Measurement of D-meson production versus multiplicity in p-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. Measurement of D-meson production versus multiplicity in p-Pb collisions at √SNN = 5.02 TeV ALICE Collaboration ALICE Collaboration. (2016). Measurement of D-meson production versus multiplicity in p-Pb collisions at √SNN = 5.02 TeV. Journal of High Energy Physics, 2016(8), Article 78. https://doi.org/10.1007/JHEP08(2016)078 2016 JHEP08(2016)078 Published for SISSA by Springer Received:February 25, 2016 Revised:June 23, 2016 Accepted:July 13, 2016 Published:August 11, 2016 Measurement of D-meson production versus multiplicity in p–Pb collisions at √sNN =5.02 TeV The ALICE collaboration E-mail: [email protected] Abstract: The measurement of prompt D-meson production as a function of multiplicity in p–Pb collisions at √sNN = 5.02 TeV with the ALICE detector at the LHC is reported. D0, D+and D∗+mesons are reconstructed via their hadronic decay channels in the centre-of-mass rapidity range −0.96 < ycms <0.04 and transverse momentum interval 1< pT<24 GeV/c. The multiplicity dependence of D-meson production is examined by either comparing yields in p–Pb collisions in different event classes, selected based on the multiplicity of produced particles or zero-degree energy, with those in pp collisions, scaled by the number of binary nucleon-nucleon collisions (nuclear modification factor); as well as by evaluating the per-event yields in p–Pb collisions in different multiplicity intervals normalised to the multiplicity-integrated ones (relative yields). The nuclear modification factors for D0, D+and D∗+are consistent with one another. The D-meson nuclear modification factors as a function of the zero-degree energy are consistent with unity within uncertainties in the measured pTregions and event classes. The relative D-meson yields, calculated in various pTintervals, increase as a function of the charged-particle multiplicity. The results are compared with the equivalent pp measurements at √s= 7 TeV as well as with EPOS 3 calculations. Keywords: Heavy Ion Experiments, Heavy-ion collision, Quark gluon plasma ArXiv ePrint: 1602.07240 Open Access, Copyright CERN, for the benefit of the ALICE Collaboration. Article funded by SCOAP3. doi:10.1007/JHEP08(2016)078 JHEP08(2016)078 Contents 1 Introduction 1 2 Experimental apparatus and data sample 4 3 Multiplicity determination 5 3.1 Centrality estimators and TpPb determination 5 3.2 Relative event multiplicity determination 7 4 D meson reconstruction 8 5 Nuclear modification factor as a function of centrality 12 5.1 Systematic uncertainties 13 5.2 Results 14 5.2.1 QpPb with CL1 and V0Aestimators 15 5.2.2 Comparison with charged-particle QpPb 17 6 Relative yields as a function of multiplicity 18 6.1 Systematic uncertainties 18 6.2 Results 19 6.2.1 Comparison of p–Pb data with pp results and models 23 7 Summary 24 A Result tables 28 The ALICE collaboration 37 1 Introduction In high-energy hadronic collisions, heavy quarks (charm and beauty) are produced in hard parton scattering processes. Due to their large masses, their production cross sections can be calculated in the framework of perturbative Quantum Chromodynamics (pQCD) down to low transverse momenta. The differential cross section for heavy-flavour hadron production in nucleon-nucleon collisions can be calculated in the factorisation approach by the convolution of parton densities in the incoming nucleon, the short-distance partonic cross section of heavy quark production, and the fragmentation function that describes the transition of the heavy quark into a heavy-flavour hadron [1]. Thus, heavy-flavour production is sensitive to the gluon and the possible heavy-quark content in the nucleon and provides constraints on the parton distribution functions (PDFs) in the proton and in – 1 – JHEP08(2016)078 the nucleus [2,3]. Measurements of heavy-flavour hadron production in hadronic collisions provide tests of pQCD and constitute a crucial baseline for the study of heavy-flavour production in heavy-ion collisions [4,5]. A suppression of heavy-flavour yields is observed in heavy-ion collisions at high transverse momentum (pT), and is interpreted as being due to the formation of a Quark-Gluon Plasma (QGP). Beauty production measurements in pp collisions at √s= 1.96 TeV at the FNAL Tevatron collider [6–8] and in pp collisions at √s= 7 TeV at the CERN LHC collider [9–13] are described by different implementations of pQCD calculations, such as the General-MassVariable-Flavour-Number Scheme (GM-VFNS) [14,15] at next-to-leading order, and the Fixed-Order plus Next-to-Leading Logarithms (FONLL) approach [16–18]. Analogously, inclusive charm meson production measurements at the LHC [19–21] are reproduced within uncertainties by the predictions of GM-VFNS, FONLL and those performed in the framework of kTfactorisation in the Leading Order (LO) approximation [22]. Recently, the study of heavy-flavour production as a function of the multiplicity of charged particles produced in the collision has attracted growing interest. Such measurements probe the interplay between hard and soft mechanisms in particle production. At LHC energies, the multiplicity dependence of heavy-flavour production is likely to be affected by the larger amount of gluon radiation associated with short-distance production processes, as well as by the contribution of Multiple-Parton Interactions (MPI) [23–25]. It has also been argued that, due to the spatial distribution of partons in the transverse plane, the probability for MPI to occur in a pp collision increases towards smaller impact parameters [26–28]. This effect might be further enhanced by quantum-mechanical fluctuations of gluon densities at small Bjorken-x[29]. The measurements of prompt D mesons, inclusive and non-prompt J/ψ in pp collisions at √s= 7 TeV [30,31], and of the three Υ states in pp collisions at √s= 2.76 TeV [32], provide evidence for a similar increase of open and hidden heavy-flavour yields as a function of charged-particle multiplicity. These results suggest that the enhancement probably originates in short-distance production processes, and is not influenced by hadronisation mechanisms. The enhancement is quantitatively described by calculations including MPI contributions, namely percolation model estimates [33,34], the EPOS 3 event generator [35, 36] and PYTHIA 8.157 calculations [37]. In proton-nucleus collisions, several so-called ‘Cold Nuclear Matter’ (CNM) effects occur due to the presence of a nucleus in the colliding system, and, possibly, to the large density of produced particles. These CNM effects can affect the production of heavy-flavour hadrons at all the stages of their formation. In particular, the PDFs of nucleons bound in nuclei are modified with respect to those of free nucleons. This modification of the PDFs in the nucleus can be described by phenomenological parameterisations (nuclear PDFs, or nPDFs) [38–40]. Alternatively, when the production process is dominated by gluons at low Bjorken-x, the nucleus can be described by the Colour-Glass Condensate (CGC) effective theory as a coherent and saturated gluonic system [41–44]. The kinematics of the partons in the initial state can be affected by multiple scatterings (transverse momentum broadening, or kTbroadening) [45–47] or by gluon radiation (energy loss) [48] before the heavy-quark pair is produced. Gluon radiation may also occur after the heavy-quark pair – 2 – JHEP08(2016)078 is formed [49]. Other measurements in p–Pb collisions at √sNN = 5.02 TeV, e.g. those of angular correlations between charged particles [50–53], of ψ(2S) suppression [54] and of the relative yields of the three Υ states [32], indicate that final-state effects also play an important role. The measured charm production cross section in minimum-bias p–Pb collisions at √sNN = 5.02 TeV [55] is consistent within uncertainties with that in pp collisions at the same energy scaled by the atomic mass number of the Pb nucleus. The nuclear modification factor was also found to be consistent with calculations considering EPS09 nPDFs [38], CGC, or transverse momentum broadening and initial-state energy loss. The influence of cold nuclear matter effects on multiplicity-integrated D-meson production in p–Pb collisions is smaller than the measurement uncertainties. Additional insight into CNM effects can be obtained by measuring the heavy-flavour hadron yields as a function of the multiplicity of charged particles produced in the p–Pb collision. The aim of these studies is to explore the dependence of heavy-flavour production on the collision geometry and on the density of final-state particles. Indeed, it is expected that the multiplicity of produced particles depends on the number of nucleons overlapping in the collision region, and therefore on the geometry of the collision (i.e. on the collision centrality). Most of the aforementioned models of CNM effects consider a dependence on the collision geometry, usually expressed through the impact parameter of the collision, the number of participant nucleons (Npart), or the number of nucleon-nucleon collisions (Ncoll). In general, CNM effects are expected to be more pronounced in central collisions, i.e. those having a small impact parameter. Some of the parameterisations of the nPDFs have studied the influence of the local nucleon density [56–59]. The spatially dependent EPS09 and EKS98 nPDF sets, EPS09s and EKS98s, are formulated as a function of the nuclear thickness [56]. The leading twist nuclear shadowing calculation [60] assumes the GlauberGribov approach of the collision geometry and predicts the dependence of the nPDF on the collision impact parameter. The estimates of the initial-state kTbroadening due to multiple soft collisions also consider a dependence on the collision impact parameter [46, 47]. Initial-state parton energy loss is also expected to evolve with the collision geometry as a consequence of the different nuclear density, though detailed calculations including this effect are not yet available. Finally, if final-state effects were to affect heavy-flavour production in p–Pb collisions, their influence would also vary with the density of produced particles. In this paper, we report the pT-differential measurements of D0, D+and D∗+production as a function of multiplicity in p–Pb collisions at √sNN = 5.02 TeV. The experimental setup and the data sample are described in section 2. The determination of the multiplicity and