Rapidity and transverse-momentum dependence of the inclusive J/ψ nuclear modification factor in p-Pb collisions at √sNN= 5.02 TeV

JHEP06(2015)055 Published for SISSA by Springer Received:March 30, 2015 Accepted:May 11, 2015 Published:June 9, 2015 Rapidity and transverse-momentum dependence of the inclusive J/ψnuclear modification factor in p-Pb collisions at √sNN = 5.02 TeV The ALICE collaboration E-mail: [email protected] Abstract: We have studied the transverse-momentum (pT) dependence of the inclusive J/ψproduction in p-Pb collisions at √sNN = 5.02 TeV, in three center-of-mass rapidity (ycms) regions, down to zero pT. Results in the forward and backward rapidity ranges (2.03 < ycms <3.53 and −4.46 < ycms <−2.96) are obtained by studying the J/ψdecay to µ+µ−, while the mid-rapidity region (−1.37 < ycms <0.43) is investigated by measuring the e+e−decay channel. The pTdependence of the J/ψproduction cross section and nuclear modification factor are presented for each of the rapidity intervals, as well as the J/ψmean pTvalues. Forward and mid-rapidity results show a suppression of the J/ψyield, with respect to pp collisions, which decreases with increasing pT. At backward rapidity no significant J/ψsuppression is observed. Theoretical models including a combination of cold nuclear matter effects such as shadowing and partonic energy loss, are in fair agreement with the data, except at forward rapidity and low transverse momentum. The implications of the p-Pb results for the evaluation of cold nuclear matter effects on J/ψproduction in Pb-Pb collisions are also discussed. Keywords: Hadron-Hadron Scattering, Relativistic heavy ion physics, Heavy ions, Charm physics ArXiv ePrint: 1503.07179 Open Access, Copyright CERN, for the benefit of the ALICE Collaboration. Article funded by SCOAP3. doi:10.1007/JHEP06(2015)055 JHEP06(2015)055 The suppression of charmonia, bound states of cand ¯cquarks, and in particular of the J/ψstate, has long been proposed as a signature for the formation of a plasma of quarks and gluons (QGP) [1] in ultrarelativistic nucleus-nucleus collisions. However, it was soon realized that charmonium production can also be modified by nuclear effects not necessarily related to QGP formation [2]. These so-called cold nuclear matter (CNM) effects can be investigated by studying charmonium production in proton-nucleus (p-A) collisions as confirmed by the analysis of results obtained by several fixed-target (SPS [3,4], HERA [5] and Tevatron [6]) and collider (RHIC [7] and LHC [8,9]) experiments. Theoretical models have studied the production of charmonium in p-A collisions and the effects of the surrounding cold nuclear medium by introducing various mechanisms which include nuclear shadowing, gluon saturation, energy loss and nuclear absorption. Models [10–12] inspired by Quantum ChromoDynamics (QCD) describe charmonium production as a two-step process, with the cc pair created in a hard parton scattering, followed by its evolution into a bound state with specific quantum numbers. The pair creation is sensitive to the Parton Distribution Functions (PDFs) in both colliding partners and, at high energy, occurs mainly via gluon fusion. Although PDFs are known to be modified in a nuclear environment, information on the dependence of such modifications on the fraction x(Bjorken-x) of the nucleon momentum carried by the gluons and on the four-momentum squared Q2transferred in the scattering is still limited [13–15]. Charmonium production measurements can therefore provide insight into the so-called nuclear shadowing, i.e., on how the nucleon gluon PDFs are modified in a nucleus. Modifications of the initial state of the nucleus are also addressed by approaches assuming that at sufficiently high energies, when the quark pair is produced from a dense gluon system carrying small x-values in the nuclear target, a coherent effect known as gluon saturation sets in. Such an effect can be described by the Color Glass Condensate (CGC) effective theory, which is characterized by a saturation momentum scale (Q2 s). When combined with a specific quarkonium production model [16,17], it is able to provide predictions for charmonium production in p-A collisions. In the context of shadowing and CGC models, a measurement of the charmonium yield as a function of transverse momentum (pT) and rapidity (y) is important as it gives access to specific ranges of values of the gluon x and/or Q2. In addition to these purely initial state effects, both the incoming partons and the cc pair propagating through the nucleus may lose energy by gluon radiation at the various stages of the charmonium formation process [18]. The interference of gluons radiated before and after the hard production vertex can lead to coherent energy loss effects, expected to induce a modification of the charmonium kinematic distributions [19]. Finally, while travelling through nuclear matter, the evolving cc pair or, if crossing times are sufficiently large, the fully formed resonance, may break-up into open charm meson pairs. Although this mechanism, known as nuclear absorption, plays an important role at lower collision energies [4], at the LHC the contribution of this effect to the production