Transverse momentum dependence of inclusive primary charged-particle production in p-Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV

The transverse momentum ($p_{\mathrm T}$) distribution of primary charged particles is measured at midrapidity in minimum-bias p-Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV with the ALICE detector at the LHC in the range $0.15<p_{\mathrm T}<50$ GeV/$c$. The spectra are compared to the expectation based on binary collision scaling of particle production in pp collisions, leading to a nuclear modification factor consistent with unity for $p_{\mathrm T}$ larger than 2 GeV/$c$, with a weak indication of a Cronin-like enhancement for $p_{\rm T}$ around 4 GeV/$c$. The measurement is compared to theoretical calculations and to data in Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}=2.76$ TeV.

Measurements of particle production in proton-nucleus collisions at high energies enable the study of fundamental properties of Quantum Chromodynamics (QCD) over a broad range of parton fractional momentum x and parton densities (see [1] for a review). They also provide reference measurements for the studies of deconfined matter created in nucleus-nucleus collisions [2].
The first measurements of charged-particle production in minimum-bias p-Pb collisions at the LHC at a centre-of-mass energy per nucleon-nucleon pair of √ s NN = 5.02 TeV [3,4] showed that: i) the charged particle multiplicity density at midrapidity scales approximately with the number of participating nucleons ( N part = 7.9 ± 0.6 for minimum-bias collisions) calculated in a Glauber model [5] and ii) the transverse momentum (p T ) spectrum, measured in the range 0.5-20 GeV/c [4], exhibits binary collision scaling above a few GeV/c, as expected in the absence of any significant nuclear modification effect. The latter is quantified by the nuclear modification factor, R pPb , the ratio of the p T spectrum in p-Pb collisions and a reference obtained by scaling the measurement in pp collisions with the number of binary nucleon-nucleon collisions in p-Pb. The preliminary result by the CMS collaboration [6] hints at an enhancement of particle production in p-Pb collisions above binary collision scaling, leading to R pPb > 1, for p T exceeding about 30 GeV/c. The preliminary result by the ATLAS collaboration [7] exhibits also, for collisions corresponding to 0-90% centrality, R pPb values above unity for p T in the range 20-100 GeV/c.
In this letter we present an update of our previously published p T spectra of primary charged particles [4] based on the 60 times larger sample size collected with the ALICE detector [8] in 2013 in minimum-bias collisions. These data allow a significant extension of the transverse momentum range. The present analysis is essentially identical to the previous and therefore we update only the information related to the enlarged data set; the reader is referred to the earlier publications [4,[9][10][11] for a more detailed and complete description.  Table 1: Systematic uncertainties on the p T -differential yields in p-Pb and pp collisions. The quoted ranges span the p T dependence of the uncertainties in the measured range, 0.15-50 GeV/c. Normalization uncertainties are also quoted.
The ALICE minimum-bias trigger is defined by a coincidence of signals in detectors covering in pseudorapidity 1 2.8 < η < 5.1 (VZERO-A) and −3.7 < η < −1.7 (VZERO-C). In the 2013 data sample, 106 million events (corresponding to an integrated luminosity of 50.7±1.6 µb −1 ) satisfy the trigger and offline event-selection criteria, which select essentially non-single-diffractive (NSD) minimum-bias col-lisions. The centre-of-mass pseudorapidity is defined as η cms = −η − |y NN |, with the proton beam at positive rapidity; |y NN | = 0.465 is the rapidity of the centre-of-mass for nucleon-nucleon collisions. This equation is exact only for massless or very high p T particles. The spectra are corrected on a statistical basis using the measurements by ALICE in p-Pb collisions of the η distribution of inclusive charged particle production [3] and of the pion, kaon, and proton yields [12]; this correction depends on the η cms range and on p T , reaching about 20% for the lowest p T bin. The systematic uncertainty of the particle composition [12] leads to a systematic uncertainty in our spectra of up to 0.4%.
The systematic uncertainties on the spectra are evaluated as in previous analyses of pp [10], Pb-Pb [9], and p-Pb [4] data. The uncertainty due to the p T scale is negligible below 20 GeV/c and reaches 1.5% at 50 GeV/c. The main contributions and the total uncertainties are listed in Table . )  Fig. 1: Transverse momentum distributions of charged particles in minimum-bias (NSD) p-Pb collisions for different pseudorapidity ranges (upper panel). The spectra are scaled by the factors indicated. The histogram represents the reference spectrum (cross section scaled by the nuclear overlap function, T pPb ) in inelastic pp collisions, determined in |η| < 0.8. The lower panel shows the ratio of spectra in p-Pb at backward pseudorapidities to that at |η cms | < 0.3. The vertical bars (boxes) represent the statistical (systematic) uncertainties.
