Measurement of electrons from heavy-flavour hadron decays in p-Pb collisions at s NN − − − √ =5.02 TeV
aa r X i v : . [ nu c l - e x ] M a r EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH
CERN-PH-EP-2015-26217 September 2015c (cid:13)
Measurement of electrons from heavy-flavour hadron decaysin p–Pb collisions at √ s NN = 5.02 TeV ALICE Collaboration ∗ Abstract
The production of electrons from heavy-flavour hadron decays was measured as a function of trans-verse momentum ( p T ) in minimum-bias p–Pb collisions at √ s NN = 5.02 TeV using the ALICE de-tector at the LHC. The measurement covers the p T interval 0 . < p T <
12 GeV / c and the rapidityrange − . < y cms < .
135 in the centre-of-mass reference frame. The contribution of electronsfrom background sources was subtracted using an invariant mass approach. The nuclear modificationfactor R pPb was calculated by comparing the p T -differential invariant cross section in p–Pb collisionsto a pp reference at the same centre-of-mass energy, which was obtained by interpolating measure-ments at √ s = 2.76 TeV and √ s = 7 TeV. The R pPb is consistent with unity within uncertainties ofabout 25%, which become larger for p T below 1 GeV / c . The measurement shows that heavy-flavourproduction is consistent with binary scaling, so that a suppression in the high- p T yield in Pb–Pb col-lisions has to be attributed to effects induced by the hot medium produced in the final state. The datain p–Pb collisions are described by recent model calculations that include cold nuclear matter effects. ∗ See Appendix A for the list of collaboration members easurement of electrons from heavy-flavour hadron decays in p–Pb collisions ALICE Collaboration
The Quark-Gluon Plasma (QGP) [1, 2], a colour-deconfined state of strongly-interacting matter, ispredicted to exist at high temperature according to lattice Quantum Chromodynamics (QCD) calcula-tions [3]. These conditions can be reached in ultra-relativistic heavy-ion collisions [4–10]. Charm andbeauty (heavy-flavour) quarks are mostly produced in initial hard scattering processes on a very shorttime scale, shorter than the formation time of the QGP medium [11], and thus experience the full tem-poral and spatial evolution of the collision. While interacting with the QGP medium, heavy quarkslose energy via elastic and radiative processes [12–14]. Heavy-flavour hadrons are therefore well-suitedprobes to study the properties of the QGP. The effect of energy loss on heavy-flavour production can becharacterised via the nuclear modification factor ( R AA ) of heavy-flavour hadrons. The R AA is definedas the ratio of the heavy-flavour hadron yield in nucleus–nucleus (A–A) collisions to that in proton–proton (pp) collisions scaled by the average number of binary nucleon–nucleon collisions. The R AA isstudied differentially as a function of transverse momentum ( p T ), rapidity ( y ) and collision centrality. Itwas measured at the Relativistic Heavy Ion Collider (RHIC) [15–18] and at the Large Hadron Collider(LHC) [19–22]. At RHIC, in central Au–Au collisions at √ s NN = 200 GeV the R AA of charmed mesonsand of electrons from heavy-flavour hadron decays shows that their production is strongly suppressed bya factor of about 5 for p T > / c at mid-rapidity. For the most central Pb–Pb collisions at √ s NN =2.76 TeV at the LHC, a suppression by a factor of 5–6 is observed for charmed mesons for p T > / c at mid-rapidity [22].The interpretation of the measurements in A–A collisions requires the study of heavy-flavour produc-tion in p–A collisions, which provides access to cold nuclear matter (CNM) effects. These effects arenot related to the formation of a colour-deconfined medium, but are present in case of colliding nuclei(or proton–nucleus). An important CNM effect in the initial state is parton-density shadowing or satu-ration, which can be described using modified parton distribution functions (PDF) in the nucleus [23]or using the Color Glass Condensate (CGC) effective theory [24]. Further CNM effects include energyloss [25] in the initial and final states and a Cronin-like enhancement [26] as a consequence of multiplescatterings [25, 27].The influence of the CNM effects can be studied by measuring the nuclear modification factor R pA . Likethe R AA , the R pA is defined such that it is unity if there are no nuclear effects. For minimum-bias p–Acollisions, it can be expressed as [28] R pA = A d s pA / d p T d s pp / d p T , (1)where d s pA / d p T and d s pp / d p T are the p T -differential production cross sections of a given particlespecies in p–A and pp collisions, respectively, and A is the number of nucleons in the nucleus.Cold nuclear matter