the estimation of the collision centrality and of the number of nucleon-nucleon collisions are discussed in section 3. The D-meson reconstruction strategy is explained in section 4. The results are reported in the form of the D-meson nuclear modification factor in different centrality classes (section 5), and the relative D-meson yields as a function of the relative charged-particle multiplicity at central and backward rapidity (section 6). – 3 – JHEP08(2016)078 2 Experimental apparatus and data sample The ALICE apparatus is described in detail in [61] and its performance in [62]. It is composed of a series of detectors in the central barrel for tracking and particle identification; the Muon Spectrometer in the forward direction for muon tracking and identification; and a further set of detectors at forward rapidity for triggering and event characterisation. The central barrel detectors are located inside a large solenoid magnet that provides a 0.5 T field parallel to the beam direction, which corresponds to the z-axis of the ALICE coordinate system. In this section, the detectors used for the D-meson analysis are briefly described. The Inner Tracking System (ITS), the Time Projection Chamber (TPC) and the Time Of Flight detector (TOF) allow the reconstruction and identification of charged particles in the central pseudorapidity region. The V0 detector, composed of two scintillator arrays located in the forward and backward pseudorapidity regions, is used for online event triggering and multiplicity determination. The Zero Degree Calorimeters (ZDC) are used for event selection and to estimate the collision centrality via the zero-degree energy. The ITS is composed of six cylindrical layers of silicon detectors, located at radii between 3.9 cm (about 1 cm from the beam vacuum tube) and 43.0 cm. The two innermost layers, which respectively cover |η|<2.0 and |η|<1.4, comprise the Silicon Pixel Detectors (SPD); the two intermediate layers, within |η|<0.9, consist of Silicon Drift Detectors (SDD); and the two outer layers, also covering |η|<0.9, consist of double-sided Silicon Strip Detectors (SSD). The low material budget, high spatial resolution, and position of the detector setup surrounding the beam vacuum tube and close to the interaction point allow it to provide a measurement of the charged-particle impact parameter in the transverse plane (d0), i.e. the distance of closest approach between the track and the primary vertex along rφ, with a resolution better than 75 µm for transverse momenta pT>1 GeV/c [63]. The TPC is a large cylindrical drift detector, extending from 85 cm to 247 cm in the radial direction and covering the range −250 < z < +250 cm along the beam axis [64]. It provides charged-particle trajectory reconstruction with up to 159 space points per track in the pseudorapidity range |η|<0.9 and in the full azimuth. The primary interaction vertex position and covariance matrix are determined from tracks reconstructed from hits in the TPC and the ITS via a χ2analytic minimisation method. The TOF detector is equipped with Multi-gap Resistive Plate Chambers (MRPCs) [62]. It is placed at radii between 377 cm and 399 cm, and has the same pseudorapidity and azimuthal coverage as the TPC. The TOF measures the flight times of charged particles from the interaction point to the detector with an overall resolution of about 85 ps. For events with the 20% lowest multiplicities, the resolution decreases to about 120 ps due to a worse start-time (collision-time) resolution. The start-time of the event is determined by combining the time estimated using the particle arrival times at the TOF and the time measured by the T0 detector, an array of Cherenkov counters located at +350 cm and −70 cm along the beamline. Particle identification (PID) is performed by comparing the measurement of the specific energy deposition dE/dxin the TPC and the time-of-flight information from the TOF with the respective expected values for each mass hypothesis. The V0 detector consists of two arrays of scintillator tiles covering the pseudorapidity regions −3.7< η < −1.7 (V0C) and 2.8< η < 5.1 (V0A) [65]. The data sample analysed – 4 – JHEP08(2016)078 in this paper was collected with a minimum-bias interaction trigger requiring at least one hit in both V0A and V0C counters coincident with the arrival time of the proton and lead bunches. The ZDC is composed of two sets of neutron (ZNA and ZNC) and proton (ZPA and ZPC) calorimeters positioned on either side of the interaction point at z=±112.5 m. Contamination from beam-background interactions was removed via offline selections based on the timing information provided by the V0 and the ZNA. The signals registered by the SPD and V0 detectors were used to determine the event charged-particle multiplicity; the SPD, V0 and ZDC detectors were also exploited to classify the events in centrality classes, as will be described in section 3. The data sample used in this paper was recorded in January 2013, during the p–Pb LHC run. Protons with an energy of 4 TeV were collided with Pb ions with an energy of 1.58 TeV per nucleon, resulting in collisions at a centre-of-mass energy per nucleon pair, √sNN, of 5.02 TeV. With this beam configuration, the centre-of-mass system moves with a rapidity of ∆ycms = 0.465 in the direction of the proton beam, due to the different energies per nucleon of the proton and the lead beams. In the case of the D-meson analyses presented here, performed in the laboratory reference interval |ylab|<0.5, this leads to a shifted centre-of-mass rapidity coverage of −0.96 < ycms <0.04. In the following, we will use the notation ηand ylab to refer to the pseudorapidity and rapidity values in the laboratory reference frame, and ηcms and ycms for the values evaluated in the centre-of-mass reference frame. A total of 108minimum-bias triggered events, corresponding to an integrated luminosity of Lint = 48.6±1.6µb−1, passed the selection criteria and were analysed. 3 Multiplicity determination The production of D mesons in p–Pb collisions has been studied as a function of chargedparticle multiplicity using two different observables. One observable is the pT-differential nuclear modification factor, which is defined as the ratio of the pT-differential yields measured in p–Pb collisions in centrality intervals to those in pp collisions, scaled by the number of binary nucleon-nucleon collisions. The centrality intervals were defined using three different estimators based on the multiplicity in the SPD and V0A detectors and the energy deposited in the zero-degree neutron calorimeter in the Pb-going side (ZNA). The procedure used to determine the number of binary nucleonnucleon collisions for each event class is described in section 3.1 and [66]. The other observable, referred to as the relative yield, is defined as the ratio of the per-event D-meson yields in p–Pb collisions in different multiplicity intervals normalised to the multiplicity-integrated yields. Details on the evaluation of the charged-particle multiplicity are discussed in section 3.2. In this analysis, the values of multiplicity measured in two different pseudorapidity intervals, namely at mid-rapidity with the SPD and at large rapidity in the Pb-going direction with the V0A, were considered. 3.1 Centrality estimators and TpPb determination A centrality-dependent measurement of the nuclear modification factor requires the p–Pb data sample to be sliced into classes according to an experimental observable related to – 5 – JHEP08(2016)078 the collision centrality, as well as a determination of the average nuclear overlap function hTpPbi, which is proportional to the number of nucleon-nucleon collisions Ncoll, for each centrality class. The minimum-bias p–Pb data sample was divided into four centrality classes by exploiting the information from: (i) V0A, the amplitude of the signal measured by the V0 scintillator array located in the Pb-going side, covering 2.8< η < 5.1, which is proportional to the number of charged particles produced in this pseudorapidity interval; (ii) CL1, the number of clusters in the outer layer of the SPD, covering |η|<1.4, which is proportional to the number of charged particles at mid-rapidity; and (iii) ZNA, the energy deposited in the Zero Degree Neutron Calorimeter positioned in the Pb-going side by the slow nucleons produced in the interaction by nuclear de-excitation processes, or knocked out by wounded nucleons. The multiplicity of these neutrons is expected to grow monotonically with the number of binary collisions, Ncoll. Centrality classes were defined as percentiles of the visible cross section, which was measured to be (2.09 ±0.07) b [67]. For the centrality classes defined using the CL1 and V0A multiplicities, a Glauber Monte Carlo was used to calculate the relevant geometrical quantities, namely the average numbers of participant nucleons hNGlauber part i, of binary collisions hNGlauber coll i, and the average nuclear overlap function hTGlauber pPb i[66]. For the case where the ZNA information was used, the values of Npart,Ncoll and TpPb were obtained using the so-called hybrid method [66]. In this approach, the determination of hTpPbiin a given ZNA-energy class relies on the assumption that the charged-particle multiplicity measured at mid-rapidity (−1< ηcms <0) scales with the number of participant nucleons, Npart. hNmult coll ii=hNmult part ii−1 = hNMB parti·hdNch/dηii hdNch/dηiMB −1<η<0−1,and hTmult pPb i=hNmult coll ii σNN , (3.1) where hNMB parti= 7.9 is the average number of participants in minimum-bias collisions and σNN = (70 ±5) mb is the interpolated inelastic nucleon-nucleon cross section at √sNN = 5.02 TeV [66]. The values of hTpPbiobtained with the three estimators in the four multiplicity (zero-degree energy) classes used for the analysis are reported in table 1. It was demonstrated by the studies of charged-particle production reported in [66] that when centrality classes are defined in p–Pb collisions, some biases are present. Firstly, there is a multiplicity selection bias due to the large multiplicity fluctuations for p–Pb interactions at a given impact parameter, which are comparable in magnitude to the full dynamic range of the minimum-bias multiplicity distribution. In addition, there is a jet-veto bias due to the contribution to the overall multiplicity from particles arising from the fragmentation of partons produced in hard-scattering processes. This causes low- (high-) multiplicity p–Pb collisions to