J/ψ suppression at forward rapidity in Pb–Pb collisions at √sNN = 5.02 TeV

This is an electronic reprint of the original article. This reprint may differ from the original in pagination and typographic detail. Author(s): Title: Year: Version: Please cite the original version: All material supplied via JYX is protected by copyright and other intellectual property rights, and duplication or sale of all or part of any of the repository collections is not permitted, except that material may be duplicated by you for your research use or educational purposes in electronic or print form. You must obtain permission for any other use. Electronic or print copies may not be offered, whether for sale or otherwise to anyone who is not an authorised user. J/ψ suppression at forward rapidity in Pb–Pb collisions at √sNN = 5.02 TeV ALICE Collaboration ALICE Collaboration. (2017). J/ψ suppression at forward rapidity in Pb–Pb collisions at √sNN = 5.02 TeV. Physics Letters B, 766, 212-224. https://doi.org/10.1016/j.physletb.2016.12.064 2017 Physics Letters B 766 (2017) 212–224 Contents lists available at ScienceDirect Physics Letters B www.elsevier.com/locate/physletb J/ψsuppression at forward rapidity in Pb–Pb collisions at √sNN =5.02 TeV ALICE Collaboration a r t i c l e i n f o a b s t r a c t Article history: Received 29 July 2016 Received in revised form 5 December 2016 Accepted 26 December 2016 Available online 10 January 2017 Editor: L. Rolandi The inclusive J/ψproduction has been studied in Pb–Pb and pp collisions at the centre-of-mass energy per nucleon pair √sNN =5.02 TeV, using the ALICE detector at the CERN LHC. The J/ψmeson is reconstructed, in the centre-of-mass rapidity interval 2.5 <y <4and in the transverse-momentum range pT<12 GeV/c, via its decay to a muon pair. In this Letter, we present results on the inclusive J/ψcross section in pp collisions at √s=5.02 TeV and on the nuclear modification factor RAA. The latter is presented as a function of the centrality of the collision and, for central collisions, as a function of the transverse momentum pTof the J/ψ. The measured RAA values indicate a suppression of the J/ψin nuclear collisions and are then compared to our previous results obtained in Pb–Pb collisions at √sNN =2.76 TeV. The ratio of the RAA values at the two energies is also computed and compared to calculations of statistical and dynamical models. The numerical value of the ratio for central events (0–10% centrality) is 1.17 ±0.04(stat)±0.20(syst). In central events, as a function of pT, a slight increase of RAA with collision energy is visible in the region 2 <pT<6GeV/c. Theoretical calculations qualitatively describe the measurements, within uncertainties. ©2017 The Author. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP3. 1. Introduction When heavy nuclei collide at ultrarelativistic energies, a state of strongly-interacting matter is formed, characterised by high temperature and density, where quarks and gluons are not confined into hadrons (Quark–Gluon Plasma, QGP [1]). A detailed characterisation of the QGP is the object, since more than 25 years, of an intense research activity at the CERN/SPS [2] and at the BNL/RHIC [3–6] and CERN/LHC [7] ion colliders. Charmonia and bottomonia, which are bound states of charm–anticharm (cc) or bottom–antibottom (bb) quarks, respectively [8], are among the most sensitive probes of the characteristics of the QGP. A suppression of their yields in nucleus–nucleus (A–A) collisions with respect to expectations from proton–proton (pp) collisions was experimentally observed. For the J/ψmeson, the ground ccstate with quantum numbers JPC =1−−, a suppression was found at the SPS, in Pb–Pb and In–In interactions at the centre-of-mass energy per nucleon pair √sNN =17.2GeV[9,10], RHIC, in Au–Au interactions at √sNN =200 GeV [11,12], and finally at the LHC, in Pb–Pb collisions at √sNN =2.76 TeV [13,14]. Early theoretical calculations predicted J/ψsuppression to be induced by the screening of the colour force in a deconfined medium and to become stronger as the QGP temperature increases [15,16]. In a complementary way to E-mail address: [email protected]. this static approach, J/ψsuppression can also be seen as the result of dynamical interactions with the surrounding partons [17–19]. The LHC results, integrated over transverse momentum (pT) down to pT=0, show a suppression of the J/ψ, quantified through the ratio between its yields in Pb–Pb and those in pp, normalised to the number of nucleon–nucleon collisions in Pb–Pb (nuclear modification factor, RAA). However, the observed suppression is smaller than at SPS and RHIC [20,21], in spite of the higher initial temperature of the QGP formed at the LHC [22]. The effect is particularly evident for head-on (central) collisions. In order to explain these observations, theoretical models require a contribution from J/ψ regeneration via a recombination mechanism [23,24] between the c and cquarks, during the deconfined phase and/or at the hadronisation of the system, which occurs when its temperature falls below the critical value Tc∼155 MeV [25]. The strength of this regeneration effect increases with the initial number of produced ccpairs relative to the total number of quarks and, therefore, increases with the collision energy, explaining the reduced suppression at the LHC. Since the bulk of charm–quark production occurs at small momenta, recombination should be more important for low-pTJ/ψ, as observed in the LHC results [21]. An important test of the suppression and regeneration picture of J/ψproduction at the LHC can be obtained by comparing the centrality and pTdependence of the J/ψRAA, measured at √sNN =2.76 TeV, to that obtained at √sNN =5.02 TeV, the highest energy available up to now in nuclear collisions. The suppression http://dx.doi.org/10.1016/j.physletb.2016.12.064 0370-2693/©2017 The Author. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP3. ALICE Collaboration / Physics Letters B 766 (2017) 212–224 213 effects related to colour screening should become stronger when increasing the collision energy, due to the higher QGP temperature, and also the recombination effects should become stronger, due to the expected increase of the ccproduction cross section. The two effects act in opposite directions and the comparison of the RAA at the different energies can provide insights in the evolution of the relative contribution of the two processes. In this Letter, we present the first results on the J/ψRAA measured by the ALICE Collaboration in Pb–Pb collisions at √sNN = 5.02 TeV and the integrated and pTdifferential J/ψproduction cross section in pp collisions at the same energy. In both Pb–Pb and pp collisions, the J/ψis reconstructed via its dimuon decay channel at forward rapidity, 2.5 <y <4 and for pT<12 GeV/c. The measurements refer to inclusive J/ψproduction, that includes both prompt J/ψ(direct J/ψand feed-down from higher-mass resonances) and non-prompt J/ψ(from decay of beauty hadrons). The nuclear modification factor is obtained by normalising the J/ψ yield in Pb–Pb collisions to the product of the nuclear overlap function times the corresponding J/ψcross section measured in pp, at the same energy and in the same kinematic window. The results on RAA are presented as a function of the J/ψpTand of the centrality of the collision. 2. Experimental apparatus and data sample The ALICE detector design and performance are extensively described in [26] and [27]. The analysis presented here is based on the detection of muons in the forward muon spectrometer [28], which covers the pseudo-rapidity range −4 <η<−2.5.1In addition, the Silicon Pixel Detector (SPD) [29] is used to reconstruct the primary vertex. The V0 detectors [30] provide a minimumbias (MB) trigger and are used to determine the centrality of the collision, while the T0 Cherenkov counters [31] are used for the luminosity determination in pp collisions. Finally, the Zero Degree Calorimeters (ZDC) are used to reject electromagnetic Pb–Pb interactions [32]. A brief description of these detectors is given hereafter. The muon spectrometer contains a front absorber, made of carbon, concrete and steel, placed between 0.9 and 5 m from the Interaction Point (IP), which filters out hadrons, thus decreasing the occupancy in the downstream tracking system. The latter is composed of five stations, each one consisting of two planes of Cathode Pad Chambers (CPC). The third tracking station is placed inside the gap of a dipole magnet with a 3Tm field integral. Two trigger stations, each one equipped with two planes of Resistive Plate Chambers (RPC), are located behind a 7.2 interaction length iron wall, which absorbs secondary hadrons escaping the front absorber and low-momentum muons. The muon trigger system delivers single-muon and dimuon triggers with a programmable transverse-momentum threshold. Finally, throughout its entire length, a conical absorber around the beam pipe (θ<2◦) made of tungsten, lead and steel shields the muon spectrometer against secondary particles produced by the interaction of large-η primary particles in the beam pipe. The primary vertex is reconstructed using hit pairs in the two cylindrical layers of the SPD [26,29], which have average radii of 3.9 and 7.6 cm, and cover the pseudo-rapidity intervals |η| <2 and |η| <1.4, respectively. The two V0 detectors [30], with 32 scintillator tiles each, are placed on each side of the IP, covering the pseudo-rapidity ranges 1In the ALICE reference frame, the muon spectrometer covers a negative ηrange and consequently a negative yrange. We have chosen to present our results with a positive ynotation. 2.8 <η<5.1 and −3.7 <η<−1.7. The coincidence of the signals from the two hodoscopes defines the MB trigger. Beam-induced background is reduced by applying timing cuts on the signals from the V0s and ZDCs. The latter are positioned along the beam direction at ±112.5 m from the IP. Finally, the T0 detectors [31], made of two arrays of quartz Cherenkov counters, are placed on both sides of the IP, covering the pseudo-rapidity intervals −3.3 < η<−3 and 4.6 <η<4.9. In Pb–Pb collisions, the centrality determination is based on a Glauber fit of the total V0 signal amplitude distribution as described in [33,34]. A selection corresponding to the most central 90% of the hadronic cross section was applied; for these events the MB trigger is fully efficient. For both Pb–Pb and pp data taking, the trigger condition used in the analysis is a μμ-MB trigger formed by the coincidence of the MB trigger and an unlike-sign (US) dimuon trigger. The latter has a trigger probability for each of the two muon candidates that increases with the muon pT, is 50% at 1.0 GeV/c(0.5 GeV/c) in Pb–Pb (pp) collisions, and saturates at pT≈2.5GeV/c, where it reaches a value of about 98%. Like-sign dimuon triggers were also collected, mainly for background normalisation purposes in the Pb–Pb analysis. The data samples used in this analysis correspond to an integrated luminosity LPb–Pb int ≈225 μb−1for Pb–Pb and Lpp int ≈ 106 nb−1for pp collisions. 