effects were recently investigated at the RHIC and the LHC [29–44]. At RHIC,the nuclear modification factor of electrons from heavy-flavour hadron decays in central d–Au collisions(0–20%) at √ s NN = 200 GeV is larger than unity at mid-rapidity in the transverse momentum interval1 . < p T < / c [42]. The corresponding measurement for muons from heavy-flavour hadron decaysin central d–Au collisions shows a suppression at forward rapidity and an enhancement at backwardrapidity [43]. Theoretical models that include the modification of the PDF in the nucleus can neitherexplain the enhancement nor the large difference between forward and backward rapidity. Possibleexplanations include the Cronin-like enhancement [26] due to radial flow of heavy mesons [45]. Atthe LHC, the p T -differential nuclear modification factor R pPb of D mesons measured in p–Pb collisionsat √ s NN = 5.02 TeV [44] is consistent with unity for p T > / c and is described by theoreticalcalculations that include gluon saturation effects. Both at RHIC and at the LHC, the p/d–A measurementsindicate that initial-state effects alone cannot explain the strong suppression seen at high- p T in nucleus–nucleus collisions. 2easurement of electrons from heavy-flavour hadron decays in p–Pb collisions ALICE CollaborationIn this Letter, the p T -differential invariant cross section and the nuclear modification factor R pPb of elec-trons from heavy-flavour hadron decays measured in minimum-bias p–Pb collisions at √ s NN = 5.02 TeVwith ALICE at the LHC are presented. The measurement covers the rapidity range − . < y cms < .
135 in the centre-of-mass system (cms) for electrons with transverse momentum 0 . < p T <
12 GeV / c .This rapidity coverage results from the same rigidity of the p and Pb beams at the LHC, leading to a ra-pidity shift of | y NN | = 0.465 between the nucleon–nucleon cms and the laboratory reference frame, inthe direction of the p beam. At low p T , the measurement probes the production of charm-hadron de-cays [46], providing sensitivity to the gluon PDF in the regime of Bjorken- x of the order of 10 − [47],where a substantial shadowing effect is expected [48].To obtain the nuclear modification factor R pPb of electrons from heavy-flavour hadron decays, the p T -differential invariant cross section in p–Pb collisions at √ s NN = 5.02 TeV was compared to a pp ref-erence multiplied by 208, the Pb mass number. The pp reference was obtained by interpolating the p T -differential cross section measurements at √ s = .
76 TeV and 7 TeV.The Letter is organised as follows. The experimental apparatus, data sample and event selection aredescribed in Section 2. The electron reconstruction strategy and the pp reference spectrum are explainedin Sections 3 and 4, respectively. The measured p T -differential invariant cross section, the nuclear mod-ification factor R pPb of electrons from heavy-flavour hadron decays and comparison of R pPb to modelcalculations are reported in Section 5. A detailed description of the ALICE apparatus can be found in [49, 50]. Electrons are reconstructed atmid-rapidity using the central barrel detectors (described below) located inside a solenoid magnet, whichgenerates a magnetic field B = . | h lab | < | h lab | < | h lab | < E /d x . The Time-Of-Flight array (TOF), based on Multi-gap Resistive PlateChambers, covers the full azimuth and | h lab | < p T ≤ / c . The collision time, used for the calculation of the time-of-flight to the TOF detector,is measured by an array of Cherenkov counters, the T0 detector, located at +
350 cm and −
70 cm fromthe interaction point along the beam direction [54]. The Electromagnetic Calorimeter (EMCal), situatedbehind the TOF, is a sampling calorimeter based on Shashlik technology [55]. Its geometrical acceptanceis 107 ◦ in azimuth and | h lab | < h coverage were limited to100 ◦ and 0.6, respectively, to ensure uniform detector performance.The minimum-bias (MB) p–Pb data sample used in this analysis was collected in 2013. The trigger con-dition required a coincidence of signals between the two V0 scintillator hodoscopes, placed on either sideof the interaction point at 2.8 < h lab < − < h lab < − ±
10 cm from the centre of the interaction region along the beam direction were rejected.About 10% of the events do not fulfil this selection criterion. A sample of 100 million events passedthe offline event selection, corresponding to an integrated luminosity L int = . ± . m b − , given thecross section s V0MB = . ± .
07 b for the minimum-bias V0 trigger condition [56]. The efficiency forthe trigger condition and offline event selection is larger than 99% for non-single-diffractive (NSD) p–Pb collisions [57].