correspond to a lower (higher) number of hard scatterings per nucleonnucleon collision. Furthermore, a purely geometrical bias was suspected to affect peripheral collisions for all centrality estimators, due to the fact that the mean impact parameter between the proton and each nucleon of the Pb nucleus, calculated from a Monte Carlo Glauber simulation, rises significantly for Npart <6, thus reducing the average number of multi-parton interactions for peripheral collisions. – 6 – JHEP08(2016)078 Centrality hTpPbiGlauber-NBD (mb−1)hTpPbihybrid method (mb−1) (%) V0A CL1 Syst. (%) ZNA Syst. (%) 0–20 0.183 0.190 11 0.164 6.5 20–40 0.134 0.136 3.7 0.136 3.9 40–60 0.092 0.088 5.0 0.101 5.9 60–100 0.037 0.037 23 0.046 6.2 Table 1.hTpPbivalues in p–Pb collisions at √sNN = 5.02 TeV obtained with a Glauber-model based approach for V0A and CL1, and from the hybrid method for ZNA, as described in [66]. These biases cause the nuclear modification factor of charged particles to differ from unity in the centrality classes even in the absence of nuclear effects. These biases decrease with increasing rapidity separation between the centrality estimator and the region where the nuclear modification factor is measured. A strong selection bias is observed for the CL1 estimator, due to the full overlap with the tracking region, which is reduced with the V0A estimator. By contrast, the selection based on the energy deposited in the ZNA is expected to be free from the biases related to the event selection, and is only affected by the geometrical bias. For these reasons, the results based on the ZNA selection, which is the least biased [66], provide insight into possible centrality-dependent nuclear effects on charm production in p–Pb collisions. Moreover, the measurements of the D-meson nuclear modification factor in centrality intervals defined with the three estimators described above offer the possibility to study these biases based on heavy-flavour production, which, due to the large mass of the charm quarks, is expected to scale with the number of binary collisions over the whole pTrange, provided that cold nuclear matter effects are negligible. This is in contrast to the charged-particle yield, where a scaling with Ncoll is expected to occur only in the high-pT region. 3.2 Relative event multiplicity determination The charged-particle multiplicity, Nch, was estimated at mid-rapidity by measuring the number of tracklets, Ntracklets, reconstructed in the SPD. A tracklet is defined as a track segment that joins a pair of space points on the two SPD layers and is aligned with the reconstructed primary vertex. Ntracklets was counted within |η|<1.0. The pseudorapidity acceptance of the SPD depends on the position of the interaction vertex along the beam line zvtx, both due to the asymmetry of the collision system and the limited coverage of the detector. In addition, the overall SPD acceptance varies as a function of time due to a varying number of active channels. A data-driven correction was applied to the Ntracklets distributions on an event-by-event basis to account for these two effects. This was done by renormalising the Ntracklets distributions to the overall minimum with a Poissonian smearing to account for the fluctuations. Multiplicity classes were then defined based on the percentiles of analysed events in each Ntracklets range. – 7 – JHEP08(2016)078 The analysis was repeated without applying the particle identification selections to the D-meson decay hadrons. The corrected yields were consistent, within statistical fluctuations, with those calculated considering particle identification selections. Therefore, no corresponding uncertainty was assigned. The systematic uncertainty due to the subtraction of feed-down D mesons from B decays was estimated by considering the FONLL uncertainties on the normalisation and factorisation scales and using a second subtraction method based on the ratio of FONLL calculations for Dand B-meson cross sections [19]. The magnitude of this systematic uncertainty depends on the meson species and on the pTinterval considered in the measurement, since it is related to the topological selections applied in each analysis. As explained in section 4, a variation of the feed-down D-meson nuclear modification factor was also taken into account as part of the systematics. The quadratic sum of the two contributions to the QpPb was found to range from a few percent up to 30%. The denominator of the QpPb has an uncertainty on the hTpPbi, which is reported in table 1, and an uncertainty on the pp reference. The latter has a contribution coming from the 7 TeV measurement (ranging from 15% up to 25%) and one from the scaling factor ranging from +17% −4% at pT= 1 GeV/c to ±3% for pT>8 GeV/c. The uncertainty on the energy scaling factor was estimated by varying the calculation parameters as described in [74]. A larger uncertainty for D0in 16 < pT<24 GeV/c was quantified due to the extrapolation procedure explained above; in that case the uncertainty is +17.5% −4% . The global QpPb uncertainties were determined by adding the pp and p–Pb uncertainties in quadrature, except for the branching ratio uncertainty, which cancels out in the ratio, and the feed-down contribution, which partially cancels out. 5.2 Results The nuclear modification factors of D0, D+and D∗+mesons were calculated according to eq. (5.1) in four centrality classes (0–20%, 20–40%, 40–60% and 60–100%) defined with the ZNA estimator, and applying the hybrid method to obtain the hTpPbiin each class. Figure 2 illustrates these results for 0–20% and 40–60% centrality classes. The QpPb of the three D-meson species were found to be consistent with one another within the statistical and systematic uncertainties for each pTand centrality class considered. Therefore, the average of the D0, D+and D∗+meson results was evaluated in each centrality class considering the inverse square of the relative statistical uncertainties as weights. The systematic uncertainties on the averages were computed considering the tracking efficiency, the B feed-down subtraction and the scaling of the pp reference as correlated uncertainty sources among the three mesons. The averages of the D0, D+and D∗+pT-differential nuclear modification factors in different centrality classes obtained with the ZNA estimator are presented in figure 3and table 4. The D-meson QpPb results in the different centrality classes are consistent with unity within the uncertainties in the measurement pTinterval. Typical values of the QpPb uncertainties are of 7% (stat.) and 16% (syst.) for 2 < pT<4 GeV/c. It should be noted that with this centrality estimator no bias is expected due to the event selection, and only a small bias in peripheral events, due to the geometrical bias in the determination of the number of hard scatterings, was observed in the studies with charged – 14 – JHEP08(2016)078 ) c (GeV/ T p 0 5 10 15 20 25 30 prompt D pPb Q 0 0.5 1 1.5 2 2.5 0 D + D *+ D ALICE = 5.02 TeV NN s p-Pb, Filled markers : pp rescaled reference -extrapolated reference T pOpen markers: pp 0-20% ZN Energy Class < 0.04 cms y-0.96 < (a) ZNA estimator, 0–20%. ) c (GeV/ T p 0 5 10 15 20 25 30 prompt D pPb Q 0 0.5 1 1.5 2 2.5 0 D + D *+ D ALICE = 5.02 TeV NN s p-Pb, Filled markers : pp rescaled reference -extrapolated reference T pOpen markers: pp 40-60% ZN Energy Class < 0.04 cms y-0.96 < (b) ZNA estimator, 40–60%. Figure 2. D0, D+and D∗+meson nuclear modification factors as a function of pTfor: (a) the 0–20% centrality class and (b) the 40–60% centrality class selected with the ZNA estimator. The vertical error bars and the empty boxes represent the statistical and systematic uncertainties, respectively. The grey-filled box at QpPb = 1 represents the normalisation uncertainty. Symbols are displaced from the bin centre for clarity. particles [66]. Therefore, with the least biased centrality estimator, the D-meson QpPb results are consistent within statistical and systematic uncertainties with binary collision scaling of the yield in pp collisions, independent of the geometry of the collision. 5.2.1 QpPb with CL1 and V0Aestimators As explained in section 3.1, the D0, D+and D∗+QpPb were also calculated with the CL1 and V0A estimators in four centrality classes to study the centrality selection biases based on heavy-flavour production from low to high pT. The QpPb results for the three D-meson species were found to be consistent with one another within the statistical and systematic uncertainties for each pTand centrality class considered. Therefore, the averages of the D0, D+and D∗+meson results and the systematic uncertainties were evaluated as explained before. The averages of the pT-differential D0, D+and D∗+nuclear modification factors in different centrality classes with CL1 and V0A estimators are presented in figure 4(see also tables 5and 6). The centrality estimation from the CL1 multiplicity suffers from a large bias introduced by multiplicity fluctuations in the central rapidity region caused by fluctuations of the number of hard scatterings per nucleon collision, which affect the hTpPbidetermination [66]. The QCL1 pPb results show an ordering from low (60–100%) to high (0–20%) multiplicity, with a difference larger than a factor of two between the most central and most peripheral classes, induced by the bias on the centrality estimator. The V0A estimator classifies the events as a function of the multiplicity in the backward rapidity region. The rapidity gap with respect to the central rapidity D-meson analyses – 15 – JHEP08(2016)078 ) c (GeV/ T p 0 5 10 15 20 25 30 prompt D pPb Q 0 0.5 1 1.5 2 2.5 0-20% 20-40% 40-60% 60-100% ALICE = 5.02 TeV NN s p-Pb, + , D* + , D 0 Average D < 0.04 cms y-0.96 < ZN Energy Classes Figure 3. Average D0, D+and D∗+meson nuclear modification factors as a function of pTin the 0–20%, 20–40%, 40–60% and 60–100% centrality classes selected with the ZNA estimator. The vertical error bars and the empty boxes represent the statistical and systematic uncertainties, respectively. The colour-filled boxes at QpPb = 1 represent the normalisation uncertainties. Symbols are displaced from the bin centre for clarity. ) c (GeV/ T p 0 5 10 15 20 25 30 prompt D pPb Q 0 0.5 1 1.5 2 2.5 0-20% 20-40% 40-60% 60-100% ALICE = 5.02 TeV NN s p-Pb, + , D* + , D 0 Average D < 0.04 cms y-0.96 < CL1 Multiplicity (a) CL1 estimator. ) c (GeV/ T p 0 5 10 15 20 25 30 prompt D pPb Q 0 0.5 1 1.5 2 2.5 0-20% 20-40% 40-60% 60-100% ALICE = 5.02 TeV NN