cross section is expected to be small, due to the very short crossing time of the pair through the nuclear environment. Understanding the role of the cold nuclear matter effects outlined above is essential to further our knowledge of various aspects of the physics of strong interactions, and it – 2 – JHEP06(2015)055 is crucial for the interpretation of the results on charmonium production in heavy-ion collisions, where the formation of a QGP is expected. In such a hot and dense deconfined medium the color screening mechanism (the QCD analogue of the Debye screening in QED) can prevent the formation of the heavy-quark bound states, leading to a suppression of quarkonium production [1]. In addition, at LHC energies, the large charm quark density may lead to a (re)generation of charmonium by (re)combination of charm quarks [20,21] in the QGP phase and/or when the system cools down and the formation of hadrons occurs. This effect enhances charmonium production and is expected to be particularly sizeable at low pT. In heavy-ion collisions, a superposition of hot and cold nuclear matter effects is expected, and a quantitative evaluation of the latter is an important prerequisite for a detailed understanding of the former. At lower energy, both at SPS [22–24] and RHIC [25,26], a suppression of J/ψproduction, in addition to the CNM effects estimated from p-A(d-A) collisions, was indeed observed. A suppression of J/ψproduction has been measured in Pb-Pb collisions at the LHC [27– 31]. It was quantified via the nuclear modification factor, i.e., the ratio of the Pb-Pb yields with respect to those measured in pp at the same energy, scaled by the number of binary nucleon-nucleon collisions. The suppression has been found to be stronger at forward rapidity and at high pT[30,31], in agreement with expectations from (re)combination models. Similar to the lower energy experiments, accurate measurements in p-A collisions are needed to quantitatively assess the contribution of hot and cold nuclear matter effects in Pb-Pb. The first measurements of inclusive J/ψproduction in p-Pb collisions at the LHC at √sNN = 5.02 TeV [8,9] have shown a sizeable suppression, with respect to binary-scaled pp collisions, at forward rapidity (p-going side) and no suppression at backward rapidity (Pb-going side). The nuclear modification factors are in fair agreement with models based on nuclear shadowing [32,33]. Calculations including a contribution from coherent energy loss [19] also reproduce the data. Corresponding measurements for the less strongly bound ψ(2S) charmonium state are presented in [34]. In addition, an extrapolation to Pb-Pb collisions of the J/ψsuppression measured in p-Pb showed that the effects observed in Pb-Pb cannot be ascribed only to CNM [8]. In this situation, a study of the transverse-momentum dependence of J/ψproduction at LHC energies for various rapidity regions is particularly interesting in order to: (i) reach a deeper understanding and better quantify the complicated interplay of CNM effects, which are expected to exhibit a well-defined kinematical dependence [33,35,36]; (ii) determine if the differential features of the Pb-Pb results that suggest the presence of (re)combination effects are still present when the contribution of CNM is considered. In this paper, we present ALICE results on the transverse-momentum dependence of the inclusive J/ψproduction in p-Pb collisions at √sNN = 5.02 TeV, measured in three center-of-mass rapidity (ycms) ranges: backward (−4.46 < ycms <−2.96), mid- (−1.37 < ycms <0.43) and forward (2.03 < ycms <3.53). The data are from the 2013 LHC p-Pb run. At mid-rapidity, J/ψare reconstructed in the e+e−decay channel with the ALICE central barrel detectors, covering the pseudorapidity range |ηlab|<0.9. For the backward and forward rapidity analysis, J/ψare detected, through their µ+µ−decay channel in the muon spectrometer, in the pseudorapidity range −4< ηlab <−2.5. – 3 – JHEP06(2015)055 Due to the energy asymmetry of the LHC beams (Ep= 4 TeV and EPb = 1.58 · APb TeV, where APb= 208 is the Pb atomic mass number), the nucleon-nucleon center-of- mass is shifted, with respect to the laboratory frame, by ∆y= 0.465 in the direction of the proton beam. Since data were collected in two configurations, interchanging the direction of the proton and the Pb beams in the LHC, the muon spectrometer acceptance covers the forward and backward ycms regions quoted above, where positive (negative) rapidities refer to the direction of the proton (Pb) beam. In the following, the notation p-Pb (Pb-p) will refer to the first (second) configuration. For the dielectron analysis, the central barrel detectors used for the J/ψreconstruction are the Inner Tracking System (ITS) [37] and the Time Projection Chamber (TPC) [38]. The ITS contains six cylindrical layers of silicon detectors, with the innermost layer at a radius of 3.9 cm with respect to the beam axis and the outermost layer at 43 cm. This detector is used for reconstructing the primary interaction vertex as well as vertices from different interactions and secondary vertices from decays of heavy-flavored particles. The TPC has a cylindrical geometry with an active volume that extends