A combination of electron identification (eID) strategies with different detectors offers the largest p T reach for the measurement of electrons from heavy-flavour hadron decays. In particular, it ensures thatthe systematic uncertainties and the hadron contamination are small over the whole transverse momen-tum range. Throughout the paper, the term ‘electron’ is used for electrons and positrons. The capa-bility of the TPC to identify electrons via specific energy loss d E / d x in the detector was used over thewhole momentum range 0 . < p T <
12 GeV / c . However, it is subject to ambiguous identification ofhadrons (pions, kaons, protons and deuterons) below 2.5 GeV / c and above 6 GeV / c in transverse mo-mentum. At low transverse momentum (0 . < p T < . / c ), these ambiguities were resolved bymeasuring the time-of-flight of the particle from the interaction region to the TOF detector and combin-ing it with the momentum measurement, to determine the particle mass. In the high momentum region(6 < p T <
12 GeV / c ), the EMCal was used to reduce the hadron contamination. Electrons are separatedfrom hadrons by calculating the ratio of the energy deposited ( E ) in the EMCal to the momentum ( p ).Since electrons deposit all of their energy in the EMCal, the ratio E / p is around unity for electrons, whilethe ratio for charged hadrons is much smaller on average.The selection criteria for charged-particle tracks are similar to those applied in previous analyses mea-suring the production of electrons from heavy-flavour hadron decays in pp collisions [58, 59]. In orderto have optimal eID performance with the TPC, the analysis was restricted to the pseudorapidity range | h lab | < . < p T <
12 GeV / c . Upto a p T of 6 GeV / c , a signal in the innermost layer of the SPD was required in order to reduce thebackground from photon conversions. In addition, this selection was further constrained by requiringhits in both SPD layers, to reduce the number of incorrect matches between candidate tracks and hitsreconstructed in the first layer of the SPD. At high p T , where the EMCal was used, tracks with hits ineither of the SPD layers were selected in order to minimise the effect of dead areas of the first SPD layerwithin the acceptance region of the EMCal, as in previous analyses [58, 59].The electron identification with TPC and TOF was based on the number of standard deviations ( n TPC s or n TOF s ) for the specific energy loss and time-of-flight measurements, respectively. The n s variable iscomputed as a difference between the measured signal and the expected one for electrons divided bythe energy loss ( s TPC ) or time-of-flight ( s TOF ) resolution. The expected signal and resolution originatefrom parametrisations of the detector signal, which are described in detail in [50]. In the transversemomentum interval 0 . < p T < . / c , particles were identified as electrons if they satisfied − . < n TPC s <
3, which yields an identification efficiency of 69%. In the transverse momentum interval 2 . < p T < / c , a tighter selection criterion of 0 < n TPC s < p T ≤ / c ), only tracks with | n TOF s | < E / d x in the TPC with respect to the expected d E / d x for electronsnormalised to the expected resolution s TPC after the eID with TOF. The solid lines indicate the selectioncriteria used for the transverse momentum interval 0 . < p T < . / c , indicating that the hadron4easurement of electrons from heavy-flavour hadron decays in p–Pb collisions ALICE Collaboration ) c (GeV/ p T P C s e æ x / d E d Æ - x / d E d -10-8-6-4-2024 10 ALICE = 5.02 TeV NN s Pb, - p < 0.135 cms y - (a) p / E C oun t s Electron candidatesHadronsALICE = 5.02 TeV NN s Pb, - p < 0.135 cms y - c < 8 GeV/ T p (b) Fig. 1: (a): Measured d E / d x in the TPC as function of momentum p expressed as a deviation from the expected en-ergy loss of electrons, normalised by the energy-loss resolution ( s TPC ) after eID with TOF. The solid lines indicatethe n TPC s selection criteria for the TPC and TOF eID strategy. (b): E / p distribution of electrons ( − < n TPC s < n TPC s < − .
5) in the transverse momentum interval 6 < p T < / c . The E / p distribution ofhadrons was normalised to that of electrons in the lower E / p range (0.4–0.6), where hadrons dominate. The solidlines indicate the applied electron selection criteria. contamination within the resulting electron candidate sample is small. In the high momentum region(6 < p T <
12 GeV / c ), electrons were selected if they satisfied − < n TPC s < . < E / p < . p T ≤ / c as done in previous analyses [58, 59]. In the transversemomentum interval 6 < p T <
12 GeV / c , the E / p distribution for hadrons identified via the specificenergy loss measured in the TPC ( n TPC s < − .
