Prompt and nonprompt J/ψ production and nuclear modification in pPb collisions at √sNN = 8.16 TeV

Physics Letters B 774 (2017) 159–178 Contents lists available at ScienceDirect Physics Letters B www.elsevier.com/locate/physletb Prompt and nonprompt J/ψ production and nuclear modification in pPb collisions at √sNN =8.16 TeV .LHCb Collaboration a r t i c l e i n f oa b s t r a c t Article history: Received 22 June 2017 Received in revised form 19 September 2017 Accepted 19 September 2017 Available online 22 September 2017 Editor: L. Rolandi The production of J/ψ mesons is studied in proton-lead collisions at the centre-of-mass energy per nucleon pair √sNN =8.16 TeV with the LHCb detector at the LHC. The double differential cross-sections of prompt and nonprompt J/ψ production are measured as a function of the J/ψ transverse momentum and rapidity in the nucleon–nucleon centre-of-mass frame. Forward-to-backward ratios and nuclear modification factors are determined. The results are compared with theoretical calculations based on collinear factorisation using nuclear parton distribution functions, on the colour glass condensate or on coherent energy loss models. ©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 The production of J/ψ mesons, and more generally of quarkonium states, has been considered as a sensitive probe of colour screening in a hot and dense medium since the proposal by Matsui and Satz in 1986 [1] of the suppression of the J/ψ meson production in heavy-ion collisions as a sign of deconfinement. The theoretical understanding of the bound-state dynamics of quarkonium by means of lattice QCD and effective field theories has progressed substantially in the last 30 years. In heavy-ion collisions, the emerging picture indicates strong modifications of the quarkonium bound-state characteristics [2]. Experimentally, measurements at the SPS, RHIC and LHC revealed interesting patterns [3]. In particular, an additional low transverse momentum (pT) component of J/ψ production was observed in PbPb collisions at the LHC [4–8]. This observation had been predicted as a sign of charmonium originating from unbound charm quarks, generated either during the lifetime of the deconfined medium [9] or at the phase boundary [10]. The limited understanding of nuclear phenomena unrelated to deconfinement, commonly called cold nuclear matter (CNM) effects, restricts the ability of phenomenological models to describe the experimental data on J/ψ production in PbPb collisions. The size of CNM effects can be quantified by measurements in protonnucleus or deuteron-nucleus collisions, which have been pursued at fixed target experiments as well as at RHIC and LHC [3]. The feature of CNM drawing the highest attention for proton-lead collisions at the LHC is the modification of the gluon flux coupling to the charm quark pair. This modification is often treated within a collinear parton distribution framework employing nuclear parton distribution functions (nPDFs) [11–15]. At low longitudinal momentum fractions xcarried by the parton, calculations within the colour glass condensate (CGC) effective field theory, describing the saturation regime of QCD [16,17], are frequently employed. Several calculations have been pursued to quantify nuclear modifications of J/ψ production in the collinear framework [18–21] or in the CGC framework [22–24]. It has to be noted that the low-xgluon content of the nucleus is largely unconstrained by experimental data at perturbative scales. In addition, small-angle gluon radiation taking into account interference between initial and final state radiation, called coherent energy loss, was proposed as the dominant nuclear modification of quarkonium production in proton-lead collisions [25]. The discrimination between these phenomena is a strong motivation for the study of the production of quarkonium as a hard-scale probe of QCD at high density. The experimental results on J/ψ production in proton-lead collisions based on the 2013 data samples at √sNN =5 TeV published by the LHC experiments ALICE, ATLAS, CMS and LHCb [26–31] can be qualitatively described by implementations of the approaches described above in the kinematic applicability range of the calculations [18–21,23–25]. No conclusion on the dominant mechanism for nuclear modification of J/ψ production could be drawn. The measurement of an additional suppression of the excited state ψ(2S)by ALICE [32,33] and LHCb [34] in proton-lead collisions at √sNN =5 TeV and by PHENIX at RHIC [35,36] in various collision systems at √sNN =0.2 TeV cannot be explained by the modification of the gluon flux or by coherent energy loss because it would affect the J/ψ and the ψ(2S)states in a similar way. These measurements motivated calculations involving hadronic and partonic interactions influencing the evolution of the c¯ cpair after the first interaction [37,38] for proton(deuteron)-nucleus collisions. Although the impact on J/ψ production is generally small https://doi.org/10.1016/j.physletb.2017.09.058 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. 160 LHCb Collaboration / Physics Letters B 774 (2017) 159–178 in these models, it can be significant in rapidity ranges with large particle densities. The measurement of the nonprompt J/ψ production provides access to the production of beauty hadrons. The modification of their kinematic distributions in nucleus–nucleus collisions carries valuable information about the created matter [3]. Similarly to direct charmonium production, the production of beauty hadrons can be subject to CNM effects altering the interpretation of nucleus–nucleus collision data. Such effects can be precisely measured in proton-lead collisions. The measurements of the production of prompt J/ψ and nonprompt J/ψ mesons, called J/ψ-from-b-hadrons in the following, presented in this letter are important ingredients for the understanding of the imprints of deconfinement in nucleus–nucleus collisions. They are based on larger integrated luminosities and on higher collision energies than the initial measurements with the 2013 proton-lead data sample by the LHCb experiment at √sNN = 5TeV[27]. 