s p-Pb, + , D* + , D 0 Average D < 0.04 cms y-0.96 < V0A Multiplicity (b) V0A estimator. Figure 4. Average D0, D+and D∗+meson nuclear modification factors as a function of pTin the 0–20%, 20–40%, 40–60% and 60–100% centrality classes selected with: (a) the CL1 estimator, and (b) the V0A estimator. The vertical error bars and the empty boxes represent the statistical and systematic uncertainties, respectively. The colour-filled boxes at QpPb = 1 represent the normalisation uncertainties. Symbols are displaced from the bin centre for clarity. removes part of the event selection bias. The QV0A pPb values evolve from higher (>1) to lower (<1) values from the 0–20% to the 60–100% centrality class. The QV0A pPb results present a – 16 – JHEP08(2016)078 Centrality (%) 0 10 20 30 40 50 60 70 80 90 100 pPb Q 0 0.5 1 1.5 2 2.5 ALICE = 5.02 TeV NN s p-Pb, Average D mesons c < 4 GeV/ T p2 < < 0.04 cms y-0.96 < mult coll ZNA N V0A CL1 (a) 2< pT<4 GeV/c. Centrality (%) 0 10 20 30 40 50 60 70 80 90 100 pPb Q 0 0.5 1 1.5 2 2.5 ALICE = 5.02 TeV NN s p-Pb, Average D mesons c < 12 GeV/ T p8 < < 0.04 cms y-0.96 < mult coll ZNA N V0A CL1 Charged particles c > 10 GeV/ T p | < 0.3 η | mult coll ZNA N V0A CL1 (b) 8< pT<12 GeV/c. Figure 5. Average D0, D+and D∗+meson QpPb as a function of centrality with the CL1, the V0A and the ZNA estimators for (a) 2 < pT<4 GeV/c and (b) 8 < pT<12 GeV/c. The average D-meson QpPb in 8 < pT<12 GeV/c is compared with the charged-particle QpPb calculated for pT>10 GeV/c [66]. The vertical error bars and the empty boxes represent, respectively, the statistical and systematic uncertainties on the D-meson results. The filled boxes at QpPb = 1 indicate the correlated systematic uncertainties: the grey-filled box represents the uncertainty on the pp reference and the p–Pb analysis PID and track selection uncertainties, common to all estimators for a given pTinterval; the red-filled box represents the correlated systematic uncertainty on Ncoll determination for the ZNA energy estimator. similar qualitative behaviour to the QCL1 pPb ones, with a smaller difference between centrality classes. This is consistent with the expectation of a smaller bias when there is a rapidity gap between the regions where the centrality and the D-meson yield are studied. 5.2.2 Comparison with charged-particle QpPb The average D-meson QpPb results obtained with the three estimators, for 2 < pT< 4 GeV/c and 8 < pT<12 GeV/c, are displayed as a function of centrality in figure 5. The D-meson QpPb for 8 < pT<12 GeV/c is compared with the analogous measurement for charged hadrons with pT>10 GeV/c [66]. In this transverse momentum region also the production of charged hadrons is expected to scale with the number of binary nucleon-nucleon collisions [66]. The measured trends of charged-particle QpPb at high pT in all the CL1 and V0A centrality classes were found to be reasonably described by an incoherent superposition of Ncoll pp collisions generated with PYTHIA, after defining the event centrality from the charged-particle multiplicity in the rapidity region covered by each estimator in the same way as in data (|η|<1.4 for CL1, 2.8< η < 5.1 for V0A) [66]. The QpPb results for D mesons and charged hadrons with pT>10 GeV/c show a similar trend as a function of centrality and estimator due to the bias in the centrality determination, as observed in [66] based on high-pTparticle production in the light flavour – 17 – JHEP08(2016)078 sector. The results presented in this paper allow these studies to be extended into the charm sector and down to low pT. 6 Relative yields as a function of multiplicity D0, D+and D∗+meson yields were also studied as a function of the charged-particle multiplicity in two pseudorapidity intervals, see section 3.2. The D-meson yields were evaluated for various multiplicity and pTintervals and the results are reported in terms of corrected per-event yields normalised to the multiplicity-integrated values (d2ND/dydpT)j hd2ND/dydpTi= 1 Nj events Nj raw D j prompt D !,1 NMB trigger/MB trigger hNraw Di hprompt Di,(6.1) where the index jidentifies the multiplicity interval, Nj raw D is the raw yield extracted from the fit to the invariant mass distribution in each multiplicity interval, j prompt D represents the reconstruction and selection efficiencies for prompt D mesons, and Nj events is the number of events analysed in each multiplicity interval. The efficiencies were estimated with Monte Carlo simulations (see section 4). Equation (6.1) holds under the assumption that the relative contribution to the raw D-meson yield due to the feed-down from beauty-hadron decays does not depend on the multiplicity of the event, and therefore cancels out in the ratio to the multiplicity-integrated values. This assumption is justified by the beauty production measurements as a function of multiplicity in pp collisions, and also by PYTHIA simulations [31]. The acceptance correction, defined as the fraction of D mesons within a given rapidity and pTinterval that decay into pairs or triplets of particles within the detector coverage, cancels out in this ratio. The number of events used for the normalisation of the multiplicity-integrated yield must be corrected for the fraction of non-single diffractive events that are not accepted by the minimum-bias trigger condition, expressed as NMB trigger/MB trigger with MB trigger = (96.4±3.1)% [67]. It was verified with PYTHIA 6.4.21 Monte Carlo simulations that the minimum-bias trigger is 100% efficient for D mesons in the kinematic range of the measurement, meaning that the number of D mesons in the minimum-bias triggered events is the same as in the sample of non-single diffractive events. 6.1 Systematic uncertainties In this section the systematic uncertainties estimated for the D-meson measurements as a function of Ntracklets and as a function of the NV0A multiplicity are outlined. The most significant source of systematic uncertainty is the one related to the signal extraction procedure. The raw D-meson yields were obtained by fixing the position of the Gaussian signal peak to the world averages of the D-meson masses, and the widths to the values obtained from the fit to the multiplicity integrated invariant mass distributions. To estimate the yield extraction uncertainty the fit parameters were varied as described in section 4. In addition to the variations listed in section 4, the fits were performed also allowing the position and the width of the Gaussian terms to remain free in the individual – 18 – JHEP08(2016)078 multiplicity intervals. The yield extraction uncertainty was estimated based on the stability of the ratio of the raw yields Nj raw D/hNraw Di, where the same raw yield extraction method was used in the multiplicity interval jand for the multiplicity-integrated result. The magnitude of this uncertainty depends on pTand meson species. The contribution of the yield extraction procedure to the systematic uncertainties varied between 4–10%. The influence of D-meson selections, due to the PID and the topological selections, were examined and found to have no significant effect on the final result, since they enter equally into the numerator and denominator of eq. (6.1). As mentioned in section 4, the contribution of feed-down from B decays to the raw yield was estimated based on FONLL calculations [18]. In this case, it was assumed that the fraction of D mesons that are not from feed-down decays, fprompt, remains constant as a function of multiplicity, causing it to cancel out in the numerator and denominator of the ratio in eq. (6.1). The feed-down contribution was therefore not explicitly subtracted from the final result. A systematic uncertainty related to this hypothesis was assigned by assuming that the fraction fj B/hfBi, where fB= 1 −fprompt, increases linearly from 1/2 to 2 from the lowest to the highest multiplicity intervals. The resulting uncertainty depends on multiplicity, pTand meson species, and ranges from +4 −0% to +10 −0% at low multiplicity and from +0 −4% to +0 −20% at high multiplicity. In the analyses as a function of Ntracklets, the relative average values of Nj tracklets/ hNtrackletsifor each interval were corrected to give relative (dNch/dη)j/hdNch/dηivalues, as described in section 3.2. The systematic uncertainty due to this correction was estimated in the simulations based on the resolution and the linearity of the correlation between the number of tracklets, Ntracklets, and the number of generated charged primary particles, Nch. The deviation from linearity was found to contribute by roughly 5% to the uncertainty on the relative multiplicity. Finally, the uncertainty on the measured hdNch/dηiin inelastic p–Pb collisions measured in [68] was considered. This contributed an uncertainty of approximately 4%. The total systematic uncertainty on the relative charged-particle density per Ntracklets interval was found to be 6.3%. In the analyses as a function of NV0A, the measurements are reported as a function of the relative multiplicity NV0AhNV0Ai. The uncertainty on the mean multiplicity values, NV0A, was determined by comparing the mean and median values of the distributions. It was found to be below 5% for each multiplicity interval, and about 30% for the multiplicityintegrated value. 6.2 Results The relative D-meson yields were calculated for each pTand multiplicity interval according to eq. (6.1). The results are reported as a function of the relative charged-particle multiplicity at both backward and central rapidity. It is worth noting that the smaller number of reconstructed D mesons in the lowest and highest pTintervals1limited the number of multiplicity intervals of the measurement for those pTintervals. 