from 85 to 247 cm in the radial direction and 500 cm longitudinally. It is the main central barrel tracking detector and also provides particle identification via the measurement of the specific energy loss (dE/dx) in the detector gas. The muon spectrometer [39] is the main detector used in the dimuon analysis. It consists of a 3 T·m dipole magnet, coupled with a tracking and a triggering system. Between the interaction point and the muon spectrometer, a ten interaction-length (λI) front absorber filters out the hadrons produced in the interaction. Muon tracking is performed by means of five tracking stations, each one made of two planes of Cathode Pad Chambers. A 7.2 λIiron wall, which stops secondary hadrons escaping the front absorber and low momentum muons, is placed after the tracking stations. It is followed by a muon trigger system, based on two stations equipped with Resistive Plate Chambers. A conical absorber made of tungsten, lead and steel protects the spectrometer against secondary particles produced by the interaction of large-ηprimary particles in the beam pipe. In the dimuon analysis, the determination of the interaction vertex is provided by the two innermost Si-pixel layers of the ITS (Silicon Pixel Detector, SPD). For both analyses, timing information from the Zero Degree Calorimeters [40], placed symmetrically at 112.5 m with respect to the interaction point, is used to remove debunched proton-lead collisions. Furthermore, two scintillator hodoscopes (VZERO) [41], with pseudorapidity coverage 2.8< ηlab <5.1 and −3.7< ηlab <−1.7, are used to remove beam-induced background. More details on the ALICE apparatus can be found in [39]. A coincidence of signals in the two VZERO detectors provides the minimum bias (MB) trigger, which has a >99% efficiency for selecting non single-diffractive p-Pb collisions [42]. While the dielectron analysis is based on MB-triggered events, the study of J/ψin the µ+µ− decay channel relies on a dimuon trigger which requires, in addition to the MB condition, the detection of two opposite-sign tracks in the trigger system. The dimuon trigger selects two muon candidates with transverse momenta pT,µ larger than 0.5 GeV/c. The trigger threshold is not sharp, and the single muon trigger efficiency reaches its plateau value (∼96%) at pT,µ ∼1.5 GeV/c. The dielectron analysis was performed on a data sample – 4 – JHEP06(2015)055 corresponding to the p-Pb configuration, with an integrated luminosity Lint = 51.4±1.9 µb−1, while for the dimuon analysis the corresponding values are 5.01±0.19 nb−1for p-Pb and 5.81 ±0.20 nb−1for Pb-p (the quoted uncertainties are systematic) [43]. The dielectron analysis is based on 1.07×108events, collected with a low MB interaction rate (∼10 kHz), with a negligible amount of events having more than one interaction per bunch crossing (pile-up events). The interaction vertex is required to lie within ±10 cm from the nominal collision point along the beam axis, in order to obtain a uniform acceptance of the central barrel detector system in the fiducial range |ηlab|<0.9. Electron candidates are selected with criteria very similar to those used in previous analyses of pp collisions at √s=7 TeV [44] and Pb-Pb collisions at √sNN = 2.76 TeV [30]. To ensure a uniform tracking efficiency and particle identification resolution in the TPC, only tracks within |ηlab|<0.9 are used. Electron identification is performed using the TPC, as shown in figure 1, by requiring the dE/dxsignal to be compatible with the electron assumption within 3σ, where σdenotes the resolution of the dE/dxmeasurement. Furthermore, the TPC tracks that are compatible with the pion and proton assumptions within 3.5σare rejected. A slightly looser rejection condition (3σ) is applied when considering tracks corresponding to dielectron candidates with pT>5 GeV/cin order to enhance the statistics. A cut on the transverse momentum (pT,e>1.0 GeV/c) is applied to remove combinatorial background from low-momentum electrons. The efficiency loss induced by this cut amounts to only ∼20%, due to the relatively large momentum of the J/ψdecay products. The electron candidates must have at least one hit in the innermost two layers of the ITS, thus rejecting a large fraction of background electrons from photon conversions. For dielectrons with pT<3 GeV/cthe electron candidates are required to have a hit in the first layer, to further reduce background. The tracks are required to have at least 70 out of a maximum of 159 clusters in the TPC and a χ2normalized to the number of clusters attached to the track smaller than 4. The J/ψyields are obtained by counting the number of entries in the invariant mass range 2.92 < me+e−<3.16 GeV/c2after background subtraction. The J/ψradiative decay channel and the energy loss of the electrons due to bremsstrahlung in the detector material produce a long tail towards low invariant masses. A fit using a Crystal Ball (CB) [45] function for the J/ψsignal gives compatible values in Monte-Carlo (MC) and data (∼20 MeV/c2for the width of the Gaussian component of the CB). Taking into account such a mass resolution and the presence of the bremsstrahlung tail, 67 −73% of the signal, depending on pT, falls