2. Detector, data sample and observables The LHCb detector [39,40] is a single-arm forward spectrometer covering the pseudorapidity range 2 <η<5, designed for the study of particles containing bor cquarks. The detector includes a high-precision tracking system consisting of a silicon-strip vertex detector surrounding the interaction region [41], a large-area silicon-strip detector located upstream of a dipole magnet with a bending power of about 4Tm, and three stations of siliconstrip detectors and straw drift tubes [42] placed downstream of the magnet. The tracking system provides a measurement of momentum of charged particles with a relative uncertainty that varies from 0.5% at low momentum to 1.0% at 200 GeV/c. The minimum distance of a track to a primary vertex (PV), the impact parameter, is measured with a resolution of (15 +29/pT)μm, where pT is the transverse momentum in the LHCb frame, in GeV/c. Different types of charged hadrons are distinguished using information from two ring-imaging Cherenkov detectors [43]. Photons, electrons and hadrons are identified by a calorimeter system consisting of scintillating-pad and preshower detectors, an electromagnetic calorimeter and a hadronic calorimeter. Muons are identified by a system composed of alternating layers of iron and multiwire proportional chambers [44]. This analysis is based on data acquired during the 2016 LHC heavy-ion run, where protons and 208Pb ions were colliding at a centre-of-mass energy per nucleon pair of √sNN =8.16 TeV. Since the energy per nucleon in the proton beam is larger than in the lead beam, the nucleon–nucleon centre-of-mass system has a rapidity in the laboratory frame of 0.465 (−0.465), when the proton (lead) beam travels from the vertex detector towards the muon chambers. Consequently, the LHCb detector covers two different acceptance regions: 1. 1.5 <y∗<4.0 when the proton beam travels from the vertex detector towards the muon chambers, 2. −5.0 <y∗<−2.5 when the proton beam travels from the muon chambers towards the vertex detector, where y∗is the rapidity in the centre-of-mass frame of the colliding nucleons, with respect to the proton beam direction. In this letter, the first configuration is denoted pPb and the second one Pbp. The data samples correspond to an integrated luminosity of 13.6 ±0.3 nb−1of pPb collisions and 20.8 ±0.5 nb−1of Pbp collisions. The instantaneous luminosity for the majority of the recorded events ranges between 0.5 and 1.0 ×1029 cm−2s−1. This luminosity corresponds on average to about 0.1 or fewer collisions per bunch crossing. In this letter, we describe the measurement of the doubledifferential production cross-sections of J/ψ mesons as a function of pTand y∗in the ranges 0 <pT<14 GeV/cand 1.5 <y∗<4.0 for pPb and −5.0 <y∗<−2.5for Pbp. The measurement is performed separately for prompt J/ψ mesons, i.e. produced directly in the initial hard scattering or from the decay of an excited charmonium state produced directly, and for J/ψ mesons coming from the decay of a long-lived b-hadron, either directly or via an excited charmonium state. Nuclear effects are quantified by the nuclear modification factor, RpPb, RpPb(pT,y∗)≡1 A d2σpPb(pT,y∗)/dpTdy∗ d2σpp(pT,y∗)/dpTdy∗,(1) where A =208 is the mass number of the Pb ion, d2σpPb(pT, y∗)/ dpTdy∗the J/ψ production cross-section in pPb or Pbpcollisions and d2σpp(pT, y∗)/dpTdy∗the J/ψ reference production crosssection in pp collisions at the same nucleon–nucleon centre-of- mass energy. The determination of the reference cross-section is described in Sec. 5.1. In the absence of nuclear effects, the nuclear modification factor is equal to unity. In addition to the nuclear modification factor, the observable RFB quantifies the relative forward-to-backward production rates. The forward-to-backward ratio is measured as the ratio of crosssections in the positive and negative y∗acceptances evaluated in the same absolute y∗value ranges, RFB(pT,y∗)≡d2σpPb(pT,+y∗)/dpTdy∗ d2σpPb(pT,−y∗)/dpTdy∗.(2) 3. Event selection and cross-section determination The J/ψ production cross-section measurement follows the approach described in Ref. [45]. The double differential J/ψ production cross-section in each kinematic bin of pTand y∗is computed as d2σ dpTdy∗=N(J/ψ →μ+μ−) L×tot ×B(J/ψ →μ+μ−)×pT×y∗,(3) where N(J/ψ →μ+μ−)is the number of reconstructed prompt J/ψ or J/ψ-from-b-hadrons signal mesons, tot is the total detection efficiency in the given kinematic bin, B(J/ψ →μ+μ−) = (5.961 ±0.033)%[46] is the branching fraction of the decay J/ψ →μ+μ−,pT=1GeV/cand y∗=0.5are the bin widths and Lis the integrated luminosity. The luminosity is determined with a van der Meer scan, which was performed for both beam configurations. The luminosity determination follows closely the approach described in Ref. [47]. 3.1. Selection An online event selection is performed by a trigger system consisting of a hardware stage, which, for this analysis, selects events containing at least one muon with pTlarger than 500 MeV/c, followed by a software stage. In the first stage of the software trigger, two muon tracks with pT>500 MeV/care required to form a J/ψ candidate with invariant mass Mμ+μ−>2.5GeV/c2. In the second stage, J/ψ candidates with an invariant mass within 120 MeV/c2 of the known value of the J/ψ mass [46] are selected. In between the two software stages, the alignment and calibration of the detector is performed in near real-time [48]. The LHCb Collaboration / Physics Letters B 774 (2017) 159–178 161 same alignment and calibration is propagated to the offline reconstruction, ensuring consistent and high-quality particle identification (PID) information between the online and offline processings. The identical performance of the online and offline reconstructions