1The number of reconstructed D mesons in the lowest and highest pTintervals is smaller than in the other pTintervals. At low pT, the strategy employed to cope with the low signal-to-background ratio was – 19 – JHEP08(2016)078 0 1 2 3 4 5 6 7 8 9 〉 T pdy/dN 2 d〈) / T pdy/dN 2 (d 2 4 6 8 10 12 14 16 |<0.5 lab y, |c<4 GeV/ T p2< meson 0 D meson + D meson + D* ALICE = 5.02 TeV NN s p-Pb not shown〉η/dNd〈) / η/dN 6.3% unc. on (d± 3.1% normalisation unc. not shown± 〉η/d ch Nd〈) / η/d ch N (d 0 1 2 3 4 5 6 7 8 9 B feed-down unc. 0.4− 0.2− 0 0.2 0.4 1/2 (2) at low (high) multiplicity×B fraction hypothesis: (a) D mesons with 2 < pT<4 GeV/c. 0 1 2 3 4 5 6 7 8 9 〉 T pdy/dN 2 d〈) / T pdy/dN 2 (d 2 4 6 8 10 12 14 16 |<0.5 lab y, |c<8 GeV/ T p4< meson 0 D meson + D meson + D* ALICE = 5.02 TeV NN s p-Pb not shown〉η/dNd〈) / η/dN 6.3% unc. on (d± 3.1% normalisation unc. not shown± 〉η/d ch Nd〈) / η/d ch N (d 0 1 2 3 4 5 6 7 8 9 B feed-down unc. 0.4− 0.2− 0 0.2 0.4 1/2 (2) at low (high) multiplicity×B fraction hypothesis: (b) D mesons with 4 < pT<8 GeV/c. Figure 6. Relative D0, D+and D∗+meson yields for two selected pTintervals as a function of charged-particle multiplicity at central rapidity. The relative yields are presented in the top panels with their statistical (vertical bars) and systematic (empty boxes) uncertainties, apart from the feed-down fraction uncertainty, which is drawn separately in the bottom panels. The position of the points on the abscissa is the average value of (dNch/dη)hdNch/dηi. For D+and D∗+mesons the points are shifted horizontally by 1.5% to improve the visibility. The diagonal (dashed) line is also shown to guide the eye. The relative D0, D+and D∗+yields were measured in five pTintervals from 1 to 24 GeV/c as a function of the charged-particle multiplicity at mid-rapidity. Figure 6 presents the measurements for selected pTintervals with their statistical (vertical bars) and systematic (boxes) uncertainties, apart from the feed-down fraction uncertainty, which is drawn separately in the bottom panels. The position of points on the abscissa is the average value of (dNch/dη)hdNch/dηi, but for some meson species they are shifted horizontally by 1.5% to improve the visibility. The relative yields of the three D-meson species are consistent with one another in all pTintervals within uncertainties. The average of the relative D0, D+and D∗+yields was evaluated considering the inverse square of their relative statistical uncertainties as weights. The yield extraction uncertainties were treated as uncorrelated systematic uncertainties, while the feed-down subtraction uncertainties were considered as correlated uncertainty sources. Figure 7a presents the average D-meson yields for each pTinterval. The results are reported in table 7. The pT evolution of the yields was examined using the results in the 2 < pT<4 GeV/c interval as reference and by computing the ratio between the average relative D-meson yields in the various pTintervals and those in 2 < pT<4 GeV/c. The results are shown in figure 7b. to apply tight topological selections, decreasing the selection efficiency and consequently the number of reconstructed D mesons. At high pT, the small number of candidates is the consequence of the steeply falling D-meson pTspectra. – 20 – JHEP08(2016)078 0 1 2 3 4 5 6 7 8 9 〉 T pdy/dN 2 d〈) / T pdy/dN 2 (d 2 4 6 8 10 12 14 16 18 c < 2 GeV/ T p 1 < c < 4 GeV/ T p 2 < c < 8 GeV/ T p 4 < c < 12 GeV/ T p 8 < c < 24 GeV/ T p 12 < = 5.02 TeV NN sALICE, p-Pb |<0.5 lab y meson, | + , D* + ,D 0 Average D not shown〉η/dNd〈) / η/dN 6.3% unc. on (d± 3.1% normalisation unc. not shown± 〉η/d ch Nd〈) / η/d ch N (d 0 1 2 3 4 5 6 7 8 9 B feed-down unc. 0.4− 0.2− 0 0.2 0.4 1/2 (2) at low (high) multiplicity×B fraction hypothesis: (a) pTdependence. 0 1 2 3 4 5 6 7 8 9 c<4 GeV/ T p) in 2< T pdy/dN 2 Ratio to (d 0.5 1 1.5 2 2.5 3 c < 2 GeV/ T p 1 < c < 8 GeV/ T p 4 < c < 12 GeV/ T p 8 < c < 24 GeV/ T p 12 < = 5.02 TeV NN sALICE, p-Pb |<0.5 lab y meson, | + , D* + ,D 0 Average D not shown〉η/dNd〈) / η/dN 6.3% unc. on (d± 〉η/d ch Nd〈) / η/d ch N (d 0 1 2 3 4 5 6 7 8 9 B feed-down unc. 0.4− 0.2− 0 0.2 0.4 1/2 (2) at low (high) multiplicity×B fraction hypothesis: (b) Ratios of pTintervals vs. 2 < pT<4 GeV/c. Figure 7. Average of relative D0, D+and D∗+yields as a function of the relative charged-particle multiplicity at central rapidity. (a) Average of relative D-meson yields in pTintervals. (b) Ratio of the average relative yields in all pTintervals with respect to that of the 2 < pT<4 GeV/c interval. The results are presented in the top panels with their statistical (vertical bars) and systematic (boxes) uncertainties, apart from the feed-down fraction uncertainty, which is drawn separately in the bottom panels. The position of the points on the abscissa is the average value of (dNch/dη)hdNch/dηi. For some pTintervals the points are shifted horizontally by 1.5% to improve the visibility. The dashed lines are also shown to guide the eye, a diagonal on (a) and a constant on (b). The yield increase is independent of transverse momentum within the uncertainties of the measurement. The D-meson yields show a faster-than-linear increase with the chargedparticle multiplicity at central rapidity. The yield increase is approximately a factor of 7 for multiplicities of 4.2 times hdNch/dηi. These results are compared with the equivalent measurements in pp collisions, as well as with model calculations, in section 6.2.1. The measurement of the relative D0, D+and D∗+yields was also performed as a function of the relative charged-particle multiplicity at large rapidity in the Pb-going direction, thus introducing an ηgap between the regions where the D mesons and the multiplicity are measured. The charge collected by the V0A detector, NV0A, was considered as a multiplicity estimator (see section 3.2). Simulations have shown that the collected charge is proportional to the charged-particle multiplicity in the measured ηrange, 2.8< η < 5.1. The relative D-meson yields measured in pTand NV0A intervals are reported as a function of the relative multiplicity in the V0A detector, NV0AhNV0Ai. The D0, D+and D∗+ yields are consistent with one another in all the measurement intervals, within uncertainties. The average D-meson yield was calculated with the same procedure used for the results as a function of charged-particle multiplicity at mid-rapidity. Figure 8and table 8 summarise these measurements. The results are independent of transverse momentum within the uncertainties of the measurement. The charmed-meson yield increases with the – 21 – JHEP08(2016)078 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 〉 T pdy/dN 2 d〈) / T pdy/dN 2 (d 1 2 3 4 5 6 7 c < 2 GeV/ T p 1 < c < 4 GeV/ T p 2 < c < 8 GeV/ T p 4 < c < 12 GeV/ T p 8 < c < 24 GeV/ T p 12 < = 5.02 TeV NN sALICE, p-Pb |<0.5 lab y meson, | + , D* + ,D 0 Average D not shown〉 V0A N〈 / V0A N 5.0% unc. on ± 3.1% normalisation unc. not shown± 〉 V0A N〈 / V0A N 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 B feed-down unc. 0.4− 0.2− 0 0.2 0.4 1/2 (2) at low (high) multiplicity×B fraction hypothesis: (a) pTdependence. 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 c<4 GeV/ T p) in 2< T pdy/dN 2 Ratio to (d 0.5 1 1.5 2 2.5 3 c < 2 GeV/ T p 1 < c < 8 GeV/ T p 4 < c < 12 GeV/ T p 8 < c < 24 GeV/ T p 12 < = 5.02 TeV NN sALICE, p-Pb |<0.5 lab y meson, | + , D* + ,D 0 Average D not shown〉 V0A N〈 / V0A N 5.0% unc. on ± 〉 V0A N〈 / V0A N 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 B feed-down unc. 0.4− 0.2− 0 0.2 0.4 1/2 (2) at low (high) multiplicity×B fraction hypothesis: (b) Ratios of pTintervals vs. 2 < pT<4 GeV/c. Figure 8. Average of relative D0, D+and D∗+yields as a function of the relative V0A multiplicity, NV0A, measured at 2.8< η < 5.1. The relative yields are presented in the top panels with their statistical (vertical bars) and systematic (boxes) uncertainties, apart from the uncertainty on the B feed-down fraction, which is drawn separately in the bottom panels. The position of the points on the abscissa is the average value of NV0AhNV0Ai. For some pTintervals the points are shifted horizontally by 1.5% to improve the visibility. The dashed lines are also shown to guide the eye, a diagonal on (a) and a constant on (b). multiplicity at backward rapidity. The yield increase is consistent with a linear growth as a function of multiplicity. The results as a function of V0A multiplicity indicate that the per-event D-meson yield increases as a function of multiplicity, regardless of the ηrange in which the multiplicity is measured. This remains the case even when the charged-particle yield is measured in a different ηinterval from the D mesons, which originate from the fragmentation of charm quarks produced in hard partonic scattering processes. One notable effect to consider when comparing the trends of D-meson production as a function of multiplicity at central and large rapidity is that the charged-particle multiplicity was observed to scale differently with the number of nucleons involved in the p–A interaction depending on η[66,75]. In particular, at central rapidity the charged-particle multiplicity is found to scale with the number of participant nucleons, Npart, while at large rapidities in the Pb-going direction (i.e. in the V0A acceptance) it scales with the number of participants of the Pb nucleus, which is equal to Npart −1 = Ncoll in p–Pb collisions. It was verified that the results of the D-meson yields as a function of multiplicity are consistent with those of the QpPb analysis (see section 5). In the QpPb analysis, D-meson production is studied by dividing the events into centrality classes equally populated by 20% of the events, whereas in this section we examine events with extremely high multiplicity (see tables 2and 3). Events with low (high) multiplicity correspond to interactions with a smaller (larger) number of hard scatterings per nucleon-nucleon collision, as well as to – 22 – JHEP08(2016)078 negative (positive) multiplicity fluctuations which affect event classification and influence both measurements. 