5) was normalised in the lower E / p range (0.4–0.6) tothe corresponding E / p distribution for identified electrons ( − < n TPC s <
3) (see Fig. 1(b)). The num-ber of hadrons with an E / p ratio between 0.8 and 1.2 was thus determined in momentum slices. Thehadron contamination ranged from 2% at 5 < p T < / c to 15% at 10 < p T <
12 GeV / c and wascorrespondingly subtracted. For p T < / c , the contamination was found to be negligible.The resulting electron candidate sample, also referred to as the ‘inclusive electron sample’ in the fol-lowing, still contains electrons from sources other than heavy-flavour hadron decays. The majority ofthe remaining background originates from photon conversions in the detector material ( g → e + e − ) andDalitz decays of neutral mesons, e.g. p → g e + e − and h → g e + e − . These electrons are hereafterdenoted as ‘photonic electrons’.In previous analyses of electrons from heavy-flavour hadron decays in pp collisions by the ALICE Col-laboration, the contribution of electrons from background sources was estimated via a data-tuned MonteCarlo cocktail and subtracted from the inclusive electron sample [58, 59]. The pion input to the cocktailwas based on pion measurements with ALICE [60, 61], while heavier mesons were implemented via m T scaling [62], and photons from hard scattering processes (direct g , g ∗ ) were obtained from next-to-leading order (NLO) calculations [63]. The resulting systematic uncertainty of the sum of all backgroundsources was large, in particular at low p T , where the signal-to-background ratio is small [58, 59]. In or-der to reduce this uncertainty, in this analysis an invariant mass technique [16] was used to estimate thenumber of electrons coming from background sources.5easurement of electrons from heavy-flavour hadron decays in p–Pb collisions ALICE Collaboration ) c (GeV/ ee m C oun t s Unlike-signLike-signDifference: Photonic
ALICE = 5.02 TeV NN s Pb, - p < 0.135 cms y - ) pairs, - , e + (e c < 0.6 GeV/ eT p Fig. 2:
Invariant mass distributions of unlike-sign and like-sign electron pairs for the inclusive electron p T interval0 . < p T < . / c . The difference between the distributions yields the photonic contribution. Photonic electrons are produced in e + e − pairs and can thus be identified using an invariant mass tech-nique (photonic method). All inclusive electrons were paired with other tracks in the same event passinglooser track selection and electron identification criteria (e.g. − < n TPC s < . < p T < . / c . The like-sign distribution estimates the uncorrelated pairs. Subtracting thesefrom the unlike-sign pairs yields the number of electrons with a photonic partner N rawphot (see Fig. 2). Aninvariant mass smaller than 0.14 GeV/ c was required. According to simulations, the peak around zero inthe photonic electron pair distribution is due to photon conversions; the exponential tail to higher valuesoriginates from Dalitz decays of neutral mesons.The efficiency e phot to find photonic electron pairs was estimated using Monte Carlo simulations. Asample of p–Pb collisions was generated with HIJING v1.36 [64]. To increase the statistical precision athigh p T , one cc or bb pair decaying semileptonically using the generator PYTHIA v6.4.21 [65] with thePerugia-0 tune [66] was added in each event. The generated particles were propagated through the ap-paratus using GEANT3 [67] and a realistic detector response was applied to reproduce the performanceof the detector system during data taking period. The simulated transverse momentum distributions ofthe p and h mesons were weighted to match the measured shapes, where the p input was based on themeasured charged-pion spectra [68, 69] assuming N p = / ( N p + + N p − ) and the h input was derivedvia m T scaling. The efficiency e phot is defined as the fraction of electrons from photonic origin for whichthe partner could be found within the defined acceptance of the analysis, i.e. the geometrical acceptanceof the ALICE apparatus together with the superimposed track selection and electron identification cri-teria. The efficiency e phot increases sharply with p T from 35% to 80% between 0.5 and 3 GeV / c andremains at 80% up to 12 GeV / c . The raw photonic electron distribution N rawphot was then corrected by theefficiency e phot as N phot ( p T ) = N rawphot ( p T ) / e phot ( p T ) and subtracted from the inclusive electron yield toobtain the yield of electrons from heavy-flavour hadron decays. The signal-to-background ratio (ratio ofnon-photonic to photonic yield) ranges from 0.2 at 0.5 GeV/ c to 4 at 10 GeV/ c .The remaining electrons are then those from semileptonic heavy-flavour hadron decays ( N rawhfe ), besides asmall residual background contribution originating from semileptonic kaon decays and dielectron decaysof J/ y mesons. The latter is the only non-negligible contribution from quarkonia. These contributionswere subtracted from the corrected invariant cross section, as described later on in this section.6easurement of electrons from heavy-flavour hadron decays in p–Pb collisions ALICE CollaborationThe p T -differential invariant cross section s hfe of electrons from heavy-flavour hadron decays, 1 / ( e + + e − ) ,was calculated as 12 p p T d s hfe d p T d y =