offers the opportunity to perform physics analyses directly using candidates reconstructed in the trigger [49,50] as well as storing all reconstructed particles in the event [51]. The present analysis exploits this feature for the first time in proton-lead collisions and is using the online reconstruction. At the analysis stage, each event is required to have at least one PV reconstructed from at least four tracks measured in the vertex detector. For events with multiple PVs, the PV that has the smallest χ2 IP with respect to the J/ψ candidate is chosen. Here, χ2 IP is defined as the difference between the vertex-fit χ2cal- culated with the J/ψ meson candidate included in or excluded from the PV fit. Each identified muon track is required to have pT>750 MeV/c, 2 <η<5 and to have a good-quality track fit. The two muon tracks of the J/ψ candidate must form a goodquality vertex, representing a tighter selection compared to the software trigger requirement. 3.2. Determination of signal yields The reconstructed vertex of the J/ψ mesons originating from b-hadron decays tends to be separated from the PVs. These J/ψ mesons can thus be distinguished from prompt J/ψ mesons by exploiting the pseudo proper time defined as tz≡(zJ/ψ −zPV)×MJ/ψ pz ,(4) where zJ/ψ and zPV are the coordinates along the beam axis of the J/ψ decay vertex position and of the PV position, pzis the zcomponent of the J/ψ momentum and MJ/ψ the known J/ψ mass. The yields of J/ψ signal candidates, for the prompt and J/ψ-from-b-hadrons categories, are determined from a simultaneous two-dimensional unbinned maximum likelihood fit to their invariant mass and pseudo proper time distributions, performed independently for each (pT, y∗)bin. In the fit function, the invariant-mass distribution of the signal is described by a Crystal Ball function [52], and the combinatorial background by an exponential function. The tzdistribution of prompt J/ψ is described by a Dirac δ-function δ(tz), and that of J/ψ-from-b-hadrons by an exponential function for tz>0. Both of them are convolved with a triple-Gaussian resolution function, modelled from simulation samples to take into account the vertex resolution. The background tzdistribution is described by an empirical function derived from the shape observed in the J/ψ upper mass sideband, 3200 <Mμ+μ−<3250 MeV/c2. This background comes from muons of semileptonic b- and c-hadron decays and from pions and kaons decaying in the detector. The distribution is parameterised as a sum of a Dirac δ-function and of five exponential functions, three for positive tzvalues and two for negative tzvalues, convolved with the sum of two Gaussian functions. An example of the invariant mass and the pseudo proper time distributions for one (pT, y∗)bin is shown in Fig. 1 for the pPb and Pbpsamples, where the one-dimensional projections of the fit result are drawn on the distributions. The width of the Gaussian part of the Crystal Ball function varies as a function of pT between 10 MeV/c2(15 MeV/c2) and 15 MeV/c2(33 MeV/c2) in the lowest (highest) rapidity bins in the laboratory frame in both beam configurations. Due to the rapidity shifts between the laboratory frame and the nucleon–nucleon centre-of-mass frames, the two examples do not correspond to the same rapidity range in the laboratory while they are in the same |y∗|range and, in this example, the mass resolution in the Pbpconfiguration is different from the one in the pPb configuration. 3.3. Efficiencies The total detection efficiency, tot, is the product of the geometrical acceptance, and the efficiencies for charged track reconstruction, particle identification, candidate and trigger selections. Samples of simulated events are used to evaluate these efficiencies except for the particle identification, which is determined in a data-driven approach. In the simulation, pPb and Pbpminimumbias collisions are generated using the Epos event generator tuned with the LHC model [53]. The J/ψ →μ+μ−signal candidates are generated separately, with the Pythia8 generator [54] in pp collisions with beams having momenta equal to the momenta per nucleon of the pand Pb beams. They are then merged with the Epos minimum bias collisions to build the samples out of which the efficiencies are computed. The decays of hadrons are generated by EvtGen [55], in which final-state electromagnetic radiation is generated with Photos [56]. The interaction of the particles with the detector, and the detector response, are implemented using the Geant4 toolkit [57] as described in Ref. [58]. The charged-track reconstruction efficiency is first evaluated in simulation and is corrected using a data-driven tag-and-probe approach. For this purpose, J/ψ candidates are formed with one fully-reconstructed “tag” track and one “probe” track reconstructed partially with a subset of the tracking sub-detectors and both identified as muons [59] in data and in simulation. The ratio of the single track efficiencies from this tag-and-probe approach is used as a correction factor. These correction factors for each track are then applied to the signal candidates in the simulation to obtain the integrated efficiency in every kinematic bin. The tag-and-probe correction evaluation is relying on the pPb and the Pbpdata samples, since the larger tracking calibration samples in pp collisions are limited in detector occupancy by an additional selection criterion on trigger level. The muon identification efficiency is determined for each track in data with a tag-and-probe method [60] taking into account the efficiency variations as function of track momentum, pseudorapidity and detector occupancy. Calibration samples of J/ψ mesons are selected applying a tight identification criterion on one of the muons and no identification requirements to the second muon. However, the sizes of the calibration samples collected in pPb and Pbpcollisions are limited. The efficiency is thus evaluated using the calibration