6.2.1 Comparison of p–Pb data with pp results and models The relative D-meson yield (average of D0, D+and D∗+) as a function of charged-particle multiplicity at central rapidity in p–Pb collisions at √sNN = 5.02 TeV is compared with the corresponding pp measurements at √s= 7 TeV for 2 < pT<4 GeV/c in figure 9a. A similar relative increase of charmed-meson yield with charged-particle multiplicity is observed in pp and p–Pb collisions. Note that the multiplicity is measured for both pp and p–Pb collisions in the same pseudorapidity range in the laboratory system, which corresponds to different ranges in the centre-of-mass frame for the two collision systems, due to the asymmetry of the beam energies in the p–Pb case. The increasing yield in pp data can be described by calculations taking into account the contribution of Multiple-Parton Interactions (MPI) [23–25], by the influence of the interactions between colour sources in the percolation model [33,34], or by the effect of the initial conditions of the collision followed by a hydrodynamic evolution computed with the EPOS 3 event generator [35,36] where the individual scatterings are identified with parton ladders. In p–Pb collisions, the multiplicity dependence of heavy-flavour production is also affected by the presence of multiple binary nucleon-nucleon interactions, and the initial conditions of the collision are modified due to CNM effects. Charmed-meson yields in pp and p–Pb collisions as a function of the relative multiplicity at large rapidity are compared in figure 9b for 2 < pT<4 GeV/c. The multiplicity in p–Pb collisions is measured in 2.8< η < 5.1 in the Pb-going direction, whereas in pp data the multiplicities at backward (2.8< η < 5.1) and forward (−3.7< η < −1.7) pseudorapidity were summed together. The D-meson yields increase faster in pp than p–Pb collisions as a function of the relative multiplicity at backward rapidity. The different pseudorapidity intervals of the multiplicity measurement may contribute to this observation. In addition, measurements in p–Pb collisions differ from those in pp interactions because the initial conditions of the collision are affected by the presence of the Pb nucleus, and because there are multiple binary nucleon-nucleon interactions per p–Pb collision. Figures 10 and 11 present comparisons of the D-meson results and EPOS 3.116 model estimates. The EPOS 3 event generator [35,36] imposes the same theoretical framework for various colliding systems: pp, p–A and A–A. The initial conditions are generated using the “Parton-based Gribov-Regge” formalism [35] of multiple scatterings. Each individual scattering is identified with a parton ladder, composed of a pQCD hard process with initialand final-state radiation. The non-linear effects of parton evolution are treated introducing a saturation scale below which those effects become important. With these initial conditions, a 3D+1 viscous hydrodynamical evolution is applied to the core of the collision [36]. The measurements agree with the EPOS 3 model calculations within uncertainties. The results at high multiplicity are better reproduced by the calculation including a viscous hydrodynamical evolution of the collision, which predicts a faster-than-linear increase of the charmed-meson yield with multiplicity at central rapidity. The same calculation evaluates an approximately linear increase of the charmed-meson yield with the multiplicity – 23 – JHEP08(2016)078 (dNch/dη)hdNch/dηi 0.56 ±0.04 1.35 ±0.09 1.72 ±0.11 2.11 ±0.13 2.85 ±0.18 4.27 ±0.27 pT(GeV/c) d2N/dydpThd2N/dydpTi 1–2 0.42 ±0.04 ±0.03+0.03 −0.00 1.46 ±0.18 ±0.08+0.07 −0.04 2.10 ±0.30 ±0.13+0.09 −0.11 2.90 ±0.44 ±0.24+0.11 −0.33 3.65 ±0.60 ±0.30+0.00 −0.70 — 2–4 0.43 ±0.01 ±0.01+0.03 −0.00 1.33 ±0.05 ±0.04+0.07 −0.03 2.05 ±0.08 ±0.06+0.08 −0.11 2.40 ±0.09 ±0.07+0.06 −0.19 3.80 ±0.13 ±0.11+0.00 −0.51 7.16 ±0.84 ±0.35+0.00 −1.02 4–8 0.42 ±0.01 ±0.01+0.03 −0.00 1.41 ±0.04 ±0.03+0.07 −0.04 1.93 ±0.06 ±0.04+0.08 −0.10 2.36 ±0.08 ±0.05+0.06 −0.18 3.86 ±0.11 ±0.08+0.00 −0.51 5.30 ±0.71 ±0.25+0.00 −0.79 8–12 0.41 ±0.02 ±0.01+0.03 −0.00 1.45 ±0.08 ±0.05+0.09 −0.05 2.01 ±0.13 ±0.07+0.10 −0.13 2.23 ±0.15 ±0.08+0.07 −0.22 3.67 ±0.21 ±0.12+0.00 −0.63 8.42 ±1.38 ±0.45+0.00 −1.49 12–24 0.40 ±0.04 ±0.02+0.03 −0.00 1.39 ±0.15 ±0.08+0.09 −0.04 1.77 ±0.26 ±0.13+0.09 −0.12 3.37 ±0.30 ±0.22+0.11 −0.32 3.53 ±0.37 ±0.21+0.00 −0.55 — Table 7. Average of relative D0, D+and D∗+meson yields for the sum of particles and antiparticles in several multiplicity and pTintervals for p–Pb collisions at √sNN = 5.02 TeV as a function of the relative charged-particle multiplicity at central rapidity. The values are reported together with their uncertainties, which are quoted in the order: statistical, systematic and feed-down contribution uncertainties. The yields reported here are per non-single diffractive event. The global normalisation uncertainty of 3.1% is not shown. – 30 – JHEP08(2016)078 NV0AhNV0Ai 0.48 ±0.02 1.32 ±0.07 1.81 ±0.09 2.36 ±0.12 3.29 ±0.16 (2.72 ±0.14) pT(GeV/c) d2N/dydpThd2N/dydpTi 1–2 0.55 ±0.05 ±0.02+0.04 −0.00 1.57 ±0.19 ±0.06+0.09 −0.05 1.92 ±0.28 ±0.08+0.09 −0.12 (2.69 ±0.39 ±0.14+0.10 −0.31) 2–4 0.52 ±0.01 ±0.01+0.03 −0.00 1.47 ±0.05 ±0.04+0.08 −0.04 1.91 ±0.07 ±0.05+0.08 −0.10 2.50 ±0.11 ±0.07+0.07 −0.20 3.25 ±0.17 ±0.09+0.00 −0.45 4–8 0.51 ±0.01 ±0.01+0.03 −0.00 1.59 ±0.04 ±0.03+0.08 −0.04 1.93 ±0.06 ±0.04+0.08 −0.10 2.43 ±0.09 ±0.05+0.06 −0.19 3.02 ±0.14 ±0.07+0.00 −0.42 8–12 0.55 ±0.02 ±0.02+0.04 −0.00 1.60 ±0.08 ±0.05+0.10 −0.05 1.84 ±0.12 ±0.06+0.09 −0.13 2.62 ±0.17 ±0.08+0.09 −0.27 3.03 ±0.27 ±0.10+0.00 −0.53 12–24 0.56 ±0.04 ±0.03+0.04 −0.00 1.53 ±0.15 ±0.07+0.09 −0.05 2.22 ±0.22 ±0.11+0.11 −0.14 (2.27 ±0.25 ±0.13+0.08 −0.24) Table 8. Average of relative D0, D+and D∗+meson yields for the sum of particles and antiparticles in several multiplicity and pTintervals for p–Pb collisions at √sNN = 5.02 TeV as a function of the relative average multiplicity in the VZERO detector, NV0AhNV0Ai. The uncertainties are shown in the following order: statistical, systematic, and feed-down contribution uncertainties. The global normalisation uncertainty of 3.1% is not shown. The yields reported here are normalised to the non-single diffractive cross section. For 1 < pT<2 GeV/c and 12 < pT<24 GeV/c, the final two multiplicity intervals are merged, and the results are shown in parentheses. – 31 – JHEP08(2016)078 Open Access. This article is distributed under the terms of the Creative Commons Attribution License (CC-BY 4.0), which permits any use, distribution and reproduction in any medium, provided the original author(s) and source are credited. References [1] A. Andronic et al., Heavy-flavour and quarkonium production in the LHC era: from proton-proton to heavy-ion collisions,Eur. Phys. J. C 76 (2016) 107 [arXiv:1506.03981] [INSPIRE]. [2] B.A. Kniehl, G. Kramer, I. Schienbein and H. Spiesberger, Open charm hadroproduction and the charm content of the proton,Phys. Rev. D 79 (2009) 094009 [arXiv:0901.4130] [INSPIRE]. [3] T. Stavreva et al., Probing gluon and heavy-quark nuclear PDFs with γ+Qproduction in pA collisions,JHEP 01 (2011) 152 [arXiv:1012.1178] [INSPIRE]. [4] ALICE collaboration, Suppression of high transverse momentum D mesons in central Pb-Pb collisions at √sNN = 2.76 TeV,JHEP 09 (2012) 112 [arXiv:1203.2160] [INSPIRE]. [5] ALICE collaboration, Production of muons from heavy flavour decays at forward rapidity in pp and Pb-Pb collisions at √sNN = 2.76 TeV,Phys. Rev. Lett. 109 (2012) 112301 [arXiv:1205.6443] [INSPIRE]. [6] CDF collaboration, D. Acosta et al., Measurement of the J/ψ meson and b-hadron production cross sections in p¯pcollisions at √s= 1960 GeV,Phys. Rev. D 71 (2005) 032001 [hep-ex/0412071] [INSPIRE]. [7] M. Cacciari, S. Frixione, M.L. Mangano, P. Nason and G. Ridolfi, QCD analysis of first b cross-section data at 1.96 TeV,JHEP 07 (2004) 033 [hep-ph/0312132] [INSPIRE]. [8] B.A. Kniehl, G. Kramer, I. Schienbein and H. Spiesberger, Finite-mass effects on inclusive B meson hadroproduction,Phys. Rev. D 77 (2008) 014011 [arXiv:0705.4392] [INSPIRE]. [9] ALICE collaboration, Measurement of prompt J/ψ and beauty hadron production cross sections at mid-rapidity in pp collisions at √s= 7 TeV,JHEP 11 (2012) 065 [arXiv:1205.5880] [INSPIRE]. [10] ALICE collaboration, Measurement of electrons from beauty hadron decays in pp collisions at √s= 7 TeV,Phys. Lett. B 721 (2013) 13 [arXiv:1208.1902] [INSPIRE]. [11] ATLAS collaboration, Measurement of the differential cross-section of B+meson production in pp collisions at √s= 7 TeV at ATLAS,JHEP 10 (2013) 042 [arXiv:1307.0126] [INSPIRE]. [12] CMS collaboration, Prompt and non-prompt J/ψ production in pp collisions at √s= 7 TeV, Eur. Phys. J. C 71 (2011) 1575 [arXiv:1011.4193] [INSPIRE]. [13] LHCb collaboration, Measurement of σ(pp →b¯ bX)at √s= 7 TeV in the forward region, Phys. Lett. B 694 (2010) 209 [arXiv:1009.2731] [INSPIRE]. [14] B.A. Kniehl, G. Kramer, I. Schienbein and H. Spiesberger, Inclusive D∗± production in p¯p collisions with massive charm quarks,Phys. Rev. D 71 (2005) 014018 [hep-ph/0410289] [INSPIRE]. [15] B.A. Kniehl, G. Kramer, I. Schienbein and H. Spiesberger, Collinear subtractions in hadroproduction of heavy quarks,Eur. Phys. J. C 41 (2005) 199 [hep-ph/0502194] [INSPIRE]. – 32 – JHEP08(2016)078 [16] M. Cacciari, M. Greco and P. Nason, The pTspectrum in heavy flavor hadroproduction, JHEP 05 (1998) 007 [hep-ph/9803400] [INSPIRE]. [17] M. Cacciari, S. Frixione and P. Nason, The pTspectrum in heavy flavor photoproduction, JHEP 03 (2001) 006 [hep-ph/0102134] [INSPIRE]. [18] M. Cacciari, S. Frixione, N. Houdeau, M.L. Mangano, P. Nason and G. Ridolfi, Theoretical predictions for charm and bottom production at the LHC,JHEP 10 (2012) 137 [arXiv:1205.6344] [INSPIRE]. [19] ALICE collaboration, Measurement of charm production at central rapidity in proton-proton collisions at √s= 7 TeV,JHEP 01 (2012) 128 [arXiv:1111.1553] [INSPIRE]. [20] ALICE collaboration, Measurement of charm production at central rapidity in proton-proton collisions at √s= 2.76 TeV,JHEP 07 (2012) 191 [arXiv:1205.4007] [INSPIRE]. [21] LHCb collaboration, Prompt charm production in pp collisions at √s= 7 TeV,Nucl. Phys. B 871 (2013) 1 [arXiv:1302.2864] [INSPIRE]. [22] R. Maciula and A. Szczurek, Open charm production at the LHC: kt-factorization approach, Phys. Rev. D 87 (2013) 094022 [arXiv:1301.3033] [INSPIRE]. [23] P. Bartalini and L. Fano, Proceedings of the First International Workshop on Multiple Partonic Interactions at the LHC (MPI08), P. Bartalini and L. Fano eds., DESY, Hamburg Germany (2009) [arXiv:1003.4220] [INSPIRE]. [24] T. Sj¨ostrand and M. van Zijl, A Multiple Interaction Model for the Event Structure in Hadron Collisions,Phys. Rev. D 36 (1987) 2019 [INSPIRE]. [25] S. Porteboeuf and R. Granier de Cassagnac, J/Ψyield vs. multiplicity in proton-proton collisions at the LHC,Nucl. Phys. Proc. Suppl. 214 (2011) 181 [arXiv:1012.0719] [INSPIRE]. [26] L. Frankfurt, M. Strikman and C. Weiss, Transverse nucleon structure and diagnostics of hard parton-parton processes at LHC,Phys. Rev. D 83 (2011) 054012 [arXiv:1009.2559] [INSPIRE]. [27] M.Y. Azarkin, I.M. Dremin and M. Strikman, Jets in multiparticle production in and beyond geometry of proton-proton collisions at the LHC,Phys. Lett. B 735 (2014) 244 [arXiv:1401.1973] [INSPIRE]. [28] M. Strikman, Transverse structure of the nucleon and multiparton interactions,Prog. Theor. Phys. Suppl. 