependence should be limited to SM processes only. Deviations observed in the theory predictions obtained with the various PDFs can often be traced back to the differences in the underlying theoretical assumptions and models in the PDF fits. With more LHC data available, tests of the compatibility of those data sets in the individual PDF fits will become more stringent. Studies to quantify the constraining power of processes like hadroproduction of \(t {\bar{t}}\) pairs, jets or \(W^\pm \) and Z bosons become possible at high precision.
For the case (ii), it is recommended to employ the PDF4LHC15 sets [8], which represent the combination of the CT14 [3], MMHT14 [6], and NNPDF3.0 [7]. The combination is performed using the Monte Carlo approach at different levels of precision, leading to the recommended sets PDF4LHC15_30 and PDF4LHC15_100. The restriction to CT14, MMHT14 and NNPDF3.0 implies a bias both for the central value and for the PDF uncertainties of BSM cross section predictions. For example, a bias is introduced by fixing the central value of \(\alpha _s(M_Z)\) to an agreed common value, currently chosen to be \(\alpha _s(M_Z) = 0.118\) at both NLO and NNLO. This choice is in contradiction with the precision determinations of \(\alpha _s(M_Z)\) at different orders in perturbation theory, as summarized in Sect. 4. Further, for searches at the highest energies, the PDFs are probed close to the hadronic threshold near \(x \simeq 1\), where nuclear corrections and other hadronic effects, considered for instance in the CJ15 [1] and JR14 [5] analyses, are important.
For the case (iii), the PDF4LHC15_30 sets are recommended to use. We would like to note, that here the balance between the computational speed and the precision of the result (in e.g. MC simulation) has to be determined by the analysers. The problem rises from the large deviations between data and theory predictions at low scales and also at the edges of the kinematical ranges of data currently used in PDF fits as illustrated in Sects. 3 and 5. The average of various GMVFNS for heavy quark production, such as ACOT [159], FONLL [162] and RT [163], leaves a large degree of arbitrariness in the theory predictions, cf. Fig. 10. Note that the PDF4LHC15_30 sets were updated in December 2015 [281] to account for an extension of their validity range below the original \(Q > 8~\,\mathrm {GeV}\) as only discussed in the later publication [282].
For the case (iv), the set PDF4LHC15_100 is recommended. Recalling that this case concerns measurements of the precision observables, it is unclear why PDFs should be treated differently than in the case (i). The differences between individual PDF sets propagate the cross section measurements directly through the acceptance corrections or extrapolation factors, as illustrated in Figs. 15, 17 and 19. Use of the PDF4LHC15_100 is worrysome, since these differences are smeared out in the combination, which, in addition, is limited to only three PDF sets. The SM parameters, determined using the precision observables obtained in this way, may be biased.
In summary, the recent PDF4LHC recommendations [8] cannot be viewed as definitive in the case of precision theory predictions, as the advocated averaging procedure introduces bias, artificially inflates the uncertainties, and makes it difficult to quantify potential discrepancies between the individual PDF sets.
6.2 New recommendations for the PDF usage at the LHC
 1.Precise theory predictions, addressing a class of predictions, within or beyond the SM, which encompasses any type of cross section prediction including radiative corrections of any kind, whether at fixedorder or via resummation to some logarithmic accuracy. This class also includes the MC simulations used for the calculation of the acceptance corrections for precision observables, e.g. cross sections which might be used further for determination of SM parameters.

Recommendation: Use the individual recent PDF sets, currently ABM12 [2], CJ15 [1], CT14 [3], JR14 [5], HERAPDF2.0 [4], MMHT14 [6], and NNPDF3.0 [7] (or as many as possible), together with the respective uncertainties for the chosen PDF set, the strong coupling \(\alpha _s(M_Z)\) and the heavy quark masses \(m_c\), \(m_b\) and \(m_t\). Once a PDF set is updated, the most recent version should be used.

Rationale: Precise theory predictions as needed for any comparisons between theory and data for processes in the SM or beyond (such as hadroproduction of jets, \(W^\pm \) or Zboson production, either singly or in pairs, heavyquark hadroproduction, or generally the production of new massive particles at the TeV scale) often depend on details of the PDF fits and the underlying theory assumptions and schemes used. Differences in the theory predictions based on the individual sets can give an indication of residual systematic uncertainties or shed light on drawbacks and need for potential improvements in the physics models used in the extraction of those PDFs. This applies in particular to measurements used for the determination of SM parameters such as the strong coupling \(\alpha _s(M_Z)\), heavy quark masses \(m_c\), \(m_b\) and \(m_t\) or the Wboson mass, because these parameters are directly correlated to the PDFs used in their extraction from the experimental observables.