3. Data analysis The analysis procedure was very similar for the two data samples described in this Letter. In the following paragraphs, the Pb– Pb analysis is first presented, followed by the description of the pp one. The J/ψcandidates were formed by combining pairs of US tracks reconstructed in the geometrical acceptance of the muon spectrometer using the tracking algorithm described in [28]. The same single-muon and dimuon selection criteria as in previous analyses [21] were applied, and tracks in the tracking system were required to match a track segment in the muon trigger system (trigger tracklet). The J/ψraw yields were determined from the invariant mass distribution of US dimuons using two methods. In the first one, the US dimuon invariant mass distributions were fitted with the sum of a signal and a background function. In the second approach, the background, estimated using an event-mixing technique and normalised using the like-sign dimuon distributions [21], was subtracted and the resulting spectra were fitted with the sum of a signal function and a (small) residual background component. Various shapes were considered for the signal and background contributions. For the J/ψsignal either an extended Crystall Ball (CB2) function or a pseudo-Gaussian with a mass-dependent width were used [35]. The non-Gaussian tails of the signal functions were fixed either (i) to the values obtained in Monte Carlo (MC) simulations, where simulated J/ψ →μ+μ−are embedded into real events to account for the effect of the detector occupancy, or (ii) to the values obtained in a high-statistics pp collision sample at √s=13 TeV, collected under similar detector conditions. The tail parameters exhibit a dependence on the pTand rapidity of the J/ψand a mild dependence on the centrality of the collision. The small contribution of the ψ(2S)signal was taken into account in the fits, its mass and width being tied to those of the J/ψ[36]. For the background, when the US dimuon mass spectrum was fitted, a variable-width-Gaussian with a mass-dependent width or the ratio of a 2nd to a 3rd order polynomial were used. When considering the US dimuon distributions after subtraction of the background obtained with the event-mixing procedure, a small 214 ALICE Collaboration / Physics Letters B 766 (2017) 212–224 Fig. 1. (Colour online.) Invariant mass distributions of US dimuons with 2.5 <y <4and pT<12 GeV/c. The top (bottom) row shows the distribution before (after) background subtraction with the event-mixing technique. The left panels correspond to the most central events (0–10%) while the right panels to a peripheral (70–80%) centrality range. The fit curves shown in blue represent the sum of the signal and background shapes, while the red lines correspond to the J/ψsignal and the grey ones to the background. dimuon continuum component is still present and was fitted using the sum of two exponentials. Several fitting sub-ranges, within the interval 2 <mμμ <5GeV/c2, were used for both signal extraction procedures. Fig. 1 shows examples of fits to the US dimuon invariant mass distributions with and without background subtraction using the event-mixing technique, for different selections in centrality. The raw J/ψyield in each centrality or pTinterval was determined as the average of the results obtained with the two fitting approaches, the various parameterisations of signal and background and the different fitting ranges, while the corresponding systematic uncertainties were defined as the RMS of these results. A further contribution to the systematic uncertainty was estimated by using a different set of resonance tails obtained using in the MC simulation a different particle transport model (GEANT4 [37] instead of GEANT3 [38]). The total number of J/ψ, integrated over centrality, pTand y, is NJ/ψ =2.77 ±0.02(stat) ±0.05(syst) ·105. The systematic uncertainty ranges from 1.6% to 2.8% as a function of centrality and from 1.2% to 3.1% as a function of pT. The nuclear modification factor, as a function of the centrality class iof the collision and for the J/ψtransverse-momentum interval pT, is calculated as Ri AA(pT)= Ni J/ψ (pT) BRJ/ψ→μ+μ−Ni MB Aεi(pT)Ti AAσpp J/ψ (pT),(1) where Ni J/ψ (pT)is the number of extracted J/ψin a given centrality and pTrange, BRJ/ψ→μ+μ−=5.96 ±0.03% is the branching ratio of the dimuon decay channel [39], Ni MB is the number of equivalent minimum-bias events, Aεi(pT)is the product of the detector acceptance times the reconstruction efficiency, Ti AAis the average of the nuclear overlap function, and σpp J/ψ (pT)is the inclusive J/ψcross section for pp collisions at the same energy and in the same kinematic range as the Pb–Pb data. The Aεvalues were determined from MC simulations, with the generated pTand ydistributions for the J/ψadjusted on data, and separately tuned for each centrality class using an iterative approach. Unpolarised J/ψproduction was assumed [21]. For