The p T spectra of charged particles measured in minimum-bias (NSD) p-Pb collisions at √ s NN = 5.02 TeV are shown in Fig. 1 for the ranges |η cms | < 0.3, −0.8 < η cms < −0.3, and −1.3 < η cms < −0.8. The pp reference spectrum, T pPb (1/2π p T )d 2 σ pp /dηdp T , is also included. T pPb is the average nuclear overlap function, calculated using the Glauber model [13], which gives T pPb = N coll /σ NN = 0.0983 ± 0.0035 mb −1 , with N coll = 6.9 ± 0.6 and σ NN = 70 ± 5 mb. Since the data in pp collisions [10] indicate only a very small η dependence of the p T spectrum in the range measured by ALICE (|η| < 0.8), our current reference spectrum is, differently than in [4,10], for |η| < 0.8. It was obtained by data interpolation at low p T and by scaling the measurement at √ s = 7 TeV with the ratio of spectra calculated with NLO pQCD at √ s = 5.02 and 7 TeV [10].
In the lower panel of Fig. 1 the ratios of the spectra for backward (−0.8 < η cms < −0.3 and −1.3 < η cms < −0.8) pseudorapidity ranges to that at |η cms | < 0.3 are shown. The indication of a slight softening of the p T spectrum when going from central to backward (Pb-side) pseudorapidity, observed already in the pilot-run data of 2012 [4] (note opposite η cms sign convention in [4]) is confirmed with better significance and extended in p T down to 0.15 GeV/c.
A good description of our earlier measurement of spectra in p-Pb collisions [4] was achieved in the EPOS3 model [14] including a hydrodynamical description of the collision, while the PHSD model [15] significantly underestimated the spectra for p T values of several GeV/c. In order to quantify nuclear effects in p-Pb collisions, the p T -differential yield relative to the pp reference, the nuclear modification factor, is calculated as: where N pPb is the charged particle yield in p-Pb collisions.
The measurement of the nuclear modification factor R pPb for charged particle production in |η cms | < 0.3 and −1.3 < η cms < 0.3 is shown in Fig. 2. The uncertainties of the p-Pb and pp spectra are added in quadrature, separately for the statistical and systematic uncertainties. The systematic uncertainties are largely correlated between adjacent p T bins. The total systematic uncertainty on the normalization, the quadratic sum of the uncertainty on T pPb , the normalization of the pp reference spectrum and The data indicate a small enhancement, R pPb above unity, barely significant within systematic errors, around 4 GeV/c, i.e. in the p T region where the much stronger Cronin enhancement is seen at lower energies [16,17].
The p-Pb data provide important constraints to models of nuclear modification effects. As an illustration, in Fig. 3 the measurement of R pPb at |η cms | < 0.3 is compared to theoretical model predictions. The predictions for shadowing [18], calculated at next-to-leading order (NLO) with the EPS09s nuclear modification of parton distribution functions describe the data for p T 6 GeV/c. The calculations are for π 0 , which may explain the differences with respect to data at low p T ; for high p T , the ALICE data on identified pions, kaons, and protons [21] give support that the comparison of our data on inclusive charged particles to EPS09s calculations for π 0 is meaningful. The LO pQCD model including cold nuclear matter effects [19] exhibits a distinct trend of decreasing R pPb , which is not supported by the data. The prediction with the HIJING 2.1 model, shown for two fragmentation schemes [20], exhibits a more pronounced trend of decreasing R pPb at high p T . It is interesting to note that calculations with the EPOS LHC model [22], not included here, show a similar trend. Several predictions based on the saturation (Color Glass Condensate) model are available [23][24][25]; they were shown previously [4] to describe, in their range of validity, namely up to several GeV/c, the R pPb data. In Fig. 4 we compare the measurement of the nuclear modification factor for inclusive primary chargedparticle (h ± ) production in p-Pb collisions to that in central (0-5% centrality) Pb-Pb collisions [9,26]. The p-Pb data demonstrate that the suppression of hadron production at high p T in Pb-Pb collisions, understood in theoretical models as a consequence of parton energy loss in (deconfined) QCD matter (see [9] and references therein), has no contribution from initial state effects. The ALICE p-Pb data show no sign of nuclear matter modification of hadron production at high p T and are therefore fully consistent with the observation of binary collision scaling in Pb-Pb of observables which are not affected by hot QCD matter (direct photons [27] and vector bosons [28, 29]) In summary, we have extended our measurements of the charged-particle p T spectra and nuclear modification factor in minimum-bias (NSD) p-Pb collisions at √ s NN = 5.02 TeV. The results, covering a substantially-extended p T range, 0.15 < p T < 50 GeV/c, exhibit, within uncertainties, no deviation from binary collision scaling at high p T ; the nuclear modification factor remains consistent with unity for p T 2 GeV/c. The data at high p T are described by a prediction based on NLO pQCD calculations with PDF shadowing and further underline our earlier observation [4] that initial state effects do not contribute to the strong suppression of hadron production at high p T observed at the LHC in Pb-Pb collisions.