 2.Theory predictions for feasibility studies, the complementary class containing all other cross section predictions where high precision is not required, such as those based on Born approximations and/or order of magnitude estimates, or in cases where precision may be sacrificed in favor of computational speed. Here, also studies of novel accelerators and detectors are addressed.

Recommendation: Use any of the recent PDF sets (listed in LHAPDFv6 or later versions).

Rationale: Often in phenomenological applications for the modern and future facilities one is interested in a quick order of magnitude estimate for the particular cross sections. These are directly proportional to the parton luminosity and to the value of \(\alpha _s(M_Z)\). In these cases, one may be willing to sacrifice precision in favor of computational speed. Here, the usage of the sets PDF4LHC15_30 and PDF4LHC15_100 may provide an efficient estimate of PDF uncertainties, although care must be taken in their interpretation depending on the observable and covered kinematic range. Restricting the recommendation to PDFs listed in the LHAPDF(v6) [283] interface excludes parton luminosities with lesser precision in the interpolation of the underlying grids (e.g., in LHAPDF(v5) [284]) or “partonometers” [285] with outdated calibration.

7 Conclusion
In this report we have reviewed recent developments in the determination of PDFs in global QCD analyses. Thanks to high precision experimental measurements and continuous theoretical improvements, the parton content of the proton is generally well constrained and PDFs, along with the strong coupling constant \(\alpha _s(M_Z)\) and the heavyquark masses \(m_c\), \(m_b\) and \(m_t\), have been determined with good accuracy, at least at NNLO in QCD. This forms the foundation for precise cross section predictions at the LHC in Run 2.
We have briefly discussed the available data used in PDF extractions and the kinematic range covered, and emphasized the importance of selecting mutually consistent sets of data in PDF fits in order to achieve acceptable \(\chi ^2\) values for the goodnessoffit estimate. The main thrust of the study has been the computation of benchmark cross sections for a variety of processes at hadron colliders, including Higgs boson production in gluon–gluon fusion. We have illustrated how different choices for the theoretical description of the hard scattering process and choices of parameters have an impact on the predicted cross sections, and lead to systematic shifts that are often significantly larger than the associated PDF and \(\alpha _s(M_Z)\) uncertainties. A particular example has been the treatment of heavy quarks in DIS, where the quality of the various scheme choices has been quantified in terms of \(\chi ^2\)/NDP values when comparing predicted cross sections to data. We have also pointed out the inconsistently low values for the pole mass of the charm quark used in some fits, and have stressed the correlation of the strong coupling constant \(\alpha _s(M_Z)\) with the PDF parameters. Ideally, \(\alpha _s(M_Z)\) should be determined simultaneously with the PDFs, and we have summarized here the state of the art in the context of PDF analyses.
Our findings expose a number of shortcomings in the recent PDF4LHC recommendations [8]. We have shown that these do not provide sufficient control over some theoretical uncertainties, and may therefore be problematic for precision predictions in Run 2 of the LHC. Instead, we suggest new recommendations for the usage of PDFs based on a theoretically consistent procedure necessary to meet the precision requirements of the LHC era.
Notes
Acknowledgments
We would like to thank S. Alioli, M. Botje, E.W.N. Glover and K. Rabbertz for discussions, K. Rabbertz also for valuable comments on the manuscript, and L. HarlandLang and R. Thorne for providing us with the Higgs and \(t{\bar{t}}\) cross sections in Tables 11 and 14. This work has been supported by Bundesministerium für Bildung und Forschung through contract (05H15GUCC1), by the DOE contract No. DEAC0506OR23177, under which Jefferson Science Associates, LLC operates Jefferson Lab, and by the European Commission through PITNGA2012316704 (HIGGSTOOLS). The work of A.A. and J.F.O. was supported in part by DOE contracts No. DESC0008791 and No. DEFG0297ER41922, respectively. Two of the authors (J.B. and S.M.) would like to thank the Mainz Institute for Theoretical Physics (MITP) for its hospitality and support.
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