12 1
D j p centreT D y D p T c unfold N rawhfe ( e geo × e reco × e eID ) s V0MB N MB , (2)where p centreT are the centres of the p T bins with widths D p T , and D j and D y denote the geometricalacceptance in azimuth and rapidity to which the analysis was restricted, respectively. N MB is the numberof events that pass the selection criteria described in Section 2 and s V0MB is the p–Pb cross section for theminimum-bias V0 trigger condition. The raw spectrum of electrons from heavy-flavour hadron decays( N rawhfe ) was corrected for the acceptance of the detectors in the selected geometrical region of the analysis( e geo ), the track reconstruction and selection efficiency ( e reco ), and the eID efficiency ( e eID ). Thesecorrections were computed using the aforementioned Monte Carlo simulations. Only the efficiency ofthe TPC electron identification selection criterion for p T < / c was determined using a data-drivenapproach based on the n TPC s distribution [59]. The measurement of the electron p T is affected by the finitemomentum resolution and by electron energy loss due to bremsstrahlung in the detector material [49],which is not corrected for in the track reconstruction algorithm. These effects distort the shape of the p T distribution, which falls steeply with increasing momentum. To determine this correction ( c unfold ), aniterative unfolding procedure based on Bayes’ theorem was applied [70, 71].The aforementioned residual background contributions, electrons from semileptonic kaon decays anddielectron decays of J/ y mesons, were estimated as an invariant cross section with Monte Carlo simula-tions and found to be less than 3% per p T bin and subtracted from the corrected invariant cross sectionof non-photonic electrons. More specifically, the contribution from J/ y mesons was implemented byusing a parametrisation for pp collisions based on the interpolation of J/ y measurements from RHICat √ s = 200 GeV, Tevatron at √ s = 1.96 TeV, and the LHC at √ s = 7 TeV according to [72]. Decaysof J/ y mesons within | y lab | < y cross section in p–Pb collisions [38].The systematic uncertainties were estimated as a function of p T by repeating the analysis with differ-ent selection criteria. The systematic uncertainties were evaluated for the spectrum obtained after thesubtraction of the photonic yield N phot from the inclusive spectrum and before removing the remain-ing background contributions originating from semileptonic kaon decays and dielectron decays of J/ y mesons. The sources of systematic uncertainty for the inclusive analysis and the determination of theelectron background are listed in Table 1.The systematic uncertainties for tracking and eID are p T dependent due to the usage of the various detec-tors in the different momentum intervals. The latter also includes the uncertainties due to the determina-tion of the hadron contamination. The 3% systematic uncertainty for the matching between ITS and TPCwas taken from [73], where the matching efficiency of charged particles in data was compared to MonteCarlo simulations. The uncertainty of the TOF-TPC matching efficiency was estimated by comparingthe matching efficiency in data and Monte Carlo simulations using electrons from photon conversions,which were identified via topological cuts. The uncertainty amounts to 3%. The TPC-EMCal matchinguncertainty was assigned to be 1%, as determined by varying the size of the matching window in h andazimuth j for charged-particle tracks that were extrapolated to the calorimeter. The resulting matchinguncertainties were combined in quadrature for the various p T intervals shown in Table 1.The listed uncertainties for the photonic method include the uncertainties on eID and tracking. In addi-tion, the Monte Carlo sample was divided into two halves. The first was treated as real data and the sec-ond was used to correct the resulting spectrum. Deviations from the expected p T spectrum of electrons7easurement of electrons from heavy-flavour hadron decays in p–Pb collisions ALICE Collaborationfrom heavy-flavour hadron decays resulted in a 2% systematic uncertainty for p T ≤ c and 4%above. The uncertainty on the re-weighting of the p - and h -meson p T distributions in Monte Carlo sim-ulations was estimated by changing the weights by ± p T ≤ c on the p T -differential invariant cross section of electrons from heavy-flavour hadron de-cays. This source of uncertainty is negligible at higher p T . The invariant mass technique gives a system-atic uncertainty smaller by a factor of ≥ p T ≤ / c and 3 < p T <