the tracking chambers, the time-dependent status of each electronic channel during the data taking period was taken into account as well as the misalignment of the detection elements. The efficiencies of the muon trigger chambers were determined from data and were then applied in the simulations. Finally, the dependence of the efficiency on the detector occupancy was taken into account by embedding MC-generated J/ψinto real minimum-bias Pb–Pb events. For J/ψproduced within 2.5 <y <4 and pT<12 GeV/c, in 0–90% most central collisions, the Aεvalue is 0.136 ±0.007(syst). A relative decrease of the efficiency by 14% was observed when going from peripheral to central collisions. As a function of pT, Aεhas a minimum value of about 0.12 at pT≈1.5GeV/c, and then steadily increases up to about 0.4 at the upper end of the considered range. The following sources of systematic uncertainty on Aεwere considered. A first contribution of 2% due to the input MC pTand ydistributions was estimated by (i) varying the input shapes that were tuned on data within their statistical uncertainties and (ii) taking into account the effect of possible pT−y correlations by comparing, as a function of centrality, the Aεvalues with the corresponding result of a 2-D acceptance calculation in classes of pTand y. A second contribution comes from the tracking efficiency and it was estimated by comparing the singlemuon tracking efficiency values obtained, in MC and data, with a procedure that exploits the redundancy of the tracking-chamber information [21]. A 3% systematic uncertainty on the dimuon tracking efficiency is obtained and is approximately constant as a function of centrality and kinematics. The systematic uncertainty on the dimuon trigger efficiency represents the third contribution and it has two origins: the intrinsic efficiencies of the muon trigger chambers and the response of the trigger algorithm. The first one was determined from the uncertainties on the trigger chamber efficiencies measured from data and applied to simulations and it amounts to 1.5%. The second one was estimated by comparing the pTdependence, at the single-muon level, of the trigger response function between data and MC and it varies between 0.2% and 4.6% as a function of pT. Combining the two sources, a systematic uncertainty ranging from 1.5% to 4.8% as a function of the J/ψpTis obtained. Finally, there is a 1% contribution related to the choice ALICE Collaboration / Physics Letters B 766 (2017) 212–224 215 Table 1 Summary of systematic uncertainties, in percentage, on RAA and d2σpp J/ψ /dydpT. Values marked with an asterisk correspond to correlated uncertainties as a function of pT (second and fifth column) or centrality (third column). There is no correlation between the uncertainties related to the analysis of the Pb–Pb and of the pp sample. The contents of the “pp reference” row correspond to the quadratic sum of the contributions indicated for d2σpp J/ψ /dydpT, excluding only the BR uncertainty which cancels out when forming the RAA. Source RAA d2σpp J/ψ /dydpT 0–90% pT<12 GeV/c vs pT (0–20%) vs centrality (pT<8GeV/c) pT<12 GeV/cvs pT Signal extr. 1.8 1.2–3.1 1.6–2.8 3 1.5–9.3 MC input 2 2 2∗2 0.7–1.5 Tracking eff. 3 3 3∗11 Trigger eff. 3.6 1.5–4.8 3.6∗1.8 1.5–1.8 Matching eff. 1 1 1∗11 F(Lpp int)0.5 0.5∗0.5∗(2.1) (2.1∗) BR – – – 0.5 0.5∗ TAA3.2 3.2∗3.1–7.6 Centrality 0 0.1∗0–6.6 pp reference 5.0 3–10 2.1∗(Lpp int)4.9∗ of the χ2cut used in defining the matching between the reconstructed tracks and the trigger tracklets. The normalisation factor to the number of equivalent MB events was obtained as Ni MB =Fi·Nμμ-MB, where Nμμ-MB is the number of μμ-MB triggered events, and Fiis the inverse of the probability of having a dimuon trigger in a MB event in the centrality range i. The Fivalues were calculated with two different methods, by applying the dimuon trigger condition in the analysis on minimum-bias events, or from the relative counting rate of the two triggers [40]. The obtained value, in the 0–90% centrality class, is F=11.84 ±0.06, where the uncertainty is dominated by a systematic contribution corresponding to the difference between the results obtained with the two approaches. As a function of centrality, Fi=F·i, where iis the fraction of the inelastic cross section of a given centrality class with respect to the whole 0–90% centrality range (e.g. 0.1/0.9 for 0–10% centrality and so on). The values for Ti AAand for the average number of participant nucleons Ni partwere obtained via a Glauber calculation [33, 34,41]. The systematic uncertainty is 3.2% for the 0–90% centrality range and was obtained by varying within uncertainties the density parameters of the Pb nucleus and the nucleon–nucleon inelastic cross section [34,41]. Finally, the effects of the uncertainty on the value of the V0 signal amplitude corresponding to 90% of the hadronic Pb–Pb cross section were estimated by varying such a value by ±0.5% [33] and redefining correspondingly the centrality intervals. The systematic effect on RAA ranges from 0.1% to 6.6% from central to peripheral collisions. The J/ψcross-section values in pp collisions at √s=5.02 TeV, both integrated and pTdifferential, were obtained with an analysis procedure similar to the one described in the previous paragraphs for Pb–Pb. In particular, the same criteria for