Constraints on nuclear parton distributions from dijet photoproduction at the LHC

Using QCD calculations of the cross section of inclusive dijet photoproduction in Pb-Pb ultraperipheral collisions in the LHC kinematics as pseudo-data, we study the effect of including these data using the Bayesian reweighting technique on nCTEQ15, nCTEQ15np, and EPPS16 nuclear parton distribution functions (nPDFs). We find that, depending on the assumed error of the pseudo-data, it leads to a significant reduction of the nPDF uncertainties at small values of the momentum fraction $x_A$. Taking the error to be 5\%, the uncertainty of nCTEQ15 and nCTEQ15np nPDFs reduces approximately by a factor of two at $x_A=10^{-3}$. At the same time, the reweighting effect on EPPS16 nPDFs is much smaller due to the higher value of the tolerance and a more flexible parametrization form.


Introduction
Collinear nuclear parton distribution functions (nPDFs) are fundamental quantities of Quantum Chromodynamics (QCD) encoding information on the one-dimensional distributions of quarks and gluons in nuclei in terms of the light-cone momentum fraction x A at a given resolution scale µ.Nuclear PDFs are essential ingredients of QCD calculations of cross sections at high energies involving charged lepton-nucleus and neutrino-nucleus deep inelastic scattering (DIS) with fixed targets and -in the future -in the collider mode and proton-nucleus and nucleus-nucleus scattering at the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC).While nPDFs are non-perturbative quantities, which cannot be calculated from first principles of QCD, the QCD collinear factorization for hard processes and the Dokshitzer-Gribov-Lipatov-Altarelli-Parisi (DGLAP) evolution equations allow one to set up a framework of global QCD fits, which enables one to extract nPDFs from available data [1,2,3,4,5,6,7].Different analyses give noticeably different predictions for nPDFs and carry significant uncertainties originating mostly from the limiting kinematic coverage of the available data, indirect determination of the gluon nPDF from the DIS data using the scaling violations, and different assumptions about the shape of nPDFs at the input scale.As a result, quark nPDFs for small x and the gluon nPDFs for essentially all x are rather poorly known.
Further progress in constraining nPDFs relies on studies of high-energy hard processes with nuclei at collider energies, notably, in proton-nucleus (pA) scattering at the LHC [8,9,10] and lepton-nucleus (eA) scattering at the future EIC [11,12] and LHeC [13].However, the QCD analyses of the data on various hard processes in pA scattering at the LHC during Run 1 [14,15,16,17] showed that the data provide only modest restrictions on nPDFs at small x.At the same time, it was proposed [18] that measurements of low-mass lepton pair production in proton-lead collisions at the LHC has a large potential to reduce the theoretical uncertainties on nPDFs in a wide range of x or even rule out some parameterizations.While the potential of hard pA scattering at the LHC will certainty continue to be explored, see, e.g.[19], it is topical to study complimentary probes of nPDFs at the LHC.
It has been realized that collisions of ultrarelativistic ions at large impact parameters, when the strong interaction is suppressed and the ions interact electromagnetically via the emission of quasi-real photons in so-called ultraperipheral collisions (UPCs), give an opportunity to study photon-photon, photon-proton, and photon-nucleus scattering at unprecedentedly high energies [20].This program was realized during Run 1 at the LHC by measuring exclusive photoproduction of charmonia (J/ψ and ψ(2S) vector mesons) in Pb-Pb UPCs at √ s N N = 2.76 TeV by the ALICE [21,22,23] and CMS [24] collaborations, which probes the small-x gluon distribution of the target [25].The analyses [26,27] of these data in leading-order (LO) QCD gave first direct and weakly model-dependent evidence of large nuclear gluon shadowing down to x ≈ 10 −3 , which agrees very well with the predictions of the leading twist nuclear shadowing model [28] and the the EPS09 [3], nCTEQ15 [5], and EPPS16 [7] nPDFs.Note that in next-toleading order (NLO) perturbative QCD, corrections for this process are large [29,30] and the relation between the gluon PDF and the generalized gluon PDF is modeldependent, which makes it challenging to include the data on J/ψ photoproduction on nuclei into global QCD fits of nPDFs.
During Run 2 at the LHC, in addition to light and heavy vector meson photoproduction in UPCs [31,32,33], the ATLAS collaboration for the first time measured inclusive dijet photoproduction in Pb-Pb UPCs [34].Predictions for rates of this process in pA and nucleus-nucleus (AA) UPCs at the LHC in LO QCD with an emphasis of heavy quark production were made in Ref. [35].It was found that the rates are very large allowing one to probe deeply into the small-x region.At NLO pQCD, the cross section of inclusive dijet photoproduction in Pb-Pb UPCs in the ATLAS kinematics was calculated in [36].It was shown that the used theoretical framework provides a good description of various kinematic distributions measured by the ATLAS collaboration and that the calculated dijet photoproduction cross section is sensitive to nuclear modifications of nPDFs at the level of 10 to 20%.
In this work, we explore the potential of inclusive dijet photoproduction in Pb-Pb UPCs in the LHC kinematics to provide new constraints on nPDFs.In particular, using the results of our NLO QCD calculations of the cross section of this process [36] as pseudo-data, we study the effect of including these data using the Bayesian reweighting technique [37,38,39] on nCTEQ15, nCTEQ15np, and EPPS16 nPDFs.We find that depending on the assumed error on the pseudo-data, it leads to a significant reduction of uncertainties of nPDFs at small x A .For instance, taking the error to be 5%, we find that the uncertainty of quark and gluon nCTEQ15 and nCTEQ15np nPDFs reduces by approximately a factor of two at x A = 10 −3 .The reweighting effect on EPPS16 nPDFs is much smaller due to the higher value of the tolerance and a more flexible parametrization form used in that analysis.
The remainder of this paper is structured as follows.In Sec. 2, we summarize key steps of the Bayesian reweighting method and define our reweighting procedure.We present and discuss our results in Sec. 3 and draw conclusions in Sec. 4.