samples collected in pp collisions, taking into account the different detector occupancies between pp, pPb and Pbp collisions, since this parameter affects the muon identification performance. The J/ψ simulation is weighted with the efficiencies determined per track in data in order to compute the muon identification efficiency in bins of J/ψ pTand y∗. The hardware and software trigger efficiencies obtained from the simulation are validated by comparing them with the efficiencies measured in control data samples recorded with minimum and unbiased trigger requirements, and containing J/ψ candidates. The total efficiency in each (pT, y∗)bin, tot, is found to be the same for prompt J/ψ and J/ψ-from-b-hadrons within uncertainties and is taken to be identical for the two components. It is shown in Fig. 2 for pPb and Pbpcollision data, as a function of the J/ψ pTin the different rapidity bins. The uncertainties are the quadratic sums of the statistical uncertainties and the uncertainties associated to the data-driven corrections and validations, described in the following section. 162 LHCb Collaboration / Physics Letters B 774 (2017) 159–178 Fig. 1. (left) Invariant mass and (right) pseudo proper time distributions for J/ψ candidates in the bin 6 <pT<7GeV/cand 3.5 <|y∗| <4.0for the (top) pPb and (bottom) Pbpsamples respectively. The black circles with error bars represent the LHCb data. The projection of the result of the fit described in the text is drawn on each distribution: the red solid line is the total fit function, the blue dashed line is the prompt J/ψ signal component, the purple solid line is the J/ψ-from-b-hadrons signal component and the green dashed line is the combinatorial background component. Fig. 2. Total J/ψ detection efficiency, tot, as a function of the J/ψ pTin different y∗bins for (left) pPb and (right) Pbp. 4. Systematic uncertainties The systematic uncertainties on the cross-section of prompt J/ψ and J/ψ-from-b-hadrons are summarised in Table 1 and described in the following. The total detection efficiency tot for prompt J/ψ and J/ψ-from-bis found to be equal within the statistical precision of the simulation and all systematic uncertainties apply both for prompt J/ψ and J/ψ-from-b. Acceptance and reconstruction efficiencies of the J/ψ vector meson depend on its polarisation at production. The ALICE and the LHCb measurements in pp collisions [61,62] indicate a polarisation consistent with zero in most of the kinematic region of the analysis pre- LHCb Collaboration / Physics Letters B 774 (2017) 159–178 163 Fig. 3. Production cross-section for (top left) prompt J/ψ in pPb, (top right) J/ψ-from-b-hadrons in pPb, (bottom left) prompt J/ψ in Pbpand (bottom right) J/ψ-from-b-hadrons in Pbp. The data points are placed at the centre of the pTbins, the horizontal error bars indicate the bin widths and the vertical error bars the total uncertainties, calculated as quadratic sums of the statistical and systematic uncertainties. Table 1 Summary of relative systematic uncertainties in pPb and Pbpon the cross-section of prompt J/ψ and J/ψ-from-b-hadrons. Uncertainties that are computed bin-by-bin are expressed as ranges giving the minimum to maximum values. The last column indicates the correlation between bins within the same beam configuration. Source pPb PbpComment Signal model 1.3% 1.3% correlated Muon identification 2.0%–11.0% 2.1%–15.3% correlated Tracking 3.0%–8.0% 5.9%–26.5% correlated Hardware trigger 1.0%–10.9% 1.0%–7.4% correlated Software trigger 2.0% 2.0% correlated Simulation statistics 0.4%–7.0% 0.4%–26.2% uncorrelated B(J/ψ →μ+μ−)0.05% 0.05% correlated Luminosity 2.6% 2.5% correlated Polarisation – – not considered sented in this letter. In this analysis, it is assumed that the J/ψ mesons are produced with no polarisation in pPb and Pbpcollisions at √sNN =8.16 TeV. No systematic uncertainty is assigned for the effects of polarisation. The uncertainty on the J/ψ-meson yields, related to the modelling of the signal mass shape in the simultaneous mass and tzfit, is studied using an alternative fit model. In this model, the signal mass shape is described by the sum of a Crystal Ball function and of a Gaussian function. The relative difference of the signal yields between the nominal and alternative fits amounts to 1.3%, which is taken as a fully correlated systematic uncertainty between bins. The uncertainty associated to the shape of the tzdistribution is negligible. The uncertainty on the muon identification has multiple contributions. The statistical uncertainty of the efficiencies is derived from the calibration sample. The impact of the finite binning in muon momentum, pseudorapidity and detector occupancy on the efficiencies is estimated by varying the binning scheme. Finally, an uncertainty due to the method to determine the number of signal candidates in the calibration samples is also considered. The total systematic uncertainty due to these three sources varies between 2% and 15%. It is assumed to be fully correlated between bins. This assumption is valid for neighbouring bins in acceptance. The bias introduced by this assumption in the evaluation of the total systematic uncertainty on integrated quantities is negligible. The data-driven corrections to the track reconstruction efficiency carry uncertainties related to the statistical uncertainties of the data, dominating in most bins. In addition, a systematic uncertainty is related to a potential bias of the selection criteria which are necessary to obtain a good signal over background ratio for the determination of the efficiency corrections. A systematic uncertainty related to the method is applied similarly to pp collisions and amounts to 0.8% per track [59]. The total uncertainty related to charged track reconstruction varies from 3.0% to 8.0% for pPb and 5.9% to 26.5% for Pbp, correlated between bins. The uncertainty in the Pbpcase is larger due to the smaller signal over background ratio