single-muon and dimuon selection were adopted. The signal extraction was then performed by fitting the spectra with the sum of a signal and a background contribution, using shapes similar to those adopted for the Pb–Pb analysis. The background subtraction via the event-mixing technique was not used, as the signal-over-background ratio is larger by a factor ∼40, in the pT-integrated spectra, with respect to central Pb–Pb collisions, making the influence of the background estimate much less important in the determination of the uncertainty on Npp J/ψ . The value Npp J/ψ =8649 ±123(stat)±297(syst)is obtained, with the systematic uncertainty determined as for the Pb–Pb analysis. The determination of Aεpp was carried out via MC simulations. Since no appreciable dependence of the tracking efficiency as a function of the hadronic multiplicity can be seen in pp, a pure MC (i.e., without embedding) was used. The input pTand ydistributions were obtained from the measured ones via an iterative procedure, and unpolarised J/ψproduction was assumed [42]. The obtained value is Aεpp =0.243 ±0.007(syst), with the systematic uncertainties on the tracking, trigger and matching efficiency calculated as in the Pb–Pb analysis. Because of the limited pp statistics, the systematic uncertainty on the MC inputs was not obtained through a 2-D acceptance calculation, as done in the Pb–Pb analysis, but it was determined comparing the Aεvalues obtained using J/ψpT(y) distributions evaluated in various y(pT) intervals in pp collisions at √s=7TeV[43]. The integrated luminosity was calculated as Lpp int =(Npp μμ-MB · Fpp)/σpp ref , where σpp ref is a reference-trigger cross section measured in a van der Meer scan, following the procedure detailed in [44], and Fpp is the ratio of the reference-trigger probability to the μμ-MB trigger probability. The corresponding numerical value is Lpp int =106.3 ±2.2(syst)nb−1, where the quoted uncertainty reflects the van der Meer scan uncertainty. Finally, the inclusive J/ψcross section in pp collisions at √s= 5.02 TeV was obtained as d2σpp J/ψ dydpT= Npp J/ψ (pT) BRJ/ψ→μ+μ−Lpp int Aεpp(pT)pTy.(2) Table 1 summarises the systematic uncertainties on the measurement of the nuclear modification factors and d2σpp J/ψ /dydpT. The RAA values presented in the following refer to inclusive J/ψ production, i.e. include both prompt and non-prompt J/ψ. Since beauty-hadron decays occur outside the QGP, the non-prompt J/ψ RAA is related to the nuclear modification of the beauty-hadron pTdistributions. The difference between the RAA of prompt and inclusive J/ψcan be estimated as in [21], using the fraction FB of non-prompt to inclusive J/ψin pp collisions and assuming two extreme cases for the Rnon-prompt AA of non-prompt J/ψ, namely no medium effects on b-quarks (Rnon-prompt AA =1) or their complete suppression (Rnon-prompt AA =0). FBwas obtained by an interpolation of the LHCb measurements in pp collisions at √s=2.76 and 7TeV [43,45,46]. The quantitative effect on the inclusive J/ψRAA is provided in the following along with the results. 4. Results The pT-differential inclusive J/ψcross section in pp collisions at √s=5.02 TeV, in the region 2.5 <y <4, is shown in Fig. 2. The cross section value, integrated over the interval 2.5 <y <4, pT<12 GeV/cis σpp J/ψ =5.61 ±0.08(stat)±0.28(syst)μb. These results are used as a reference in the determination of the nuclear modification factor for Pb–Pb collisions. Both the differential 216 ALICE Collaboration / Physics Letters B 766 (2017) 212–224 Fig. 2. (Colour online.) The differential cross section d2σpp J/ψ /dydpTfor inclusive J/ψ production in pp collisions at √s=5.02 TeV. The error bars represent the statistical uncertainties, the boxes around the points the uncorrelated systematic uncertainties. The uncertainty on the luminosity measurement represents a correlated global uncertainty. Fig. 3. (Colour online.) The nuclear modification factor for inclusive J/ψproduction, as a function of centrality, at √sNN =5.02 TeV, compared to published results at √sNN =2.76 TeV [20]. The error bars represent statistical uncertainties, the boxes around the points uncorrelated systematic uncertainties, while the centralitycorrelated global uncertainties are shown as a filled box around RAA =1. The widths of the centrality classes used in the J/ψanalysis at √sNN =5.02 TeV are 2% from 0 to 12%, then 3% up to 30% and 5% for more peripheral collisions. and integrated pp cross section values are consistent with those obtained via an interpolation [45,47] of the measured values at √s=2.76 and 7 TeV [48,49], which were used for the determination of the nuclear modification factor in p–Pb collisions at √sNN =5.02 TeV [40,47,50]. The nuclear modification factor for inclusive J/ψproduction in Pb–Pb collisions at √sNN =5.02 TeV, integrated over the centrality range 0–90%, and for the interval 2.5 <y <4, pT<12 GeV/c is RAA(pT<12 GeV/c) =0.65 ±0.01(stat) ±0.05(syst), showing a significant suppression of the J/ψwith respect to pp collisions at the same energy. When restricting the pTrange to 8 GeV/c, corresponding to the interval covered in the √sNN =2.76 TeV results, one obtains RAA(pT<8GeV/c)=0.66 ±0.01(stat) ±0.05(syst). The ratio between the latter value and the corresponding one at √sNN =2.76 TeV, RAA(pT<8GeV/c)=0.58 ±0.01(stat) ± 0.09(syst)[20], is 1.13 ±0.02(stat)±0.18(syst). When calculating the ratio, the quoted uncertainties on the two values are considered as uncorrelated, except for the TAAcontribution. Fig. 3 shows the centrality dependence of RAA at √sNN = 5.02 TeV. The results are compared to the values obtained at √sNN =2.76 TeV [20], and correspond to the same transverseFig. 