Reweighting of the dijet photoproduction cross section
To quantity the power of inclusive dijet photoproduction in Pb-Pb UPCs at the LHC to constrain nPDFs, we use the standard Bayesian reweighting procedure outlined in the literature [37,38,39].In detail, starting with 2N error sets of nPDFs (N = 16 for nCTEQ15 [5] and N = 20 for EPPS16 [7]), one generates Nrep = 10, 000 replicas labeled by the index k as follows where f 0 j/A and f i± j/A (x, Q 2 ) are the central fit and the plus and minus error nPDFs corresponding to the eigenvector direction i and R ki is a random number from the normal distribution centered at zero with the standard deviation of unity.
Next, for each PDF replica, one calculates the observable of interest, which in our case is the dijet photoproduction cross section as a function of x A (x A is the hadron-level estimate for the momentum fraction carried by the interacting nuclear parton) [36]: In Eq. ( 2), a, b are parton flavors; f γ/A (y) is the flux of equivalent photons emitted by ion A, which depends on the photon light-cone momentum fraction y; f a/γ (xγ, is the PDF of the photon for the resolved-photon contribution, which depends on the momentum fraction xγ and the factorization scale µ; f b/B (x A , µ 2 ) is the nuclear PDF with x A being the corresponding parton momentum fraction; and dσ(ab → jets) is the elementary cross section for production of two-and three-parton final states emerging as jets in hard scattering of partons a and b.The sum over a involves quarks and gluons for the resolved photon contribution and the photon for the direct photon contribution dominating at xγ ≈ 1.The integration limits are determined by the rapidities and transverse momenta of the produced jets, see [36] for details.Note that since the inclusive dijet cross section is linear in the nPDFs, which in turn are linear in R ki , it is sufficient to evaluate it 2N times for each error nPDF.
The essence of the reweighting procedure is the calculation of statistical weights for each replica w k , which quantify how well the calculation using Eq. ( 2) reproduces data or pseudo-data.In our case, for the pseudo-data, we use the results our calculation of the dijet cross section [36] where f 0 b/B (x A , µ 2 ) corresponds to the central value of the nCTEQ15 nPDFs.Then, where the sum runs over the pseudo-data points; σ j is the assumed uncertainty of the pseudo-data.In our case, N data = 9 corresponding to different bins in x A and we take σ j = ǫdσ 0 /dx A with ǫ = 0.05, 0.1, 0.15, and 0.2.Finally, with the help of χ 2 k , one can introduce the weights w k using the following relation: where T is the tolerance associated with a given set of nPDFs.Note that In our analysis, we use T = 35 for nCTEQ15 and nCTEQ15np [5] and T = 52 for EPPS16 [7].
Using the weights w k , one can calculate the new, weighted average cross section and its error: Similarly, one can evaluate the reweighted nPDFs and their uncertainties: Equations ( 6) and ( 7) quantify the effect of the pseudo-data on the calculation of the cross section of inclusive dijet photoproduction and the central value and uncertainties of nPDFs, respectively.The effective number of replicas contributing to Eqs. ( 6) and ( 7) can be estimated using the following expression: Table 1 summarizes our values of N eff for ǫ = 0.05, 0.1, 0.15, and 0.2 and nCTEQ15, nCTEQ15np, and EPPS16 nPDFs.Figures 1, 6, and 11 show the dijet cross section as a function of x A : the pseudodata points labeled "nCTEQ15" and given by black open squares are the results of the calculation using the central nCTEQ15 fit; red crosses with the associated error bands are the results of the calculations using a given set of nPDFs (the crosses coincide with the open squares in Fig. 1, and, hence, are not shown); finally, the blue filled circles and the associated error bands show the reweighted cross section and its uncertainty, see Eq. ( 6).The four panels correspond to our four choices of the assumed error ǫ.One can see from these figures that while the reweighting does not noticeably change the central values of the cross section, it reduces its theoretical uncertainty: the effect is largest for the smallest ǫ and the first small-x A bin. 1The remaining figures (Figs. 2-5, 7-10, and 12-13) demonstrate the effect of the reweighting on nPDFs: different panels show gluon, u-quark, d-quark, and s-quark nPDFs and their uncertainties before (red, outer band) and after (blue, inner band) the reweighting as a function of the momentum fraction x A at Q 2 = 400 GeV 2 .This is a characteristic value of Q 2 probed in dijet photoproduction in the ATLAS kinematics.While the central values of nPDFs are essentially not affected by the reweighting, the uncertainty bands for nCTEQ15 and nCTEQ15np are noticeably reduced.As expected, the effect is largest at ǫ = 0.05 and much smaller at ǫ = 0.2.For instance, the uncertainty in the gluon and quark nPDFs at x A = 0.001 reduces by approximately a factor of two.It is interesting to note that the uncertainty of the small-x A gluon distribution in the case of nCTEQ15np after the reweighting is similar to that of nCTEQ15 before the reweighting -it is of the order of 15% in both cases.Therefore, dijet photoproduction should have a similar impact on nCTEQ15 nPDFs as the RHIC inclusive pion production data, which was included in nCTEQ15 and excluded in nCTEQ15np.The advantage of dijet photoproduction is that it does not involve the pion fragmentation functions, which necessarily brings an additional uncertainty in analyses of nPDFs.
In the case of EPPS16 nPDFs (see Figs. 12 and 13), the effect of reweighting is much smaller due to several reasons.First, these nPDFs have been obtained with a higher value of the tolerance T , which allows for significantly more replicas to contribute to the reweighted quantities (see Table 1) and which reduces the effectiveness of the reweighting.Second, a more flexible form of the EPPS16 nPDF parametrization also significantly reduces the reweighting effect, which is negligibly small in the ǫ = 0.15 and ǫ = 0.2 cases.We therefore do not show them here, since the blue and red solid lines and bands completely overlap.