12 GeV / c ,respectively, compared to the one of the cocktail subtraction method [59]. The reduction in uncertainty,in particular at low p T , proves the advantage of using the invariant mass technique for the estimation ofelectrons from background sources.The uncertainty of the p T unfolding procedure was determined by employing an alternative unfoldingmethod (matrix inversion) and, as described in [59], by correcting the data with two different Monte Carlosamples corresponding to different p T distributions. In addition to the aforementioned signal-enhancedMonte Carlo sample, a minimum-bias sample was used. The comparison of the resulting p T spectrarevealed an uncertainty of 1% for p T ≤ / c , and smaller than 1% above 6 GeV / c . The systematicuncertainties of the heavy-flavour electron yield due to the subtraction of the remaining backgroundoriginating from semileptonic kaon decays and dielectron decays from J/ y mesons are smaller than0.5%. This was estimated by changing the particle yields by ±
50% and ± y meson andthe semileptonic kaon decays, respectively.The individual sources of systematic uncertainties are uncorrelated. Therefore, they were added inquadrature to give a total systematic uncertainty ranging from 5.8% to 16.4% depending on the p T bin.The normalisation uncertainty on the luminosity is of 3.7% [56]. Variable 0 . < p T < . / c . < p T < / c < p T <
12 GeV / c Tracking 4.3% 2.2% 3%Matching 4.2% 3% 3.2%eID 3.6% 3.6% 3.2% (6–8 GeV / c )5.1% (8–10 GeV / c )15.1% (10–12 GeV / c )Photonicmethod 6.9% (0.5–1 GeV / c )3.7% (1–2.5 GeV / c ) 2.4% 4.5%Unfolding 1% 1% < / c )8.0% (1–2.5 GeV / c ) 5.8% 7.1% (6–8 GeV / c )8.1% (8–10 GeV / c )16.4% (10–12 GeV / c ) Table 1:
Systematic uncertainties for the different momentum intervals.
Figure 3 shows the interval 2 . < p T < / c of the p T -differential invariant cross section of electronsfrom heavy-flavour hadron decays in minimum-bias p–Pb collisions at √ s NN = 5.02 TeV, comparingthe results of the various eID strategies in the two transition regions at 2.5 GeV / c and 6 GeV / c . Aconsistency within 1% is found. In order to calculate the nuclear modification factor R pPb , a reference cross section for pp collisions atthe same centre-of-mass energy is needed. Since pp data at √ s = .
02 TeV are currently not available,the reference was obtained by interpolating the p T -differential cross sections of electrons from heavy-flavour hadron decays measured in pp collisions at √ s = .
76 TeV and at √ s = . < p T <
12 GeV / c . While the √ s = .
76 TeV analysis was carried out in this p T range, the √ s = p T interval 0 . < p T < / c . Thus, to extend the p T interval up to 12 GeV / c a measurement8easurement of electrons from heavy-flavour hadron decays in p–Pb collisions ALICE Collaboration ) c (GeV/ T p ) ) c ) ( m b / ( G e V / y d T p / ( d s ) d T p p / ( -4 -3 -2 -1 TPC+TOF eIDTPC eIDTPC+EMCal eID
ALICE = 5.02 TeV NN s Pb, - p < 0.135 cms y - )/2, - + e + (e fi c,b Fig. 3:
The p T -differential invariant cross section of electrons from heavy-flavour hadron decays in minimum-biasp–Pb collisions at √ s NN = 5.02 TeV, comparing the results of the eID strategies in the two transition regions at 2.5and 6 GeV / c . The centre values are slightly shifted along the p T -axis in the transition regions for better visibility.The results agree within 1%. Details on the eID strategies can be found in the text. by the ATLAS Collaboration in the p T interval 7 < p T <
12 GeV / c was used [74]. The publishedATLAS measurement, d s /d p T , was divided by 1/(2 p p centreT D y ), where p centreT denotes the central valuesof the p T bins, and D y the rapidity range covered by the measurement. In the overlap interval 7 < p T < / c the mboxALICE and ATLAS measurements, which agree within uncertainties, werecombined as a weighted average. The inverse quadratic sum of statistical and systematic uncertainties ofthe two spectra were used as weights. Perturbative QCD (pQCD) calculations at fixed order with next-to-leading-log (FONLL) resummation [75–77] describe all aforementioned pp results [58, 59] withinexperimental and theoretical uncertainties. The pp references are measured in a symmetric rapiditywindow ( | y cms | < . √ s = .
76 TeV and | y cms | < . √ s = √ s dependence of the heavy-flavour production cross sections is required for theinterpolation. Calculations based on pQCD are consistent with a power-law scaling of the heavy-flavourproduction cross section with √ s [78]. Therefore, this scaling was used to calculate the interpolateddata points. The statistical uncertainties of the spectra at √ s = .