4. (Colour online.) Comparison of the centrality dependence (with 10% width centrality classes) of the inclusive J/ψRAA for 0.3 <pT<8GeV/cwith theoretical models [17–19,52–55]. The model calculations do not include the pTcut (except for TM1), which was anyway found to have a negligible impact, since they only include hadronic J/ψproduction. The error bars represent the statistical uncertainties, the boxes around the data points the uncorrelated systematic uncertainties, while the centrality-correlated global uncertainty is shown as a filled box around RAA =1. The brackets shown in the three most peripheral centrality intervals represent the range of variation of the hadronic J/ψRAA under extreme hypothesis on the photoproduction contamination on the inclusive RAA. momentum range, pT<8GeV/c. The centrality dependence, characterised by an increasing suppression with centrality up to Npart ∼100, followed by an approximately constant RAA value, is similar at the two energies. A systematic difference by about 15% is visible when comparing the two sets of results, even if the effect is within the total uncertainty of the measurements. The RAA of prompt J/ψwould be about 10% higher if Rnon-prompt AA =0 and about 5% (1%) smaller if Rnon-prompt AA =1for central (peripheral) collisions. An excess of very-low pTJ/ψ, compared to the yield expected assuming a smooth evolution of the J/ψhadro-production and nuclear modification factor was observed in peripheral Pb–Pb collisions at √sNN =2.76 TeV [51]. This excess might originate from the photo-production of J/ψand could influence the RAA in peripheral collisions. To quantify the expected difference between the hadronic J/ψRAA and the measured values the method described in [21] was adopted. The hadronic J/ψRAA, for 0 <pT< 8GeV/c, is estimated to be about 34%, 17% and 9% smaller than the measured values in the 80–90%, 70–80% and 60–70% centrality classes, respectively. The variation decreases to about 9%, 4% and 2%, respectively, when considering the RAA for J/ψwith 0.3 <pT<8GeV/c, due to the remaining small contribution of photo-produced J/ψ. Fig. 4 shows RAA as a function of centrality, for 0.3 <pT<8GeV/c. Comparing the results of Fig. 3 and Fig. 4, a less pronounced increase of RAA for peripheral events can indeed be seen when such a selection is introduced. The same extreme hypotheses as in [21] were made to define upper and lower limits, represented with brackets on Fig. 4. Thus, the selection of J/ψwith pT>0.3GeV/c makes the results more suitable for a comparison with theoretical models that only include hadronic J/ψproduction. We start by comparing the results to a calculation based on a statistical model approach [52], where J/ψare created, like all other hadrons, only at chemical freeze-out according to their statistical weights. In this model, the nucleon–nucleon ccproduction cross section is extrapolated from LHCb pp measurements at √s=7TeV[56] using FONLL calculations [57], obtaining dσcc/dy =0.45 mb in the yrange covered by the data. Then, the nuclear modification of the parton distribution functions (shadowing) is accounted for via the EPS09 NLO parameterisation [58]. ALICE Collaboration / Physics Letters B 766 (2017) 212–224 217 Fig. 5. (Colour online.) The ratio of the inclusive J/ψRAA for 0.3 <pT<8GeV/cbetween √sNN =5.02 and 2.76 TeV, compared to theoretical models [17–19,52–55], shown as a function of centrality. The model calculations do not include the pT cut (except for TM1), which was anyway found to have a negligible impact, since they only include hadronic J/ψproduction. The error bars represent the statistical uncertainties and the boxes around the data points the uncorrelated systematic uncertainties. The centrality-correlated global uncertainty is shown as a filled box around r=1and is obtained as the quadratic sum of the corresponding global uncertainties at √sNN =2.76 and 5.02 TeV. The corresponding 17% uncertainty on the extrapolated dσcc/dy plus shadowing is used when calculating the uncertainty bands for this model. The results are also compared to the calculations of a transport model (TM1) [18,54,55] based on a thermal rate equation, which includes continuous dissociation and regeneration of the J/ψboth in the QGP and in the hadronic phase. The inclusive cccross section is taken as dσcc/dy =0.57 mb, consistent with FONLL calculations, while the J/ψproduction cross section value in N–N collisions is dσJ/ψ /dy =3.14 μb. The results of this model are shown as a band including a variation of the shadowing contribution between 10% and 25% and a 5% uncertainty on the cc cross section. The results are then compared to the calculations of a second transport model (TM2) [19], which implements a hydrodynamic description of the medium evolution. The input nucleon– nucleon cross sections for cc and J/ψare taken as dσcc/dy = 0.82 mb, corresponding to the upper limit of FONLL calculations, and dσJ/ψ /dy =3.5μb. Also for this model the band corresponds to the choice of