Conclusions
In this work, we studied the potential of inclusive dijet photoproduction in Pb-Pb UPCs in the LHC kinematics to give new constraints on nPDFs.Using the results of our NLO QCD calculations of the cross section of this process as pseudo-data, we analyzed the effect of including these data using the Bayesian reweighting technique on nCTEQ15, nCTEQ15np, and EPPS16 nPDFs.We found that depending on the assumed error on the pseudo-data, it leads to a significant reduction of the nPDF uncertainties at small x A .For instance, taking the error to be 5%, we find that the uncertainty of quark and Fig. 2 The gluon, u-quark, d-quark, and s-quark nCTEQ15 nPDFs as a function of x A at Q 2 = 400 GeV 2 with (blue, inner band) and without (red, outer band) the Bayesian reweighting.The case of ǫ = 0.05.gluon nCTEQ15 and nCTEQ15np nPDFs reduces by a factor of two at x A = 10 −3 .We observed that the uncertainty of the small-x A gluon distribution in the nCTEQ15np case after the reweighting is similar to that of nCTEQ15 before the reweighting, which indicates that dijet photoproduction should have a similar impact on nCTEQ15 nPDFs as the RHIC inclusive pion production data with the advantage that dijet photoproduction is free from the uncertainty associated with the pion fragmentation functions.At the same time, the reweighting effect on EPPS16 nPDFs is much smaller due to the higher value of the tolerance and a more flexible parametrization form used in the EPPS16 analysis.Fig. 3 The same as Fig. 3, but with ǫ = 0.1.Fig. 4 The same as Fig. 3, but with ǫ = 0.15.Fig. 5 The same as Fig. 3, but with ǫ = 0.2.Fig. 6 The dijet photoproduction cross section as a function of x A with (blue solid circles and error bands) and without (red crosses and error bands) the Bayesian reweighting calculated using the nCTEQ15np nPDFs; the cross section used as pseudo-data is calculated with nCTEQ15 (open black squares).Different panels show the results for the four considered cases of the assumed error ǫ.Fig. 8 The same as Fig. 7, but with ǫ = 0.1.Fig. 9 The same as Fig. 7, but with ǫ = 0.15.Fig. 10 The same as Fig. 7, but with ǫ = 0.2.Fig. 11 The dijet photoproduction cross section as a function of x A with (blue solid circles and error bands) and without (red crosses and error bands) the Bayesian reweighting calculated using the EPPS16 nPDFs; the cross section used as pseudo-data is calculated with nCTEQ15 (open black squares).Different panels show the results for the four considered cases of the assumed error ǫ.Fig. 13 The same as Fig. 12, but with ǫ = 0.1.

Fig. 1
Fig.1The dijet photoproduction cross section as a function of x A with (blue solid circles and error bands) and without (black open squares and red error bands) the Bayesian reweighting.The calculations correspond to the nCTEQ15 nPDFs.Different panels show the results for the four considered cases of the assumed error ǫ = 0.05, 0.1, 0.15, and 0.2.

Table 1
The effective number of replicas N eff for different choices of the experimental error and sets of nPDFs.Using the procedure outlined in Sec. 2, we perform the Bayesian reweighting of the pseudo-data on the cross section of inclusive dijet photoproduction on nuclei in Pb-Pb UPCs in the LHC kinematics.Our results are shown in Figs.1-5 for nCTEQ15 nPDFs, Figs.6-10 for nCTEQ15np nPDFs, and Figs.11-13 for EPPS16 nPDFs.