76 TeV and √ s = √ s interpolation. The weighted correlated systematic un-certainties (tracking, matching and eID) of the spectra at √ s = .
76 TeV and √ s = √ s interpolation.The uncorrelated and correlated uncertainties were then added in quadrature.The systematic uncertainty of the bin-by-bin interpolation procedure was added in quadrature to theprevious ones. It was estimated by using a linear or exponential dependence on √ s instead of a powerlaw. The ratios of the resulting p T spectra to the baseline pp reference were used to estimate a systematicuncertainty of + − %.The resulting pp reference cross section is well described by FONLL calculations. The systematic uncer-9easurement of electrons from heavy-flavour hadron decays in p–Pb collisions ALICE Collaboration ) c (GeV/ T p ) ) c ) ( m b / ( G e V / y d T p / ( d s ) d T p p / ( - - - - - - - - = 5.02 TeV NN s Pb, - p = 5.02 TeV s pp interpolated to ALICE)/2 - + e + (e fi c,b Fig. 4:
The p T -differential invariant cross section of electrons from heavy-flavour hadron decays in minimum-biasp–Pb collisions at √ s NN = 5.02 TeV. The pp reference obtained via the interpolation method is shown, not scaledby A , for comparison. The statistical uncertainties are indicated for both spectra by error bars, the systematicuncertainties are shown as boxes. tainties of the normalisations related to the determination of the minimum-bias nucleon–nucleon crosssections of the input spectra were likewise interpolated, yielding a normalisation uncertainty of 2.3% forthe pp reference spectrum, assuming that they are uncorrelated. The p T -differential invariant cross section of electrons from heavy-flavour hadron decays in the rapidityrange − . < y cms < .
135 for p–Pb collisions at √ s NN = 5.02 TeV is shown in Fig. 4 and comparedwith the pp reference cross section. The vertical bars represent the statistical uncertainties, while theboxes indicate the systematic uncertainties. The systematic uncertainties of the p–Pb cross section aresmaller than those of the pp cross section, in particular at low transverse momentum, mainly as a conse-quence of the estimation of the electron background via the invariant mass technique. For the pp analysis,the background was subtracted via the cocktail method. At low p T , the electrons mainly originate fromcharm-hadron decays, while for p T ≥ c beauty-hadron decays are the dominant source in ppcollisions [46].The nuclear modification factor R pPb of electrons from heavy-flavour hadron decays as a function oftransverse momentum is shown in Fig. 5. The statistical and systematic uncertainties of the spectrain p–Pb and pp were propagated as independent uncertainties. The normalisation uncertainties of thepp reference and the p–Pb spectrum were added in quadrature and are shown as a filled box at hightransverse momentum in Fig. 5.The R pPb is consistent with unity within uncertainties over the whole p T range of the measurement. Theproduction of electrons from heavy-flavour hadron decays is thus consistent with binary collision scalingof the reference spectrum for pp collisions at the same centre-of-mass energy. The suppression of theyield of heavy-flavour production in Pb–Pb collisions at high- p T is therefore a final state effect inducedby the produced hot medium. 10easurement of electrons from heavy-flavour hadron decays in p–Pb collisions ALICE Collaboration ) c (GeV/ T p p P b R Kang et al.: incoherent multiple scatteringSharma et al.: coherent scattering + CNMFONLL + EPS09NLO shad.Blast wave calculationNormalisation uncertaintyKang et al.: incoherent multiple scatteringSharma et al.: coherent scattering + CNMFONLL + EPS09NLO shad.Blast wave calculationNormalisation uncertainty ALICE = 5.02 TeV NN s Pb, - p < 0.135 cms y - )/2, - + e + (e fi c,b Fig. 5:
Nuclear modification factor R pPb of electrons from heavy-flavour hadron decays as a function of transversemomentum for minimum-bias p–Pb collisions at √ s NN = 5.02 TeV, compared with theoretical models [25, 27, 45,48, 75], as described in the text. The vertical bars represent the statistical uncertainties, and the boxes indicate thesystematic uncertainties. The systematic uncertainty from the normalisation, common to all points, is shown as afilled box at high p T . Given the large systematic uncertainties, our measurement is also compatible with an enhancement in thetransverse momentum interval 1 < p T < / c as seen at mid-rapidity in d–Au collisions at √ s NN =
200 GeV [42]. Such an enhancement might be caused by radial flow as suggested by studies on the mean p T as a function of the identified particle multiplicity [68].The data are described within the uncertainties by pQCD calculations including initial-state effects(FONLL [75] + EPS09NLO [48] nuclear shadowing parametrisation). The results suggest that initial-state effects are small at high transverse momentum in Pb–Pb collisions. Calculations by Sharma etal. which include CNM energy loss, nuclear shadowing and coherent multiple scattering at the partoniclevel also describe the data [27]. Calculations based on incoherent multiple scatterings by Kang et al. predict an enhancement at low p T [25]. The formation of a hydrodynamically expanding medium andconsequently flow of charm and beauty quarks are expected to result in an enhancement in the nuclearmodification factor R pPb [45]. To quantify the possible effect on R pPb , a blast wave calculation with pa-rameters extracted from fits to the p T spectra of light-flavour hadrons [68] measured in p–Pb collisionswas employed. The model calculation agrees with the data. However, the present uncertainties of themeasurement do not allow us to discriminate among the aforementioned theoretical approaches. The p T -differential invariant cross section for electrons from heavy-flavour hadron decays in minimum-bias p–Pb collisions at √ s NN = 5.02 TeV was measured in the rapidity range − . < y cms < .