187 (2011) 289 [INSPIRE]. [29] M. Strikman, Comments on the observation of high multiplicity events at the LHC,Phys. Rev. D 84 (2011) 011501 [arXiv:1105.2285] [INSPIRE]. [30] ALICE collaboration, J/ψ Production as a Function of Charged Particle Multiplicity in pp Collisions at √s= 7 TeV,Phys. Lett. B 712 (2012) 165 [arXiv:1202.2816] [INSPIRE]. [31] ALICE collaboration, Measurement of charm and beauty production at central rapidity versus charged-particle multiplicity in proton-proton collisions at √s= 7 TeV,JHEP 09 (2015) 148 [arXiv:1505.00664] [INSPIRE]. [32] CMS collaboration, Event activity dependence of Y(nS)production in √sNN = 5.02 TeV pPb and √s= 2.76 TeV pp collisions,JHEP 04 (2014) 103 [arXiv:1312.6300] [INSPIRE]. [33] E.G. Ferreiro and C. Pajares, High multiplicity pp events and J/ψ production at LHC,Phys. Rev. C 86 (2012) 034903 [arXiv:1203.5936] [INSPIRE]. – 33 – JHEP08(2016)078 [34] E.G. Ferreiro and C. Pajares, Open charm production in high multiplicity proton-proton events at the LHC,arXiv:1501.03381 [INSPIRE]. [35] H.J. Drescher, M. Hladik, S. Ostapchenko, T. Pierog and K. Werner, Parton based Gribov-Regge theory,Phys. Rept. 350 (2001) 93 [hep-ph/0007198] [INSPIRE]. [36] K. Werner, B. Guiot, I. Karpenko and T. Pierog, Analysing radial flow features in p-Pb and p-p collisions at several TeV by studying identified particle production in EPOS3,Phys. Rev. C 89 (2014) 064903 [arXiv:1312.1233] [INSPIRE]. [37] T. Sj¨ostrand, S. Mrenna and P.Z. Skands, A Brief Introduction to PYTHIA 8.1,Comput. Phys. Commun. 178 (2008) 852 [arXiv:0710.3820] [INSPIRE]. [38] K.J. Eskola, H. Paukkunen and C.A. Salgado, EPS09: A New Generation of NLO and LO Nuclear Parton Distribution Functions,JHEP 04 (2009) 065 [arXiv:0902.4154] [INSPIRE]. [39] D. de Florian and R. Sassot, Nuclear parton distributions at next-to-leading order,Phys. Rev. D 69 (2004) 074028 [hep-ph/0311227] [INSPIRE]. [40] M. Hirai, S. Kumano and T.-H. Nagai, Determination of nuclear parton distribution functions and their uncertainties in next-to-leading order,Phys. Rev. C 76 (2007) 065207 [arXiv:0709.3038] [INSPIRE]. [41] H. Fujii and K. Watanabe, Heavy quark pair production in high energy pA collisions: Open heavy flavors,Nucl. Phys. A 920 (2013) 78 [arXiv:1308.1258] [INSPIRE]. [42] P. Tribedy and R. Venugopalan, QCD saturation at the LHC: Comparisons of models to p+pand A+Adata and predictions for p+P b collisions,Phys. Lett. B 710 (2012) 125 [Erratum ibid. B 718 (2013) 1154] [arXiv:1112.2445] [INSPIRE]. [43] J.L. Albacete, A. Dumitru, H. Fujii and Y. Nara, CGC predictions for p+Pb collisions at the LHC,Nucl. Phys. A 897 (2013) 1 [arXiv:1209.2001] [INSPIRE]. [44] A.H. Rezaeian, CGC predictions for p+Acollisions at the LHC and signature of QCD saturation,Phys. Lett. B 718 (2013) 1058 [arXiv:1210.2385] [INSPIRE]. [45] M. Lev and B. Petersson, Nuclear Effects at Large Transverse Momentum in a QCD Parton Model,Z. Phys. C 21 (1983) 155 [INSPIRE]. [46] X.-N. Wang, Systematic study of high pThadron spectra in pp,pA and AA collisions at ultrarelativistic energies,Phys. Rev. C 61 (2000) 064910 [nucl-th/9812021] [INSPIRE]. [47] B.Z. Kopeliovich, J. Nemchik, A. Schafer and A.V. Tarasov, Cronin effect in hadron production off nuclei,Phys. Rev. Lett. 88 (2002) 232303 [hep-ph/0201010] [INSPIRE]. [48] I. Vitev, Non-Abelian energy loss in cold nuclear matter,Phys. Rev. C 75 (2007) 064906 [hep-ph/0703002] [INSPIRE]. [49] F. Arleo, S. Peigne and T. Sami, Revisiting scaling properties of medium-induced gluon radiation,Phys. Rev. D 83 (2011) 114036 [arXiv:1006.0818] [INSPIRE]. [50] CMS collaboration, Observation of long-range near-side angular correlations in proton-lead collisions at the LHC,Phys. Lett. B 718 (2013) 795 [arXiv:1210.5482] [INSPIRE]. [51] ALICE collaboration, Long-range angular correlations on the near and away side in p-Pb collisions at √sNN = 5.02 TeV,Phys. Lett. B 719 (2013) 29 [arXiv:1212.2001] [INSPIRE]. [52] ALICE collaboration, Long-range angular correlations of π,Kand pin p-Pb collisions at √sNN = 5.02 TeV,Phys. Lett. B 726 (2013) 164 [arXiv:1307.3237] [INSPIRE]. – 34 – JHEP08(2016)078 [53] ATLAS collaboration, Observation of Associated Near-Side and Away-Side Long-Range Correlations in √sNN = 5.02 TeV Proton-Lead Collisions with the ATLAS Detector,Phys. Rev. Lett. 110 (2013) 182302 [arXiv:1212.5198] [INSPIRE]. [54] ALICE collaboration, Suppression of ψ(2S)production in p-Pb collisions at √sNN = 5.02 TeV,JHEP 12 (2014) 073 [arXiv:1405.3796] [INSPIRE]. [55] ALICE collaboration, Measurement of prompt D-meson production in p-Pb collisions at √sNN = 5.02 TeV,Phys. Rev. Lett. 113 (2014) 232301 [arXiv:1405.3452] [INSPIRE]. [56] I. Helenius, Spatially dependent parton distribution functions and hard processes in nuclear collisions,arXiv:1408.6660 [INSPIRE]. [57] I. Helenius, K.J. Eskola, H. Honkanen and C.A. Salgado, Impact-Parameter Dependent Nuclear Parton Distribution Functions: EPS09s and EKS98s and Their Applications in Nuclear Hard Processes,JHEP 07 (2012) 073 [arXiv:1205.5359] [INSPIRE]. [58] V. Emel’yanov, A. Khodinov, S.R. Klein and R. Vogt, Spatial variation of nuclear structure functions and heavy quark production,Phys. Rev. Lett. 81 (1998) 1801 [nucl-th/9805027] [INSPIRE]. [59] V. Emel’yanov, A. Khodinov, S.R. Klein and R. Vogt, The Effect of shadowing on initial conditions, transverse energy and hard probes in ultrarelativistic heavy ion collisions,Phys. Rev. C 61 (2000) 044904 [hep-ph/9909427] [INSPIRE]. [60] L. Frankfurt, V. Guzey and M. Strikman, Leading Twist Nuclear Shadowing Phenomena in Hard Processes with Nuclei,Phys. Rept. 512 (2012) 255 [arXiv:1106.2091] [INSPIRE]. [61] ALICE collaboration, The ALICE experiment at the CERN LHC,2008 JINST 3S08002 [INSPIRE]. [62] ALICE collaboration, Performance of the ALICE Experiment at the CERN LHC,Int. J. Mod. Phys. A 29 (2014) 1430044 [arXiv:1402.4476] [INSPIRE]. [63] ALICE collaboration, Alignment of the ALICE Inner Tracking System with cosmic-ray tracks,2010 JINST 5P03003 [arXiv:1001.0502] [INSPIRE]. [64] J. Alme et al., The ALICE TPC, a large 3-dimensional tracking device with fast readout for ultra-high multiplicity events,Nucl. Instrum. Meth. A 622 (2010) 316 [arXiv:1001.1950] [INSPIRE]. [65] ALICE collaboration, Performance of the ALICE VZERO system,2013 JINST 8P10016 [arXiv:1306.3130] [INSPIRE]. [66] ALICE collaboration, Centrality dependence of particle production in p-Pb collisions at √sNN = 5.02 TeV,Phys. Rev. C 91 (2015) 064905 [arXiv:1412.6828] [INSPIRE]. [67] ALICE collaboration, Measurement of visible cross sections in proton-lead collisions at √sNN = 5.02 TeV in van der Meer scans with the ALICE detector,2014 JINST 9P11003 [arXiv:1405.1849] [INSPIRE]. [68] ALICE collaboration, Pseudorapidity density of charged particles in p+P b collisions at √sNN = 5.02 TeV,Phys. Rev. Lett. 110 (2013) 032301 [arXiv:1210.3615] [INSPIRE]. [69] Particle Data Group collaboration, K.A. Olive et al., Review of Particle Physics,Chin. Phys. C 38 (2014) 090001 [INSPIRE]. [70] T. Sj¨ostrand, S. Mrenna and P.Z. Skands, PYTHIA 6.4 Physics and Manual,JHEP 05 (2006) 026 [hep-ph/0603175] [INSPIRE]. – 35 – JHEP08(2016)078 [71] P.Z. Skands, The Perugia Tunes, in proceedings of the 1st International Workshop on Multiple Partonic Interactions at the LHC (MPI@LHC 08), Perugia, Italy, October 27–31 2008, p. 284 [arXiv:0905.3418] [INSPIRE]. [72] X.-N. Wang and M. Gyulassy, HIJING: A Monte Carlo model for multiple jet production in pp,pA and AA collisions,Phys. Rev. D 44 (1991) 3501 [INSPIRE]. [73] D.J. Lange, The EvtGen particle decay simulation package,Nucl. Instrum. Meth. A 462 (2001) 152 [INSPIRE]. [74] R. Averbeck, N. Bastid, Z. Conesa del Valle, P. Crochet, A. Dainese and X. Zhang, Reference Heavy Flavour Cross Sections in pp Collisions at √s= 2.76 TeV, using a pQCD-Driven √s-Scaling of ALICE Measurements at √s= 7 TeV,arXiv:1107.3243 [INSPIRE]. [75] PHENIX collaboration, A. Adare et al., Centrality categorization for Rp(d)+Ain high-energy collisions,Phys. Rev. C 90 (2014) 034902 [arXiv:1310.4793] [INSPIRE]. – 36 – JHEP08(2016)078 The ALICE collaboration J. Adam40, D. Adamov´a84, M.M. Aggarwal88, G. Aglieri Rinella36, M. Agnello110, N. Agrawal48, Z. Ahammed132, S.U. Ahn68, S. Aiola136, A. Akindinov58, S.N. Alam132, D. Aleksandrov80, B. Alessandro110, D. Alexandre101, R. Alfaro Molina64, A. Alici104,12, A. Alkin3, J.R.M. Almaraz119, J. Alme38, T. Alt43, S. Altinpinar18, I. Altsybeev131, C. Alves Garcia Prado120, C. Andrei78, A. Andronic97, V. Anguelov94, J. Anielski54, T. Antiˇci´c98, F. Antinori107, P. Antonioli104, L. Aphecetche113, H. Appelsh¨auser53, S. Arcelli28, R. Arnaldi110, O.W. Arnold37,93, I.C. Arsene22, M. Arslandok53, B. Audurier113, A. Augustinus36, R. Averbeck97, M.D. Azmi19, A. Badal`a106, Y.W. Baek67, S. Bagnasco110, R. Bailhache53, R. Bala91, S. Balasubramanian136, A. Baldisseri15, R.C. Baral61, A.M. Barbano27, R. Barbera29, F. Barile33, G.G. Barnaf¨oldi135, L.S. Barnby101, V. Barret70, P. Bartalini7, K. Barth36, J. Bartke117, E. Bartsch53, M. Basile28, N. Bastid70, S. Basu132, B. Bathen54, G. Batigne113, A. Batista Camejo70, B. Batyunya66, P.C. Batzing22, I.G. Bearden81, H. Beck53, C. Bedda110, N.K. Behera50, I. Belikov55, F. Bellini28, H. Bello Martinez2, R. Bellwied122, R. Belmont134, E. Belmont-Moreno64, V. Belyaev75, P. Benacek84, G. Bencedi135, S. Beole27, I. Berceanu78, A. Bercuci78, Y. Berdnikov86, D. Berenyi135, R.A. Bertens57, D. Berzano36, L. Betev36, A. Bhasin91, I.R. Bhat91, A.K. Bhati88, B. Bhattacharjee45, J. Bhom128, L. Bianchi122, N. Bianchi72, C. Bianchin134,57, J. Bielˇc´ık40, J. Bielˇc´ıkov´a84, A. Bilandzic81,37,93, G. Biro135, R. Biswas4, S. Biswas79, S. Bjelogrlic57, J.T. Blair118, D. Blau80, C. Blume53, F. Bock74,94, A. Bogdanov75, H. Bøggild81, L. Boldizs´ar135, M. Bombara41, J. Book53, H. Borel15, A. Borissov96, M. Borri83,124, F. Boss´u65, E. Botta27, C. Bourjau81, P. Braun-Munzinger97, M. Bregant120, T. Breitner52, T.A. Broker53, T.A. Browning95, M. Broz40, E.J. Brucken46, E. Bruna110, G.E. Bruno33, D. Budnikov99, H. Buesching53, S. Bufalino27,36, P. Buncic36, O. Busch94,128, Z. Buthelezi65, J.B. Butt16, J.T. Buxton20, D. Caffarri36, X. Cai7, H. Caines136, L. Calero Diaz72, A. Caliva57, E. Calvo Villar102, P. Camerini26, F. Carena36, W. Carena36, F. Carnesecchi28, J. Castillo Castellanos15, A.J. Castro125, E.A.R. Casula25, C. Ceballos Sanchez9, P. Cerello110, J. Cerkala115, B. Chang123, S. Chapeland36, M. Chartier124, J.L. Charvet15, S. Chattopadhyay132, S. Chattopadhyay100, A. Chauvin93,37, V. Chelnokov3, M. Cherney87, C. Cheshkov130, B. Cheynis130, V. Chibante Barroso36, D.D. Chinellato121, S. Cho50, P. Chochula36, K. Choi96, M. Chojnacki81, S. Choudhury132, P. Christakoglou82, C.H. Christensen81, P. Christiansen34, T. Chujo128, S.U. Chung96, C. Cicalo105, L. Cifarelli12,28, F. Cindolo104, J. Cleymans90, F. Colamaria33, D. Colella59,36, A. Collu74,25, M. Colocci28, G. Conesa