within the counting window. The background shape is obtained from event mixing. Event mixing is performed by pairing leptons from different events having similar global characteristics such as the primary-vertex position and the track multiplicity (the result being quite insensitive to the rapidity range, either forward or central, chosen for the multiplicity measurements). The mixed-event background is then scaled to match the same-event opposite-sign distribution in the mass ranges 2.0< me+e−<2.5 GeV/c2and 3.2< me+e−<3.7 GeV/c2(the contribution of the bremsstrahlung tail in the former range and of the ψ(2S) in the latter are negligible). Consistent results are found when the sameevent like-sign distributions are used, instead of event mixing, to estimate the background. The systematic uncertainty on the signal extraction comes from the variation of the mass – 5 – JHEP06(2015)055 )c (GeV/p 1 10 in TPC (arb. units)x/dEd 20 40 60 80 100 120 ALICE = 5.02 TeV NN s p-Pb p d e K π Figure 1. Charged particle specific energy loss (dE/dx) as a function of momentum, as measured in the TPC in p-Pb collisions. The black lines are the corresponding Bethe-Bloch parametrizations for the various particle species. 2.5 3 3.5 4 4.5 2 cCounts per 40 MeV/ 20 40 60 80 = 5.02 TeV NN sALICE, p-Pb <0.43 cms y -1.37< c < 1.3 GeV/ T p 0 < 2.5 3 3.5 4 4.5 20 40 60 80 c < 3 GeV/ T p1.3 < 2.5 3 3.5 4 4.5 20 40 60 80 c < 5 GeV/ T p3 < 2.5 3 3.5 4 4.5 20 40 60 80 c < 7 GeV/ T p5 < ) 2 c (GeV/ - e + e m 2.5 3 3.5 4 4.5 20 40 60 80 c < 10 GeV/ T p7 < Figure 2. Opposite-sign dielectron invariant mass spectra (blue symbols) for various pTintervals, compared to the background (black curve) estimated through mixed events. The background is scaled to match the data in the mass ranges 2.0< me+e−<2.5 GeV/c2and 3.2< me+e−< 3.7 GeV/c2. range where the normalization of the mixed-event background shape is performed and from the choice of the mass window where the signal is counted. The signal extraction has been performed in five transverse-momentum bins, pT<1.3, 1.3< pT<3, 3 < pT<5, 5< pT<7 and 7 < pT<10 GeV/c. The J/ψcounts in these bins vary from 25 to 132, with a significance, computed in the 2.92 < me+e−<3.16 GeV/c2mass region, ranging from 4.6 to 8.7. An analysis of the pT-integrated data sample, using the procedure detailed above, gives 465±37(stat.)±16(syst.) J/ψsignal counts. The systematic uncertainty on the signal extraction is largest at low pT(10% for pT<1.3 GeV/cand 12% for 1.3< pT<3 GeV/c), – 6 – JHEP06(2015)055 due to a less favorable signal over background ratio, and decreases to ∼5.5–8.4% in the other three pTbins. Figure 2shows the invariant mass distributions for the opposite-sign dielectrons compared with the mixed-event background for the different intervals of pT. The dimuon analysis is performed as detailed in [8], and is shortly summarized hereafter. Data were collected with the dimuon trigger, and the MB interaction rate (up to 200 kHz) was much higher than in the sample used for the dielectron analysis. This leads to a∼2% interaction pile-up probability. However, the probability of having more than one dimuon in the same bunch crossing satisfying the trigger condition is negligible. Muon candidate tracks are reconstructed in the tracking system by using the standard reconstruction algorithm [44]. The quality of the tracks is ensured by requiring the single muon pseudorapidity to be in the range −4< ηlab,µ <−2.5, in order to remove particles at the edges of the muon spectrometer acceptance. In addition, a cut on the radial coordinate of the track at the end of the front absorber (17.6< Rabs <89.5 cm) is performed, ensuring rejection of muons crossing its high-density part, where energy loss and multiple scattering effects are more important. The tracks reconstructed in the tracking system that are not matched to a corresponding track in the triggering system are rejected [44]. Finally, the reconstructed dimuons are required to be in 2.03 < ycms <3.53 (−4.46 < ycms <−2.96) for the forward (backward) rapidity analysis. The number of J/ψis extracted in transverse-momentum bins, in the range pT<15 GeV/c, through fits to the invariant mass spectra of opposite-sign dimuons. The spectra are fitted with a superposition of background and resonance shapes. The background is described with a Gaussian function with a mass-dependent width or, alternatively, with an exponential function times a fourth-order polynomial function. For the J/ψshape an extended Crystal Ball function, which accommodates a non-Gaussian tail both on the right and on the left side of the resonance peak, is adopted. Alternatively, a pseudo-Gaussian function [46] is used, corresponding to a Gaussian core around the J/ψ pole, and tails on the right and left side of it, parameterized by varying the width of the Gaussian as a function of the mass. The value of the J/ψmass and its width (σ) at the pole position are free parameters of the fit. The mass coincides with the PDG value within less than 5 MeV/c2and the width is ∼70 MeV/c2, slightly increasing with pT, due to a small relative decrease in the tracking resolution for harder muons. Although the signal over background ratios, calculated for a ±3σinterval