for the partially reconstructed candidates used in the data-driven tag-and-probe method compared to the pPb case. The assumption on the correlation is valid for neighbouring bins. The introduced bias in the evaluation of the total systematic uncertainty on integrated quantities is negligible. The largest uncertainties appear at low track momenta and hence low J/ψ pT. The trigger efficiency is determined in data and in simulation by the data-driven method described in the previous section and in Ref. [49]. The uncertainties related to the trigger are estimated by comparing the results in simulation and in data. The uncertainty on the hardware trigger efficiency is found to vary between 1% and 11%, and the uncertainty on the software trigger efficiency is 164 LHCb Collaboration / Physics Letters B 774 (2017) 159–178 estimated to amount to 2%. The trigger uncertainties are assumed to be fully correlated between bins. The finite size of the simulation event sample used for the efficiency determination introduces a systematic uncertainty, which varies between 0.4% and 26.2% between the kinematic bins of the pPb and the Pbpsimulation. The largest relative values appear at high pTand large rapidities and do not dominate the overall uncertainties. They differ between the pPb and Pbpcase due to the different rapidity coverage in the centre-of-mass system. The branching fraction contributes to the cross-section uncertainty with 0.05%. The luminosity measurement uncertainty amounts to 2.6% in pPb and to 2.5% in Pbpcollisions. The uncertainty on all other applied selections is found to be negligible based on comparisons between data and simulation signal distributions of selection and kinematics variables. 5. Results 5.1. Cross-sections The measured double-differential cross-sections of prompt J/ψ and J/ψ-from-b-hadrons in the pPb and Pbpdata samples are shown in Fig. 3, as a function of pTfor the considered y∗bins. The numerical values are presented in Appendices A.1–A.4. The total cross-sections, integrated over the measurement ranges, amount to σprompt J/ψ (1.5<y∗<4.0,pT<14 GeV/c) =1625 ±4±117μb, σJ/ψ-from-b-hadrons(1.5<y∗<4.0,pT<14 GeV/c) =276 ±2±20μb, σprompt J/ψ (−5.0<y∗<−2.5,pT<14 GeV/c) =1692 ±4±182μb, σJ/ψ-from-b-hadrons(−5.0<y∗<−2.5,pT<14 GeV/c) =209 ±1±22μb, where the first uncertainties are statistical and the second systematic. The fraction of J/ψ-from-b-hadrons, fb, is derived from the cross-section measurements. The fraction fbis defined as fb(pT,y∗)≡(5) d2σJ/ψ-from-b-hadrons/dpTdy∗ d2σprompt J/ψ /dpTdy∗+d2σJ/ψ-from-b-hadrons/dpTdy∗. Most of the systematic uncertainties cancel in the determination of fb, which can thus be measured precisely. The values of fbas a function of pTin the different y∗bins are shown in Fig. 4 for pPb and Pbpand listed in Appendices A.5 and A.6. The values of fb measured in pp collisions at a centre-of-mass energy of 8TeV[63], are shown on the same figure for comparison. The differences that appear between the measurements performed in the two collision systems indicate, particularly at low pT, different nuclear modifications for prompt J/ψ and b-quark production. The focus of this publication is the quantification of the nuclear effects, comparing in particular the J/ψ production in proton-lead collisions with that in pp collisions at the same energy. Following the same approach as in the previous LHCb publication on J/ψ production in pPb collisions at √sNN =5TeV[27], a pp reference cross-section at √s=8.16 TeV is determined from an interpolation of the LHCb cross-section measurements at 7TeV[64], 8TeV[63] and 13 TeV [45]. The extracted reference cross-section is in agreement with the measured cross-section at √s=8TeV. For Fig. 4. Fraction of J/ψ-from-b-hadrons, fb, as a function of pTfor (from top to bottom) 1.5 <|y∗| <2.0, 2.0 <|y∗| <2.5, 2.5 <|y∗| <3.0, 3.0 <|y∗| <3.5, 3.5 <|y∗| <4.0, 4.0 <|y∗| <4.5and 4.5 <|y∗| <5.0. The data points are placed at the centre of the pTbins, the horizontal error bars indicate the bin widths and the vertical error bars the total uncertainties, calculated as quadratic sums of the statistical and systematic uncertainties. Blue circles are for pPb collisions, red squares for Pbpcollisions and black triangles for pp collisions at 8TeVtaken from Ref. [63]. the edges of the rapidity range in pPb collisions (1.5 <y∗<2.0) and in Pbpcollisions (4.5 <y∗<5.0), which are not covered by the measurements in pp collisions, an extrapolation is used based on the experimental measurements. The interpolation and the extrapolation methods were validated with ALICE and LHCb data and are described in Ref. [65]. The cross-section as a function of y∗, integrated over pTin the range 0 <pT<14 GeV/cin pPb and Pbpcollisions, is shown in Fig. 5. The cross-section is compared with the reference crosssection for prompt J/ψ and J/ψ-from-b-hadrons production in pp collisions at √s=8.16 TeV, multiplied by the Pb mass number A =208. The total relative uncertainties on the pp cross-section range between 3% and 11% and are largest in the bins based on extrapolations. The cross-sections as a function of pT, integrated over the range 1.5 <y∗<4.0for pPb and −5.0 <y∗<−2.5for Pbp, and the corresponding scaled pp cross-sections are represented in Fig. 6. In this case, the total relative uncertainties on the pp reference cross-section vary between 3% and 18%. LHCb Collaboration / Physics Letters B 774 (2017) 159–178 165 Fig. 5. Absolute production cross-sections of (left) prompt J/ψ and (right) J/ψ-from-b-hadrons, as a function of y∗, integrated over the range 0 <pT<14 GeV/c. The black circles are the pPb and Pbpvalues and the red open squares the values for pp collisions at the same energy, multiplied by the Pb mass number A =208. The horizontal error bars are the bin widths and vertical error bars the total uncertainties. Fig. 6. Absolute production cross-sections of (top left) prompt J/ψ in pPb, (top right) prompt J/ψ in Pbp, (bottom left) J/ψ-from-b-hadrons in Pbpand (bottom left) J/ψ-from-b-hadrons in Pbp, as a function of pTand integrated over the rapidity range of the analysis. The black circles are the pPb and Pbpvalues and the red open squares the values for pp collisions at the same energy, multiplied by the Pb mass number A =208, integrated over the same rapidity ranges. The horizontal error bars are the bin widths and vertical error bars the total uncertainties. 