either no shadowing, or a shadowing effect estimated with the EPS09 NLO parameterisation. Finally, the data are compared to a ‘co-mover’ model [17,53], where the J/ψare dissociated via interactions with the partons/hadrons produced in the same rapidity range, using an effective interaction cross section σco-J/ψ =0.65 mb, based on calculations that described lower energy experimental results. Regeneration effects are included, based on dσcc/dyvalues ranging from 0.45 to 0.7 mb, which correspond to the uncertainty band shown for the model. Shadowing effects, calculated within the Glauber–Gribov theory [59], are included and are consistent with EKS98/nDSg predictions [60,61]. Finally, the contribution of non-prompt production is taken into account in the transport models TM1 and TM2, while it is not considered in the other calculations. The data are described by the various calculations, the latter having rather large uncertainties, due to the choice of the corresponding input parameters, and in particular of dσcc/dy. It can be noted that for most calculations a better description is found when considering their upper limit. For transport models this corresponds to a minimum contribution or even absence of nuclear shadowing, which can be clearly considered as an extreme assumption for primary J/ψ, considering the J/ψmeasurements in p–Pb collisions [47,50]. Fig. 6. (Colour online.) The pTdependence of the inclusive J/ψRAA at √sNN = 5.02 TeV, compared to the corresponding result at √sNN =2.76 TeV [20] and to the calculation of a transport model [18,54,55] (TM1), in the centrality interval 0–20%. The pTdependence of ris also shown for both data and theory. The error bars represent statistical uncertainties, the boxes around the points uncorrelated systematic uncertainties, while pT-correlated global uncertainties are shown as a filled box around RAA =1. A correlation between the parameters of the models is present when comparing their calculations for √sNN =2.76 and 5.02 TeV. Therefore, the theoretical uncertainties can be reduced by forming the ratio r=RAA(5.02 TeV)/RAA(2.76 TeV). Concerning data, the uncertainties on TAAcancel. In Fig. 5 the centrality dependence of r, calculated for 0.3 <pT<8GeV/c, is shown and compared to models. For prompt J/ψthe ratio rwould be about 2% (1–2%) higher if beauty hadrons were fully (not) suppressed by the medium. The transport model of Ref. [18,54,55] (TM1) shows a decrease of rwith increasing centrality, due to the larger suppression effects at high energy, followed by an increase, related to the effect of regeneration, which acts in the opposite direction and becomes dominant for central collisions. The other transport model (TM2) [19] also exhibits an increase for central collisions, while for peripheral collisions the behaviour is different. In the co-mover model [17,53], no structure is visible as a function of centrality, and the calculation favours r-values slightly below unity, implying that in this model the increase of the suppression effects with energy may be dominant over the regeneration effects for all centralities. Finally, the statistical model [52] shows a continuous increase of rwith centrality, dominated by the increase in the cccross section with energy. The uncertainty bands shown in Fig. 5 correspond to variations of about 5% in the cccross section at √sNN =5.02 TeV, plus a 10% relative variation of the shadowing contribution between the two energies in the case of TM1. The data are, within uncertainties, compatible with the theoretical models, and show no clear centrality dependence. The ratio for central collisions and 0.3 <pT<8GeV/cis r0–10% =1.17 ±0.04(stat)±0.20(syst). Finally, the study of the pTdependence of RAA has proven to be a sensitive test of the presence of a regeneration component which, in calculations, leads to an increase at low pT. Fig. 6 shows, for the centrality interval 0–20%, RAA as a function of transverse momentum, compared to the corresponding results obtained at √sNN =2.76 TeV, and to a theoretical model calculation. The region pT<0.3GeV/cwas not excluded, because the contribution of J/ψphoto-production is negligible with respect to the hadronic one for central events [51]. In the same figure the pTdependence of ris also shown. A hint for an increase of RAA with √sNN is visible in the region 2 <pT<6GeV/c, while the r-ratio is consistent 218 ALICE Collaboration / Physics Letters B 766 (2017) 212–224 with unity elsewhere. This feature is qualitatively described by the theoretical model (TM1) also shown in the figure. The prompt J/ψ RAA is expected to be 7% larger (2% smaller) for pT<1GeV/cand 30% larger (55% smaller) for 10 <pT<12 GeV/cwhen the beauty contribution is fully (not) suppressed. Assuming that Rnon-prompt AA does not vary significantly between the two collision energies, the ratio rappears to be less sensitive to the non-prompt J/ψ contribution. The effect is negligible for the case of full suppression of beauty hadrons, while it varies from no increase at low transverse momentum up to a maximum increase of about 15% for 5 <pT<6GeV/cif no suppression is assumed. The transport model of Ref. [18,54,55] (TM1) fairly describes the overall shape of the RAA pTdependence. 