around the resonance peak, are relatively large (ranging from 1.4 to ∼6 moving from low to high pT), the parameters of the tails of the J/ψdistributions cannot be reliably tuned on the data (in particular at large pT, where statistics is limited), but are fixed, for each pTbin, to the values extracted from fits to reconstructed samples from a signal-only MC generation. The contribution of the ψ(2S) resonance is also included in the fitting procedure, even if its influence on the determination of the J/ψyield is negligible. Finally, all the fits are performed in two different invariant mass ranges, either 2 < mµµ <5 GeV/c2or 2.2< mµµ <4.5 GeV/c2. Examples of fits to the invariant mass spectra, in the pTbins under study, are shown in figure 3. For each pTbin, the number of J/ψis evaluated as the average of the integrals of the resonance functions obtained in the various fits. The RMS of the corresponding yield distributions (0.2−3%, depending on pT) provides the systematic uncertainty on the signal extraction. Additional sets of tails, obtained from the MC, but referring to other ycms and – 7 – JHEP06(2015)055 2 Counts per 50 MeV/c 2000 4000 < 1 GeV/c T p0 < = 5.02 TeV NN s ALICE, p-Pb <3.53 cms y2.03< 2000 4000 < 2 GeV/c T p1 < 2000 4000 < 3 GeV/c T p2 < 2000 4000 < 4 GeV/c T p3 < 2000 4000 < 5 GeV/c T p4 < 2.5 3 3.5 4 4.5 2 Counts per 50 MeV/c 500 1000 < 6 GeV/c T p5 < 2.5 3 3.5 4 4.5 500 1000 < 7 GeV/c T p6 < 2.5 3 3.5 4 4.5 500 1000 < 8 GeV/c T p7 < 2.5 3 3.5 4 4.5 500 1000 < 10 GeV/c T p8 < ) 2 c (GeV/ - µ + µ m 2.5 3 3.5 4 4.5 500 1000 < 15 GeV/c T p10 < Figure 3. The opposite-sign dimuon invariant mass spectra for the various pTbins, relative to the p-Pb data sample (blue symbols). The fits shown in this figure (blue curves) were performed by using the sum of extended Crystal Ball functions for the J/ψand ψ(2S) signals, and a variable width Gaussian for the background. The signal and background components are shown separately as red curves. pTphase space regions, have also been tested and the dependence of the extracted yields on the variation of the tails (2%) is included in the systematic uncertainty on the signal extraction. As a function of pT, the number of J/ψin the p-Pb (Pb-p) configuration ranges between ∼16100 (∼16000) in the most populated bin (1 < pT<2 GeV/c) and less than ∼900 (∼300) in the highest pTbin (10 < pT<15 GeV/c). The J/ψyields are then corrected for the product of acceptance times efficiency (A×ε), evaluated by means of a MC simulation. J/ψproduction is assumed to be unpolarized, as motivated by the small degree of polarization measured in pp collisions at √s= 7 TeV [47– 49]. In the e+e−decay channel, A×εis calculated using a MC simulation where J/ψ are injected into p-Pb collisions simulated with HIJING [50]. The decay products of the J/ψare then propagated through a realistic description of the ALICE set-up, based on GEANT3.21 [51], taking into account the time evolution of the detector performance. Finally, J/ψcandidates are reconstructed with the same procedure applied to data. The pT- integrated A×εfactor amounts to 8.9%. Its pT-dependence exhibits a minimum (∼7.5%) around pT= 2 GeV/c, due to the kinematical acceptance, and it reaches ∼12% at high pT. The integrated value of A×εis affected by a 3% systematic uncertainty related to the choice of the J/ψ pT- and y-distributions used in the MC simulation. This value is obtained using as input several distributions, determined by varying within uncertainties the differential spectra extracted from the ALICE p-Pb data themselves. For pT-differential studies, the values of A×εare found to be sensitive only at a sub-percent level to the adopted input pT- and y-distributions. A further small systematic uncertainty reaching 1.5% in the highest pTinterval and related to the statistical uncertainty of the MC sample is also introduced. The systematic uncertainty on the dielectron reconstruction efficiency – 8 – JHEP06(2015)055 is strongly dominated by the particle identification uncertainty and amounts to 4%. It was obtained by comparing the single track reconstruction efficiency for topologically identified positrons and electrons from photon conversions with the corresponding MC quantities. In the dimuon analysis, the J/ψ A ×εis obtained with a MC simulation, by generating signal-only samples, tracking them in the experimental set-up modeled with GEANT3.21 and using the same reconstruction procedure applied to data. The use of a pure signal MC is justified, since the tracking efficiency does not show a dependence on the hadronic multiplicity of the collision. A realistic description of the set-up is adopted, including the time evolution of the efficiencies of tracking and triggering detectors. As for the dielectron analysis, the differential distributions used as an input to the MC are tuned directly on the data. The J/ψ A ×εvalues, integrated over pT, are 25.4% and 17.1% for p-Pb and Pb-p respectively [8], and exhibit a dependence on transverse momentum, being of the order of ∼24% (∼16%) for p-Pb (Pb-p) at low pTand reaches ∼50% (∼35%) in the highest pTbin (10 < pT<15 GeV/c). The systematically lower A×εvalues in Pb-p reflect the smaller detector efficiency in the