5.2. Nuclear modification factors The nuclear modification factor RpPb defined in Eq. (1) is computed from the prompt J/ψ and J/ψ-from-b-hadrons production cross-sections in pp and pPb or Pbpcollisions. The systematic uncertainties are assumed to be uncorrelated between the measurements in proton-lead and in pp collisions. The nuclear modification factors for prompt J/ψ and J/ψ-from-b-hadrons production as functions of pTor y∗, integrating over the other variable, are shown in Figs. 7 and 8, respectively. The numerical values are available in Appendix B. The results at √sNN =5TeV[27] are also depicted on Fig. 8 and are in good agreement with the new and more precise results at √sNN =8.16 TeV. At forward rapidity, 1.5 <y∗<4.0, a strong suppression of up to 50% is observed in the case of prompt J/ψ production at low pT(Fig. 7). This behaviour results in a strong suppression in the nuclear modification factor as a function of rapidity shown in Fig. 8. With increasing pT, RpPb approaches unity and the suppression is stronger at more forward rapidities. The production of J/ψ-from-b-hadrons is also suppressed compared to 166 LHCb Collaboration / Physics Letters B 774 (2017) 159–178 Fig. 7. J/ψ nuclear modification factor, RpPb, integrated over y∗in the analysis range, as a function of pTfor (top left) prompt J/ψ in pPb, (bottom left) J/ψ-from-b-hadrons in pPb, (top right) prompt J/ψ in Pbpand (bottom right) J/ψ-from-b-hadrons in Pbp. Horizontal error bars are the bin widths, vertical error bars the total uncertainties. The black circles are the values measured in this letter and the coloured areas the theoretical predictions from the models detailed in the text with their uncertainties. Fig. 8. J/ψ nuclear modification factor, RpPb, integrated over pTin the range 0 <pT<14 GeV/c, as a function of y∗for (left) prompt J/ψ and (right) J/ψ-from-b-hadrons. The horizontal error bars are the bin widths and vertical error bars the total uncertainties. The black circles are the values measured in this letter, the red squares the values measured at √sNN =5TeVfrom Ref. [27] and the coloured areas the theoretical computations from the models detailed in the text, with their uncertainties. that in pp collisions at forward rapidities, although to a lesser degree, as shown in Fig. 8. No dependence as a function of rapidity can be observed within the experimental uncertainties. The dependence as a function of the transverse momentum is weaker for J/ψ-from-b-hadrons compared to prompt J/ψ, but the nuclear modification factor is also approaching unity at high transverse momentum. At backward rapidity, −5.0 <y∗<−2.5, a weaker suppression of prompt J/ψ production at low pTis observed, of up to 25%. Similarly to the forward-rapidity region, the suppression is weakening and the nuclear modification factor is approaching values consistent with unity at high transverse momentum. The nuclear modification factor as a function of rapidity shows a weak suppression with no visible rapidity dependence within experimental uncertainties. The nuclear modification factor of J/ψ-from-b-hadrons at backward rapidity is consistent with unity over the full kinematic region. The measurements of prompt J/ψ nuclear modification factors are compared in Figs. 7 and 8with three groups of calculations: LHCb Collaboration / Physics Letters B 774 (2017) 159–178 167 1. collinear factorisation using different nPDFs [66,67] (labelled “HELAC-Onia with EPS09LO”, “HELAC-Onia with nCTEQ15” and “HELAC-Onia with EPS09NLO” on the figures), 2. CGC effective field theory in the dilute-dense approximation taking into account the dense nature of the Pb nucleus, but approximating the proton as a dilute parton source [24,68] (labelled “CGC”), 3. coherent energy loss calculating the impact of low angle coherent gluon radiation during the crossing of the nucleus [25] (labelled “Energy Loss”). The CGC calculations [24,68] describe well the behaviour of the prompt J/ψ data at forward rapidity. At backward rapidity, this approach is not available due to the breakdown of the dilute approximation for the partons in the proton. The uncertainties take into account the variation of the charm-quark mass and the factorisation scale. These uncertainties largely cancel in this ratio of cross-sections. The collinear calculations are based on the HELAC- Onia event generator [66,67], tuned to reproduce prompt J/ψ cross-section measurements in pp collision [21] and combined with different sets of nPDFs: nCTEQ15 [14] and EPS09 at leading (LO) and at next-to-leading order (NLO) [12]. However, the large uncertainties reveal the missing experimental constraints on the gluon density in the nucleus at low xprobed by the measurements in the LHCb detector acceptance. At backward rapidities, the experimental points are found at the lower bound or slightly below the theoretical uncertainty bands and exhibit a different rapidity shape from the calculations. The coherent energy loss model [25] is able to provide the overall shape of the suppression, but overestimates the experimental data at forward rapidities. The uncertainty of this calculation reflects the allowed variation of the parameterisation of pp data used in the model and the allowed variation of the only free model parameter from fits to other measurements. The measurements of J/ψ-from-b-hadrons nuclear modification factors are compared in Figs. 7 and 8with a perturbative QCD calculation at fixed-order next-to-leading-logarithms (FONLL) [69, 70] coupled with the EPS09 nPDF set at