5. Conclusion We reported the ALICE measurement of inclusive J/ψproduction in pp and Pb–Pb collisions at √sNN =5.02 TeV at the LHC. Asystematic difference by about 15% is visible when comparing the RAA measured at √sNN =5.02 TeV to the one obtained at √sNN =2.76 TeV, even if such an effect is within the total uncertainty of the measurements. When removing very-low pTJ/ψ (pT<0.3GeV/c), the RAA shows a less pronounced increase for peripheral events, which can be ascribed to the removal of a large fraction of electromagnetic J/ψproduction [51]. These results, as well as those on the ratio of the nuclear modification factors between √sNN =5.02 and 2.76 TeV, are described by theoretical calculations, and closer to their upper limits. The pTdependence of RAA exhibits an increase at low pT, a feature that in the model which is compared to the data is related to an important contribution of regenerated J/ψ. A hint for an increase of RAA between √sNN =2.76 and 5.02 TeV is visible in the region 2 <pT<6GeV/c, while the results are consistent elsewhere. The results presented in this paper confirm that also at the highest energies reached today at the LHC, data on J/ψproduction support a picture where a combination of suppression and regeneration takes place in the QGP, the two mechanisms being dominant at high and low pT, respectively. Acknowledgements 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: A. I. Alikhanyan National Science Laboratory (Yerevan Physics Institute) Foundation (ANSL), State Committee of Science and World Federation of Scientists (WFS), Armenia; Austrian Academy of Sciences and Nationalstiftung für Forschung, Technologie und Entwicklung, Austria; Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Universidade Federal do Rio Grande do Sul (UFRGS), Financiadora de Estudos e Projetos (Finep) and Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP), Brazil; Ministry of Science and Technology of the People’s Republic of China (MOST), National Natural Science Foundation of China (NSFC) and Ministry of Education of China (MOE), China; Ministry of Science, Education and Sport and Croatian Science Foundation, Croatia; Ministry of Education, Youth and Sports of the Czech Republic, Czech Republic; The Danish Council for Independent Research – Natural Sciences, the Carlsberg Foundation and Danish National Research Foundation (DNRF), Denmark; Helsinki Institute of Physics (HIP), Finland; Commissariat à l’Énergie Atomique et aux Énergies Alternatives (CEA) and Institut National de Physique Nucléaire et de Physique des Particules (IN2P3) and Centre National de la Recherche Scientifique (CNRS), France; Bundesministerium für Bildung, Wissenschaft, Forschung und Technologie (BMBF) and GSI Helmholtzzentrum für Schwerionenforschung GmbH, Germany; Ministry of Education, Research and Religious Affairs, Greece; National Research, Development and Innovation Office, Hungary; Department of Atomic Energy, Government of India (DAE) and Council of Scientific and Industrial Research (CSIR), New Delhi, India; Indonesian Institute of Science, Indonesia; Centro Fermi – Museo Storico della Fisica e Centro Studi e Ricerche Enrico Fermi and Istituto Nazionale di Fisica Nucleare (INFN), Italy; Institute for Innovative Science and Technology, Nagasaki Institute of Applied Science (IIST), Japan Society for the Promotion of Science (JSPS), KAKENHI and Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan; Consejo Nacional de Ciencia y Tecnología (CONACYT), through Fondo de Cooperación Internacional en Ciencia y Tecnología (FONCICYT) and Dirección General de Asuntos del Personal Academico (DGAPA), Mexico; Nationaal instituut voor subatomaire fysica (Nikhef), Netherlands; The Research Council of Norway, Norway; Commission on Science and Technology for Sustainable Development in the South (COMSATS), Pakistan; Pontificia Universidad Católica del Perú, Peru; Ministry of Science and Higher Education and National Science Centre, Poland; Korea Institute of Science and Technology Information and National Research Foundation of Korea (NRF), Republic of Korea; Ministry of Education and Scientific Research, Institute of Atomic Physics and Romanian National Agency for Science, Technology and Innovation, Romania; Joint Institute for Nuclear Research (JINR), Ministry of Education and Science of the Russian Federation and National Research Centre Kurchatov Institute, Russia; Ministry of Education, Science, Research and Sport of the Slovak Republic, Slovakia; National Research Foundation of South Africa, South Africa; Centro de Aplicaciones Tecnológicas y Desarrollo Nuclear (CEADEN), Cubaenergía, Cuba; Ministerio de Ciencia e Innovación and Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas (CIEMAT), Spain; Swedish Research Council (VR) and Knut & Alice Wallenberg Foundation (KAW), Sweden; European Organization for Nuclear Research, Switzerland; National Science and Technology Development Agency (NSDTA), Suranaree University of Technology (SUT) and Office of the Higher Education Commission under NRU project of Thailand, Thailand; Turkish Atomic Energy Agency (TAEK), Turkey; National Academy of Sciences of Ukraine, Ukraine; Science and Technology Facilities Council (STFC), United Kingdom; National Science Foundation of the United States of America (NSF) and United States Department of Energy, Office of Nuclear Physics (DOE NP), United States of America. 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