corresponding data taking period. The systematic uncertainty on the integrated A×εdue to the input shapes is 1.5% for both p-Pb and Pb-p, and has been estimated using various distributions obtained from data and corresponding to smaller intervals in y,pTand centrality (see [8] for details). For pT-differential studies, the corresponding uncertainties are below 1.5%. The uncertainty on the dimuon tracking efficiency amounts to 4% (6%) for p-Pb (Pb-p) and is taken as constant for the full pT range. It is evaluated by combining the uncertainties on single muon tracking efficiencies, considered as uncorrelated. The efficiency of each tracking plane is obtained using the redundancy of the tracking system (two independent planes per station) and then single muon efficiencies for the full tracking system are calculated according to the tracking algorithm [52]. Their uncertainty is determined by comparing the efficiency obtained with tracks from MC and real data. The systematic uncertainty on the dimuon trigger efficiency includes: (i) a contribution due to the uncertainty in the evaluation of the trigger detector efficiency (∼2%, independent of pT); (ii) a 0.5−3% pT-dependent contribution (2% for the integrated efficiency), related to small differences in the trigger response function between data and MC in the region close to the trigger threshold; (iii) a 0.5−3.5% pT-dependent contribution due to a small fraction of opposite-sign pairs which were misidentified as likesign by the trigger system. Finally, a ∼1% uncertainty, independent of pT, is included, due to the choice of the value of the χ2cut applied to the matching of tracks reconstructed in the muon tracking and triggering systems. The differential cross section for inclusive J/ψproduction is defined as: d2σJ/ψ pPb dydpT =NJ/ψ(∆y, ∆pT) LpPb int ·(A×ε)(∆y,∆pT)·B.R.(J/ψ →l+l−)·∆y·∆pT (1) where NJ/ψ(∆y, ∆pT) is the number of J/ψfor a given ∆yand ∆pTinterval. The branching ratio to dileptons, B.R.(J/ψ →l+l−), is 5.94 ±0.06% (5.93 ±0.06%) for the dielectron (dimuon) decay [53]. The integrated luminosity, LpPb int , is the ratio between NMB, the number of MB collisions, and σMB pPb, the corresponding cross section, measured in a van der – 9 – JHEP06(2015)055 other combines coherent energy loss with EPS09 shadowing. The uncertainty bands include, for the coherent energy loss mechanism, a variation of both the q0parameter (gluon transport coefficient evaluated at x= 0.01) and the parametrization of the production cross section. At forward rapidity the pure energy loss scenario predicts a much steeper pT-dependence, while better agreement is found when the EPS09 contribution is included. However, at low pT, a discrepancy between data and both calculations is observed. Also at mid-rapidity the coherent energy loss model including the EPS09 contribution better describes the data, although the larger uncertainties prevent a firm conclusion. The same features can be observed at backward rapidity, where the calculation including coherent energy loss and shadowing agrees with the data in showing weak nuclear effects on J/ψ production. Finally, the results at central and forward rapidities are compared with a prediction based on the CGC framework and using CEM for the prompt J/ψproduction [36]. In the backward rapidity region, higher gluon xin the nucleus are probed and the CGC model is out of its range of applicability. The quoted uncertainties are related to the choices of Q2 sand of the charm quark mass. While the model is in fair agreement with mid-rapidity data, it clearly underpredicts the J/ψ RpPb in the full pTrange at forward rapidity. The theoretical calculations discussed above are carried out for prompt J/ψ(i.e., direct J/ψand the contribution from χcand ψ(2S) decays), while the measurements are for inclusive J/ψwhich include a non-prompt contribution from B-hadron decays. The contribution of the latter source to Rincl pPb can be evaluated from the measured fraction fBof non-prompt to prompt J/ψproduction in pp collisions and on the suppression Rnon−prompt pPb of non-prompt J/ψin p-Pb collisions. More in detail, in the range 2 < ycms <4.5, the fraction fBmeasured by LHCb in pp collisions at √s= 7 TeV, increases from 0.08 to 0.22 from pT= 0 to 8 GeV/c[48]. This quantity has a small variation within the ycms range covered and is also not strongly √s-dependent (similar values are obtained for √s= 8 TeV [58]). At mid-rapidity, fBwas measured by ALICE in pp collisions at √s= 7 TeV and ranges from 0.10 to 0.44 for pTincreasing from 1.3 to 10 GeV/c[61]. Rnon−prompt pPb was measured at √sNN = 5.02 TeV by LHCb, integrated over pT, obtaining 0.83 ±0.02 ±0.08 for 2.5< ycms <4 and 0.98 ±0.06 ±0.10 for −4< ycms <−2.5 [9]. Assuming for each pT-bin a variation of Rnon−prompt pPb between 0.6 and 1.3, a conservative choice due to the unavailability of a pT-differential result, and considering the pT-dependence of fBat √s= 7 TeV, one can extract Rprompt pPb as Rprompt pPb =Rincl pPb +fB·(Rincl pPb −Rnon−prompt pPb ). The maximum differences between the inclusive and prompt RpPb obtained in this way are, for low and high pT: (i) 3 and 10% at backward rapidity; (ii) 11 and 16% at central rapidity; (iii) 10 and 