next-to-leading order [12] (labelled “FONLL with EPS09NLO” on the figures). The displayed uncertainties correspond to the uncertainties from the nPDF, which are of similar size to or smaller than the total experimental uncertainties. The pTdependence of the experimental data is described within uncertainties by the model. However, the calculation tends to show larger nuclear modification factors than the data. This tendency is confirmed by the nuclear modification factor as a function of rapidity, where the most precise experimental data points are below the model uncertainty band. Furthermore, at backward rapidity, the slope of the theoretical curve is not seen in the experimental data. Finally, recent measurements have shown that long-range collective effects, which have previously been observed in relatively large nucleus–nucleus collision systems, may also be present in smaller collision systems at large charged-particle multiplicites [71–74]. If these effects have a hydrodynamic origin, momentum anisotropies at the quark level can arise and may modify the distribution of observed heavy-quark hadrons [75]. However, the expected magnitude of these effects on prompt J/ψ or J/ψ-from-b-hadrons production has not yet been calculated. Since the measurements in this letter are integrated over chargedparticle multiplicity, potential modifications in high-multiplicity events are diluted. 5.3. Forward-to-backward ratios Figs. 9 and 10 show the forward-to-backward ratio, RFB, of the production of prompt J/ψ and J/ψ-from-b-hadrons, in the overlapping acceptance between the two beam configurations, as functions of transverse momentum and rapidity, respectively. The numerical results are listed in Appendix C. In the RFB ratio, most of the systematic uncertainties cancel. The measurements of RFB at √sNN =5TeV[27] are compared with the measurements at 8.16 TeV and are found to be in agreement. They are compared with the theoretical computations based on collinear factorisation with different nPDFs described in the previous section. The calculations with different nPDFs do not fully cover the experimental points within uncertainties in particular at low pTwith the exception of the EPS09LO combination, which has considerably larger uncertainties. However, a detailed analysis of theoretical correlations in the pT-dependent RFB may be interesting for future studies in order to quantify more precisely the discrepancies. The coherent energy loss calculation is compared with the rapidity dependence of the experimental data points in Fig. 10. It shows within its small uncertainties a slightly different slope from the experimental data points and predicts larger values in the bin at smallest |y∗|. The RFB ratio of J/ψ-from-b-hadrons in Fig. 9 shows a rising trend as a function of transverse momentum starting from a value 0.7 at low pTtowards values consistent with unity at high pT. The rapidity dependence of RFB in Fig. 10 is consistent with a flat behaviour with a central value of 0.8. 6. Conclusions The differential production cross-sections of prompt J/ψ and J/ψ-from-b-hadrons in pPb and Pbpcollisions at √sNN =8.16 TeV are measured in the range 0 <pT<14 GeV/c. The nuclear modification factors are similar to the findings at a collision energy of √sNN =5TeV, but with increased precision thanks to 10 and 40 times larger data sets in pPb and Pbpcollisions, respectively. A suppression of prompt J/ψ production compared to pp collisions of up to 50% (25%) in pPb (Pbp) at the lowest transverse momentum is observed. In both configurations, the nuclear modification factor approaches unity asymptotically at the highest pT. Theoretical calculations for the nuclear modification factor based on collinear factorisation with different nuclear parton distribution functions, coherent energy loss as well as the colour glass condensate model can account for the majority of the observed dependences. For the first time, beauty-hadron production is measured precisely down to pT=0at the LHC in pPb and Pbp collisions. In pPb, a weak suppression at the lowest transverse momenta is observed, whereas in Pbpno significant deviation from unity in the nuclear modification factor is found. This weak modification of beauty production in proton-ion collisions is an important ingredient for the investigation of the modifications of beauty production in heavy-ion collisions. Although the presented measurements have improved precision, it is not possible to single out the main nuclear modification mechanism between different phenomenological approaches for charmonium production in proton-lead collisions at the TeV scale. This measurement of J/ψ production is the first step towards measurements of other charmonium states as well as complementary observables like Drell–Yan production, to improve the understanding of quantum chromodynamics at low xand in dense nuclear environments. Acknowledgements We thank G. Bruno, B. Ducloué, H. Shao, J.-Ph. Lansberg and F. 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Zucchelli 15 1Centro Brasileiro de Pesquisas Físicas (CBPF), Rio de Janeiro, Brazil 2Universidade Federal do Rio de Janeiro (UFRJ), Rio de Janeiro, Brazil 3Center for High Energy Physics, Tsinghua University, Beijing, China 4LAPP, Université Savoie Mont-Blanc, CNRS/IN2P3, Annecy-Le-Vieux, France 5Clermont Université, Université Blaise Pascal, CNRS/IN2P3, LPC, Clermont-Ferrand, France 6CPPM, Aix-Marseille Université, CNRS/IN2P3, Marseille, France 7LAL, Université Paris-Sud, CNRS/IN2P3, Orsay, France 8LPNHE, Université Pierre et Marie Curie, Université Paris Diderot, CNRS/IN2P3, Paris, France 9I. Physikalisches Institut, RWTH Aachen University, Aachen, Germany 10 Fakultät Physik, Technische Universität Dortmund, Dortmund, Germany 11 Max-Planck-Institut für Kernphysik (MPIK), Heidelberg, Germany 12 Physikalisches Institut, Ruprecht-Karls-Universität Heidelberg, Heidelberg, Germany 13 School of Physics, University College