Balbastre71, Z. Conesa del Valle51, M.E. ConnorsII,136, J.G. Contreras40, T.M. Cormier85, Y. Corrales Morales110, I. Cort´es Maldonado2, P. Cortese32, M.R. Cosentino120, F. Costa36, P. Crochet70, R. Cruz Albino11, E. Cuautle63, L. Cunqueiro54,36, T. Dahms93,37, A. Dainese107, A. Danu62, D. Das100, I. Das51,100, S. Das4, A. Dash121,79, S. Dash48, S. De120, A. De Caro31,12, G. de Cataldo103, C. de Conti120, J. de Cuveland43, A. De Falco25, D. De Gruttola12,31, N. De Marco110, S. De Pasquale31, A. Deisting97,94, A. Deloff77, E. D´enesI,135, C. Deplano82, P. Dhankher48, D. Di Bari33, A. Di Mauro36, P. Di Nezza72, M.A. Diaz Corchero10, T. Dietel90, P. Dillenseger53, R. Divi`a36, Ø. Djuvsland18, A. Dobrin82,57, D. Domenicis Gimenez120, B. D¨onigus53, O. Dordic22, T. Drozhzhova53, A.K. Dubey132, A. Dubla57, L. Ducroux130, P. Dupieux70, R.J. Ehlers136, D. Elia103, E. Endress102, H. Engel52, E. Epple136, B. Erazmus113, I. Erdemir53, F. Erhardt129, B. Espagnon51, M. Estienne113, S. Esumi128, J. Eum96, D. Evans101, S. Evdokimov111, G. Eyyubova40, L. Fabbietti93,37, D. Fabris107, J. Faivre71, A. Fantoni72, M. Fasel74, L. Feldkamp54, A. Feliciello110, G. Feofilov131, J. Ferencei84, A. Fern´andez T´ellez2, E.G. Ferreiro17, A. Ferretti27, A. Festanti30, V.J.G. Feuillard15,70, J. Figiel117, M.A.S. Figueredo124,120, S. Filchagin99, D. Finogeev56, F.M. Fionda25, E.M. Fiore33, M.G. Fleck94, M. Floris36, S. Foertsch65, P. Foka97, S. Fokin80, E. Fragiacomo109, – 37 – JHEP08(2016)078 A. Francescon36,30, U. Frankenfeld97, G.G. Fronze27, U. Fuchs36, C. Furget71, A. Furs56, M. Fusco Girard31, J.J. Gaardhøje81, M. Gagliardi27, A.M. Gago102, M. Gallio27, D.R. Gangadharan74, P. Ganoti89, C. Gao7, C. Garabatos97, E. Garcia-Solis13, C. Gargiulo36, P. Gasik93,37, E.F. Gauger118, M. Germain113, A. Gheata36, M. Gheata36,62, P. Ghosh132, S.K. Ghosh4, P. Gianotti72, P. Giubellino110,36, P. Giubilato30, E. Gladysz-Dziadus117, P. Gl¨assel94, D.M. Gom´ez Coral64, A. Gomez Ramirez52, V. Gonzalez10, P. Gonz´alez-Zamora10, S. Gorbunov43, L. G¨orlich117, S. Gotovac116, V. Grabski64, O.A. Grachov136, L.K. Graczykowski133, K.L. Graham101, A. Grelli57, A. Grigoras36, C. Grigoras36, V. Grigoriev75, A. Grigoryan1, S. Grigoryan66, B. Grinyov3, N. Grion109, J.M. Gronefeld97, J.F. Grosse-Oetringhaus36, J.-Y. Grossiord130, R. Grosso97, F. Guber56, R. Guernane71, B. Guerzoni28, K. Gulbrandsen81, T. Gunji127, A. Gupta91, R. Gupta91, R. Haake54, Ø. Haaland18, C. Hadjidakis51, M. Haiduc62, H. Hamagaki127, G. Hamar135, J.C. Hamon55, J.W. Harris136, A. Harton13, D. Hatzifotiadou104, S. Hayashi127, S.T. Heckel53, H. Helstrup38, A. Herghelegiu78, G. Herrera Corral11, B.A. Hess35, K.F. Hetland38, H. Hillemanns36, B. Hippolyte55, D. Horak40, R. Hosokawa128, P. Hristov36, M. Huang18, T.J. Humanic20, N. Hussain45, T. Hussain19, D. Hutter43, D.S. Hwang21, R. Ilkaev99, M. Inaba128, E. Incani25, M. Ippolitov75,80, M. Irfan19, M. Ivanov97, V. Ivanov86, V. Izucheev111, N. Jacazio28, P.M. Jacobs74, M.B. Jadhav48, S. Jadlovska115, J. Jadlovsky115,59, C. Jahnke120, M.J. Jakubowska133, H.J. Jang68, M.A. Janik133, P.H.S.Y. Jayarathna122, C. Jena30, S. Jena122, R.T. Jimenez Bustamante97, P.G. Jones101, H. Jung44, A. Jusko101, P. Kalinak59, A. Kalweit36, J. Kamin53, J.H. Kang137, V. Kaplin75, S. Kar132, A. Karasu Uysal69, O. Karavichev56, T. Karavicheva56, L. Karayan97,94, E. Karpechev56, U. Kebschull52, R. Keidel138, D.L.D. Keijdener57, M. Keil36, M. Mohisin KhanIII,19, P. Khan100, S.A. Khan132, A. Khanzadeev86, Y. Kharlov111, B. Kileng38, D.W. Kim44, D.J. Kim123, D. Kim137, H. Kim137, J.S. Kim44, M. Kim44, M. Kim137, S. Kim21, T. Kim137, S. Kirsch43, I. Kisel43, S. Kiselev58, A. Kisiel133, G. Kiss135, J.L. Klay6, C. Klein53, J. Klein36, C. Klein-B¨osing54, S. Klewin94, A. Kluge36, M.L. Knichel94, A.G. Knospe118,122, C. Kobdaj114, M. Kofarago36, T. Kollegger97, A. Kolojvari131, V. Kondratiev131, N. Kondratyeva75, E. Kondratyuk111, A. Konevskikh56, M. Kopcik115, M. Kour91, C. Kouzinopoulos36, O. Kovalenko77, V. Kovalenko131, M. Kowalski117, G. Koyithatta Meethaleveedu48, I. Kr´alik59, A. Kravˇc´akov´a41, M. Kretz43, M. Krivda59,101, F. Krizek84, E. Kryshen86,36, M. Krzewicki43, A.M. Kubera20, V. Kuˇcera84, C. Kuhn55, P.G. Kuijer82, A. Kumar91, J. Kumar48, L. Kumar88, S. Kumar48, P. Kurashvili77, A. Kurepin56, A.B. Kurepin56, A. Kuryakin99, M.J. Kweon50, Y. Kwon137, S.L. La Pointe110, P. La Rocca29, P. Ladron de Guevara11, C. Lagana Fernandes120, I. Lakomov36, R. Langoy42, C. Lara52, A. Lardeux15, A. Lattuca27, E. Laudi36, R. Lea26, L. Leardini94, G.R. Lee101, S. Lee137, F. Lehas82, R.C. Lemmon83, V. Lenti103, E. Leogrande57, I. Le´on Monz´on119, H. Le´on Vargas64, M. Leoncino27, P. L´evai135, S. Li7,70, X. Li14, J. Lien42, R. Lietava101, S. Lindal22, V. Lindenstruth43, C. Lippmann97, M.A. Lisa20, H.M. Ljunggren34, D.F. Lodato57, P.I. Loenne18, V. Loginov75, C. Loizides74, X. Lopez70, E. L´opez Torres9, A. Lowe135, P. Luettig53, M. Lunardon30, G. Luparello26, T.H. Lutz136, A. Maevskaya56, M. Mager36, S. Mahajan91, S.M. Mahmood22, A. Maire55, R.D. Majka136, M. Malaev86, I. Maldonado Cervantes63, L. MalininaIV,66, D. Mal’Kevich58, P. Malzacher97, A. Mamonov99, V. Manko80, F. Manso70, V. Manzari36,103, M. Marchisone27,65,126, J. Mareˇs60, G.V. Margagliotti26, A. Margotti104, J. Margutti57, A. Mar´ın97, C. Markert118, M. Marquard53, N.A. Martin97, J. Martin Blanco113, P. Martinengo36, M.I. Mart´ınez2, G. Mart´ınez Garc´ıa113, M. Martinez Pedreira36, A. Mas120, S. Masciocchi97, M. Masera27, A. Masoni105, L. Massacrier113, A. Mastroserio33, A. Matyja117, C. Mayer117,36, J. Mazer125, M.A. Mazzoni108, D. Mcdonald122, F. Meddi24, Y. Melikyan75, A. Menchaca-Rocha64, E. Meninno31, J. Mercado P´erez94, M. Meres39, Y. Miake128, M.M. Mieskolainen46, K. Mikhaylov66,58, L. Milano74,36, J. Milosevic22, L.M. Minervini103,23, A. Mischke57, A.N. Mishra49, D. Mi´skowiec97, J. Mitra132, C.M. Mitu62, N. Mohammadi57, – 38 – JHEP08(2016)078 B. Mohanty79, L. Molnar55,113, L. Monta˜no Zetina11, E. Montes10, D.A. Moreira De Godoy54,113, L.A.P. Moreno2, S. Moretto30, A. Morreale113, A. Morsch36, V. Muccifora72, E. Mudnic116, D. M¨uhlheim54, S. Muhuri132, M. Mukherjee132, J.D. Mulligan136, M.G. Munhoz120, R.H. Munzer93,37, H. Murakami127, S. Murray65, L. Musa36, J. Musinsky59, B. Naik48, R. Nair77, B.K. Nandi48, R. Nania104, E. Nappi103, M.U. Naru16, H. Natal da Luz120, C. Nattrass125, S.R. Navarro2, K. Nayak79, R. Nayak48, T.K. Nayak132, S. Nazarenko99, A. Nedosekin58, L. Nellen63, F. Ng122, M. Nicassio97, M. Niculescu62, J. Niedziela36, B.S. Nielsen81, S. Nikolaev80, S. Nikulin80, V. Nikulin86, F. Noferini104,12, P. Nomokonov66, G. Nooren57, J.C.C. Noris2, J. Norman124, A. Nyanin80, J. Nystrand18, H. Oeschler94, S. Oh136, S.K. Oh67, A. Ohlson36, A. Okatan69, T. Okubo47, L. Olah135, J. Oleniacz133, A.C. Oliveira Da Silva120, M.H. Oliver136, J. Onderwaater97, C. Oppedisano110, R. Orava46, A. Ortiz Velasquez63, A. Oskarsson34, J. Otwinowski117, K. Oyama94,76, M. Ozdemir53, Y. Pachmayer94, P. Pagano31, G. Pai´c63, S.K. Pal132, J. Pan134, A.K. Pandey48, P. Papcun115, V. Papikyan1, G.S. Pappalardo106, P. Pareek49, W.J. Park97, S. Parmar88, A. Passfeld54, V. Paticchio103, R.N. Patra132, B. Paul110,100, H. Pei7, T. Peitzmann57, H. Pereira Da Costa15, D. Peresunko80,75, C.E. P´erez Lara82, E. Perez Lezama53, V. Peskov53, Y. Pestov5, V. Petr´aˇcek40, V. Petrov111, M. Petrovici78, C. Petta29, S. Piano109, M. Pikna39, P. Pillot113, L.O.D.L. Pimentel81, O. Pinazza104,36, L. Pinsky122, D.B. Piyarathna122, M. P losko´n74, M. Planinic129, J. Pluta133, S. Pochybova135, P.L.M. Podesta-Lerma119, M.G. Poghosyan85,87, B. Polichtchouk111, N. Poljak129, W. Poonsawat114, A. Pop78, S. Porteboeuf-Houssais70, J. Porter74, J. Pospisil84, S.K. Prasad4, R. Preghenella104,36, F. Prino110, C.A. Pruneau134, I. Pshenichnov56, M. Puccio27, G. Puddu25, P. Pujahari134, V. Punin99, J. Putschke134, H. Qvigstad22, A. Rachevski109, S. Raha4, S. Rajput91, J. Rak123, A. Rakotozafindrabe15, L. Ramello32, F. Rami55, R. Raniwala92, S. Raniwala92, S.S. R¨as¨anen46, B.T. Rascanu53, D. Rathee88, K.F. Read85,125, K. Redlich77, R.J. Reed134, A. Rehman18, P. Reichelt53, F. Reidt94,36, X. Ren7, R. Renfordt53, A.R. Reolon72, A. Reshetin56, J.-P. Revol12, K. Reygers94, V. Riabov86, R.A. Ricci73, T. Richert34, M. Richter22, P. Riedler36, W. Riegler36, F. Riggi29, C. Ristea62, E. Rocco57, M. Rodr´ıguez Cahuantzi11,2, A. Rodriguez Manso82, K. Røed22, E. Rogochaya66, D. Rohr43, D. R¨ohrich18, R. Romita124, F. Ronchetti72,36, L. Ronflette113, P. Rosnet70, A. Rossi36,30, F. Roukoutakis89, A. Roy49, C. Roy55, P. Roy100, A.J. Rubio Montero10, R. Rui26, R. Russo27, E. Ryabinkin80, Y. Ryabov86, A. Rybicki117, S. Sadovsky111, K. ˇ Safaˇr´ık36, B. Sahlmuller53, P. Sahoo49, R. Sahoo49, S. Sahoo61, P.K. Sahu61, J. Saini132, S. Sakai72, M.A. Saleh134, J. Salzwedel20, S. Sambyal91, V. Samsonov86, L. ˇ S´andor59, A. Sandoval64, M. Sano128, D. Sarkar132, P. Sarma45, E. Scapparone104, F. Scarlassara30, C. Schiaua78, R. Schicker94, C. Schmidt97, H.R. Schmidt35, S. Schuchmann53, J. Schukraft36, M. Schulc40, T. Schuster136, Y. Schutz36,113, K. Schwarz97, K. Schweda97, G. Scioli28, E. Scomparin110, R. Scott125, M. ˇ Sefˇc´ık41, J.E. Seger87, Y. Sekiguchi127, D. Sekihata47, I. Selyuzhenkov97, K. Senosi65, S. Senyukov3,36, E. Serradilla10,64, A. Sevcenco62, A. Shabanov56, A. Shabetai113, O. Shadura3, R. Shahoyan36, A. Shangaraev111, A. Sharma91, M. Sharma91, M. Sharma91, N. Sharma125, K. Shigaki47, K. Shtejer27,9, Y. Sibiriak80, S. Siddhanta105, K.M. Sielewicz36, T. Siemiarczuk77, D. Silvermyr34, C. Silvestre71, G. Simatovic129, G. Simonetti36, R. Singaraju132, R. Singh79, S. Singha132,79, V. Singhal132, B.C. Sinha132, T. Sinha100, B. Sitar39, M. Sitta32, T.B. Skaali22, M. Slupecki123, N. Smirnov136, R.J.M. Snellings57, T.W. Snellman123, C. Søgaard34, J. Song96, M. Song137, Z. Song7, F. Soramel30, S. Sorensen125, R.D.de Souza121, F. Sozzi97, M. Spacek40, E. Spiriti72, I. Sputowska117, M. Spyropoulou-Stassinaki89, J. Stachel94, I. Stan62, P. Stankus85, G. Stefanek77, E. Stenlund34, G. Steyn65, J.H. Stiller94, D. Stocco113, P. Strmen39, A.A.P. Suaide120, T. Sugitate47, C. Suire51, M. Suleymanov16, M. SuljicI,26, R. Sultanov58, M. ˇ Sumbera84, A. Szabo39, A. Szanto de ToledoI,120, I. Szarka39, A. Szczepankiewicz36, M. Szymanski133, U. Tabassam16, J. Takahashi121, G.J. Tambave18, N. Tanaka128, M.A. Tangaro33, M. Tarhini51, – 39 –