8% at forward rapidity. These variations are, at most, of the same order of magnitude as the quoted uncertainties on inclusive RpPb. The RpPb results shown in this paper can be considered as a valuable tool to improve our understanding of the contribution of CNM to the suppression of the J/ψyields observed in Pb-Pb [30,31]. Indeed, as verified in [8] for the dimuon analysis, in Pb-Pb collisions the Bjorken-xranges probed by the J/ψproduction process in the two colliding nuclei, assuming a gg →J/ψ (2→1) [62] mechanism, are shifted by only ∼10% with respect to the corresponding intervals for p-Pb and Pb-p, despite the different energy (√sNN = 2.76 TeV) – 16 – JHEP06(2015)055 and the slightly different ycms range (2.5< y < 4) for Pb-Pb. A similar conclusion holds at mid-rapidity, where the covered x-intervals, calculated for pT=hpTi, are 6.1×10−4< x < 3.0×10−3and 7.0×10−4<x<3.5×10−3for p-Pb and Pb-Pb collisions, respectively. Under the assumption that shadowing is the main CNM-related mechanism that plays a role in the J/ψproduction and that its effect on the two colliding nuclei in Pb-Pb collisions can be factorized, the product RpPb ×RPbp (R2 pPb) can be considered as an estimate of CNM effects in Pb-Pb collisions at forward (central) rapidity [63,64]. This conclusion holds not only for the 2 →1 production process but also when the more general 2 →2 mechanism (gg →J/ψg) is considered. In figure 6the comparison of the measured RPbPb with the quantities defined above is carried out. Such a comparison should be considered as qualitative, in view of the slight xmismatch detailed above and of the fact that, at mid-rapidity, the centrality ranges probed in p-Pb and Pb-Pb are not the same (0-100% and 0-50%, respectively). In both rapidity regions, the extrapolation of CNM effects shows a clear pT-dependence, corresponding to a strong suppression at low pT, which vanishes for large transverse momenta. At low pTand central rapidity, there might be an indication for a Pb-Pb suppression smaller than the CNM extrapolation, consistent with the presence of a contribution related to the (re)combination of c¯cpairs [30], taking place in the hot medium. A similar effect can be seen at forward rapidity. At large pTand forward rapidity, the observed suppression in Pb-Pb collisions is much larger than CNM extrapolations, showing that, in this transversemomentum region, suppression effects in hot matter, possibly related to color screening, become dominant. Finally, a more direct comparison of Pb-Pb results with the CNM extrapolation can be obtained by defining the ratio SJ/ψ =RPbPb/(RpPb ×RPbp). Such a quantity, for forward rapidity results, is shown in figure 7and confirms the main features detailed above, i.e., a strong suppression of J/ψat large pT, and a hint for an enhancement at low pT. At central rapidity, due to the sizeable uncertainties on both p-Pb and Pb-Pb results, only the pT- integrated ratio can be obtained. Using the RPbPb in the 0-90% centrality range [30], and the integrated RpPb given above, one gets 1.43 ±0.26(stat) ±0.56(syst). More precise measurements are needed to draw a firm conclusion in this rapidity range. In summary, we have presented results on the inclusive J/ψproduction in p-Pb collisions at √sNN = 5.02 TeV. The pT-differential cross sections, the hpTiand the nuclear modification factors have been evaluated in three rapidity regions: −4.46 < ycms <−2.96, −1.37 < ycms <0.43 and 2.03 < ycms <3.53. At forward and mid-rapidity a significant suppression is observed at low pT, with a vanishing trend at high pT. At backward rapidity no significant suppression or enhancement is visible. Comparisons with theoretical models based on a combination of nuclear shadowing and coherent energy loss effects provide a fair description of the observed patterns, except at forward rapidity and low transverse momentum. These results can be used to provide a qualitative estimate of the influence of cold nuclear matter effects on the J/ψsuppression observed in Pb-Pb collisions. Under the assumption that shadowing represents the main CNM contribution, we find that it cannot account for the observed suppression in Pb-Pb at high pT. At low pT, the observed CNM effects alone may suggest a suppression larger than that observed in Pb-Pb, which – 17 – JHEP06(2015)055 )c (GeV/ T p 0 2 4 6 8 10 PbPb R, 2 pPb R 0 0.2 0.4 0.6 0.8 1 1.2 1.4 - e + e→ ψALICE inclusive J/ = 5.02 TeV NN s <0.43), cms y(-1.37< 2 pPb R = 2.76 TeV, 0-50 % NN s|<0.8), cms y(| PbPb R )c (GeV/ T p 0 1 2 3 4 5 6 7 8 PbPb R, backw pPb R x forw pPb R 0 0.2 0.4 0.6 0.8 1 1.2 1.4 - µ + µ→ψALICE inclusive J/ = 5.02 TeV NN s <-2.96), cms y (-4.46< backw pPb R<3.53) x cms y (2.03< forw pPb R = 2.76 TeV, 0-90% NN s<4), cms y (2.5< PbPb R (Phys. Lett. B734 (2014) 314) Figure 6. The estimate of the pT-dependence of CNM effects in Pb-Pb, calculated as R2 pPb for midrapidity data (top) and as RpPb ×RPbp (bottom) at forward rapidity. The quantities are compared to RPbPb measured in Pb-Pb collisions in the (approximately) corresponding y-ranges [30,31]. The vertical error bars correspond to the statistical uncertainties, the open boxes (shaded areas) represent pT-uncorrelated (partially correlated) systematic