Dublin, Dublin, Ireland 14 Sezione INFN di Bari, Bari, Italy 15 Sezione INFN di Bologna, Bologna, Italy 16 Sezione INFN di Cagliari, Cagliari, Italy 17 Universita e INFN, Ferrara, Ferrara, Italy 18 Sezione INFN di Firenze, Firenze, Italy 19 Laboratori Nazionali dell’INFN di Frascati, Frascati, Italy 20 Sezione INFN di Genova, Genova, Italy 21 Universita & INFN, Milano-Bicocca, Milano, Italy 22 Sezione di Milano, Milano, Italy 23 Sezione INFN di Padova, Padova, Italy 24 Sezione INFN di Pisa, Pisa, Italy 25 Sezione INFN di Roma Tor Vergata, Roma, Italy 26 Sezione INFN di Roma La Sapienza, Roma, Italy 27 Henryk Niewodniczanski Institute of Nuclear Physics Polish Academy of Sciences, Kraków, Poland 28 AGH – University of Science and Technology, Faculty of Physics and Applied Computer Science, Kraków, Poland 29 National Center for Nuclear Research (NCBJ), Warsaw, Poland 30 Horia Hulubei National Institute of Physics and Nuclear Engineering, Bucharest-Magurele, Romania 31 Petersburg Nuclear Physics Institute (PNPI), Gatchina, Russia 32 Institute of Theoretical and Experimental Physics (ITEP), Moscow, Russia 33 Institute of Nuclear Physics, Moscow State University (SINP MSU), Moscow, Russia 34 Institute for Nuclear Research of the Russian Academy of Sciences (INR RAN), Moscow, Russia 35 Yandex School of Data Analysis, Moscow, Russia 36 Budker Institute of Nuclear Physics (SB RAS), Novosibirsk, Russia 37 Institute for High Energy Physics (IHEP), Protvino, Russia 38 ICCUB, Universitat de Barcelona, Barcelona, Spain 39 Universidad de Santiago de Compostela, Santiago de Compostela, Spain 40 European Organization for Nuclear Research (CERN), Geneva, Switzerland 41 Institute of Physics, Ecole Polytechnique Fédérale de Lausanne (EPFL), Lausanne, Switzerland 42 Physik-Institut, Universität Zürich, Zürich, Switzerland 43 Nikhef National Institute for Subatomic Physics, Amsterdam, The Netherlands 44 Nikhef National Institute for Subatomic Physics and VU University Amsterdam, Amsterdam, The Netherlands 45 NSC Kharkiv Institute of Physics and Technology (NSC KIPT), Kharkiv, Ukraine 46 Institute for Nuclear Research of the National Academy of Sciences (KINR), Kyiv, Ukraine 47 University of Birmingham, Birmingham, United Kingdom 48 H.H. Wills Physics Laboratory, University of Bristol, Bristol, United Kingdom 49 Cavendish Laboratory, University of Cambridge, Cambridge, United Kingdom 50 Department of Physics, University of Warwick, Coventry, United Kingdom 51 STFC Rutherford Appleton Laboratory, Didcot, United Kingdom 52 School of Physics and Astronomy, University of Edinburgh, Edinburgh, United Kingdom 53 School of Physics and Astronomy, University of Glasgow, Glasgow, United Kingdom 178 LHCb Collaboration / Physics Letters B 774 (2017) 159–178 54 Oliver Lodge Laboratory, University of Liverpool, Liverpool, United Kingdom 55 Imperial College London, London, United Kingdom 56 School of Physics and Astronomy, University of Manchester, Manchester, United Kingdom 57 Department of Physics, University of Oxford, Oxford, United Kingdom 58 Massachusetts Institute of Technology, Cambridge, MA, United States 59 University of Cincinnati, Cincinnati, OH, United States 60 University of Maryland, College Park, MD, United States 61 Syracuse University, Syracuse, NY, United States 62 Pontifícia Universidade Católica do Rio de Janeiro (PUC-Rio), Rio de Janeiro, Brazil x 63 University of Chinese Academy of Sciences, Beijing, China y 64 School of Physics and Technology, Wuhan University, Wuhan, China y 65 Institute of Particle Physics, Central China Normal University, Wuhan, Hubei, China y 66 Departamento de Fisica, Universidad Nacional de Colombia, Bogota, Colombia z 67 Institut für Physik, Universität Rostock, Rostock, Germany aa 68 National Research Centre Kurchatov Institute, Moscow, Russia ab 69 National Research Tomsk Polytechnic University, Tomsk, Russia ab 70 Instituto de Fisica Corpuscular, Centro Mixto Universidad de Valencia – CSIC, Valencia, Spain ac 71 Van Swinderen Institute, University of Groningen, Groningen, The Netherlands ad *Corresponding author. E-mail address: [email protected]h (M.A. Winn). aUniversidade Federal do Triângulo Mineiro (UFTM), Uberaba-MG, Brazil. bLaboratoire Leprince-Ringuet, Palaiseau, France. cP.N. Lebedev Physical Institute, Russian Academy of Science (LPI RAS), Moscow, Russia. dUniversità di Bari, Bari, Italy. eUniversità di Bologna, Bologna, Italy. fUniversità di Cagliari, Cagliari, Italy. gUniversità di Ferrara, Ferrara, Italy. hUniversità di Genova, Genova, Italy. iUniversità di Milano Bicocca, Milano, Italy. jUniversità di Roma Tor Vergata, Roma, Italy. kUniversità di Roma La Sapienza, Roma, Italy. lAGH -University of Science and Technology, Faculty of Computer Science, Electronics and Telecommunications, Kraków, Poland. mLIFAELS, La Salle, Universitat Ramon Llull, Barcelona, Spain. nHanoi University of Science, Hanoi, Viet Nam. oUniversità di Padova, Padova, Italy. pUniversità di Pisa, Pisa, Italy. qUniversità degli Studi di Milano, Milano, Italy. rUniversità di Urbino, Urbino, Italy. sUniversità della Basilicata, Potenza, Italy. tScuola Normale Superiore, Pisa, Italy. uUniversità di Modena e Reggio Emilia, Modena, Italy. vIligan Institute of Technology (IIT), Iligan, Philippines. wNovosibirsk State University, Novosibirsk, Russia. xAssociated to Universidade Federal do Rio de Janeiro (UFRJ), Rio de Janeiro, Brazil. yAssociated to Center for High Energy Physics, Tsinghua University, Beijing, China. zAssociated to LPNHE, Université Pierre et Marie Curie, Université Paris Diderot, CNRS/IN2P3, Paris, France. aa Associated to Physikalisches Institut, Ruprecht-Karls-Universität Heidelberg, Heidelberg, Germany. ab Associated to Institute of Theoretical and Experimental Physics (ITEP), Moscow, Russia. ac Associated to ICCUB, Universitat de Barcelona, Barcelona, Spain. ad Associated to Nikhef National Institute for Subatomic Physics, Amsterdam, The Netherlands. †Deceased.