uncertainties, while the boxes around RpPb = 1 show the size of the correlated uncertainties. The horizontal bars correspond to the widths of the pTbins. The Pb-Pb points in the bottom panel were slightly displaced in pT, to improve visibility. – 18 – JHEP06(2015)055 )c(GeV/ T p 0 1 2 3 4 5 6 7 8 ψJ/ S 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 - µ + µ→ψALICE inclusive J/ <3.53 cms y = 5.02 TeV, 2.03< NN s: forw pPb R <-2.96 cms y = 5.02 TeV, -4.46< NN s : backw pPb R <4, 0-90% cms y = 2.76 TeV, 2.5< NN s : PbPb R (Phys. Lett. B734 (2014) 314) backw pPb R × forw pPb R PbPb R = ψJ/ S Figure 7. The ratio between RPbPb for inclusive J/ψat forward rapidity and the product RpPb ×RPbp of the nuclear modification factors at forward and backward rapidity. None of the uncertainties cancels out in the ratio. Statistical uncertainties are shown as vertical error bars, while the boxes around the points represent a quadratic combination of uncorrelated and partially correlated systematic uncertainties. The box around SJ/ψ = 1 corresponds to correlated uncertainties. The horizontal bars coincide with the widths of the pTbins. is consistent with the presence of a charm quark (re)combination component to the J/ψ production in nucleus-nucleus collisions. Acknowledgments The ALICE Collaboration would like to thank all its engineers and technicians for their invaluable contributions to the construction of the experiment and the CERN accelerator teams for the outstanding performance of the LHC complex. The ALICE Collaboration gratefully acknowledges the resources and support provided by all Grid centres and the Worldwide LHC Computing Grid (WLCG) collaboration. The ALICE Collaboration acknowledges the following funding agencies for their support in building and running the ALICE detector: State Committee of Science, World Federation of Scientists (WFS) and Swiss Fonds Kidagan, Armenia, Conselho Nacional de Desenvolvimento Cient´ıfico e Tecnol´ogico (CNPq), Financiadora de Estudos e Projetos (FINEP), Funda¸c˜ao de Amparo `a Pesquisa do Estado de S˜ao Paulo (FAPESP); National Natural Science Foundation of China (NSFC), the Chinese Ministry of Education (CMOE) and the Ministry of Science and Technology of China (MSTC); Ministry of Education and Youth of the Czech Republic; Danish Natural Science Research Council, the Carlsberg Foundation and the Danish National Research Foundation; The European Research Council under the European Community’s Seventh Framework Programme; Helsinki Institute of Physics and the Academy of Finland; French CNRS-IN2P3, the ‘Region Pays de Loire’, ‘Region Alsace’, ‘Region Auvergne’ – 19 – JHEP06(2015)055 and CEA, France; German Bundesministerium fur Bildung, Wissenschaft, Forschung und Technologie (BMBF) and the Helmholtz Association; General Secretariat for Research and Technology, Ministry of Development, Greece; Hungarian Orszagos Tudomanyos Kutatasi Alappgrammok (OTKA) and National Office for Research and Technology (NKTH); Department of Atomic Energy and Department of Science and Technology of the Government of India; Istituto Nazionale di Fisica Nucleare (INFN) and Centro Fermi — Museo Storico della Fisica e Centro Studi e Ricerche “Enrico Fermi”, Italy; MEXT Grant-in-Aid for Specially Promoted Research, Japan; Joint Institute for Nuclear Research, Dubna; National Research Foundation of Korea (NRF); Consejo Nacional de Cienca y Tecnologia (CONACYT), Direccion General de Asuntos del Personal Academico (DGAPA), M´exico; Amerique Latine Formation academique — European Commission (ALFA-EC) and the EPLANET Program (European Particle Physics Latin American Network) Stichting voor Fundamenteel Onderzoek der Materie (FOM) and the Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO), Netherlands; Research Council of Norway (NFR); National Science Centre, Poland; Ministry of National Education/Institute for Atomic Physics and Consiliul Nat ,ional al Cercet˘arii S¸tiint ,ifice — Executive Agency for Higher Education Research Development and Innovation Funding (CNCS-UEFISCDI) — Romania; Ministry of Education and Science of Russian Federation, Russian Academy of Sciences, Russian Federal Agency of Atomic Energy, Russian Federal Agency for Science and Innovations and The Russian Foundation for Basic Research; Ministry of Education of Slovakia; Department of Science and Technology, South Africa; Centro de Investigaciones Energeticas, Medioambientales y Tecnologicas (CIEMAT), E-Infrastructure shared between Europe and Latin America (EELA), Ministerio de Econom´ıa y Competitividad (MINECO) of Spain, Xunta de Galicia (Conseller´ıa de Educaci´on), Centro de Aplicaciones Tecnol´ogicas y Desarrollo Nuclear (CEADEN), Cubaenerg´ıa, Cuba, and IAEA (International Atomic Energy Agency); Swedish Research Council (VR) and Knut & Alice Wallenberg Foundation (KAW); Ukraine Ministry of Education and Science; United Kingdom Science and Technology Facilities Council (STFC); The United States Department of Energy, the United States National Science Foundation, the State of Texas, and the State of Ohio; Ministry of Science, Education and Sports of Croatia and Unity through Knowledge Fund, Croatia. 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