135 andthe transverse momentum interval 0 . < p T <
12 GeV / c using the combination of three electron iden-tification methods. The application of the invariant mass technique to subtract electrons not originatingfrom open heavy-flavour hadron decays largely reduced the systematic uncertainties with respect to thecocktail subtraction method, in particular at low transverse momentum. The pp reference for the nuclearmodification factor R pPb was obtained by interpolating the measured p T -differential cross sections of11easurement of electrons from heavy-flavour hadron decays in p–Pb collisions ALICE Collaborationelectrons from heavy-flavour hadron decays at √ s = 2.76 TeV and √ s = 7 TeV. The R pPb is consistentwith unity within uncertainties of about 25%, which become larger for p T below 1 GeV / c . The presentedcalculations describe the data within uncertainties. The results suggest that heavy-flavour production inminimum-bias p–Pb collisions scales with the number of binary collisions, although within uncertain-ties the data are also consistent with an enhancement above this scaling. The consistency with unity ofthe R pPb at high p T indicates that the suppression of heavy-flavour production in Pb–Pb collisions is ofdifferent origin than cold nuclear matter effects. Acknowledgements
The ALICE Collaboration would like to thank all its engineers and technicians for their invaluable con-tributions to the construction of the experiment and the CERN accelerator teams for the outstandingperformance of the LHC complex. The ALICE Collaboration gratefully acknowledges the resources andsupport provided by all Grid centres and the Worldwide LHC Computing Grid (WLCG) Collaboration.The ALICE Collaboration acknowledges the following funding agencies for their support in buildingand running the ALICE detector: State Committee of Science, World Federation of Scientists (WFS)and Swiss Fonds Kidagan, Armenia; Conselho Nacional de Desenvolvimento Cient´ıfico e Tecnol´ogico(CNPq), Financiadora de Estudos e Projetos (FINEP), Fundac¸ ˜ao de Amparo `a Pesquisa do Estado de S˜aoPaulo (FAPESP); National Natural Science Foundation of China (NSFC), the Chinese Ministry of Edu-cation (CMOE) and the Ministry of Science and Technology of China (MSTC); Ministry of Educationand Youth of the Czech Republic; Danish Natural Science Research Council, the Carlsberg Foundationand the Danish National Research Foundation; The European Research Council under the EuropeanCommunity’s Seventh Framework Programme; Helsinki Institute of Physics and the Academy of Fin-land; French CNRS-IN2P3, the ‘Region Pays de Loire’, ‘Region Alsace’, ‘Region Auvergne’ and CEA,France; German Bundesministerium fur Bildung, Wissenschaft, Forschung und Technologie (BMBF)and the Helmholtz Association; General Secretariat for Research and Technology, Ministry of Develop-ment, Greece; Hungarian Orszagos Tudomanyos Kutatasi Alappgrammok (OTKA) and National Officefor Research and Technology (NKTH); Department of Atomic Energy and Department of Science andTechnology of the Government of India; Istituto Nazionale di Fisica Nucleare (INFN) and Centro Fermi- Museo Storico della Fisica e Centro Studi e Ricerche “Enrico Fermi”, Italy; MEXT Grant-in-Aidfor Specially Promoted Research, Japan; Joint Institute for Nuclear Research, Dubna; National ResearchFoundation of Korea (NRF); Consejo Nacional de Cienca y Tecnologia (CONACYT), Direccion Generalde Asuntos del Personal Academico(DGAPA), M´exico, Amerique Latine Formation academique - Euro-pean Commission (ALFA-EC) and the EPLANET Program (European Particle Physics Latin AmericanNetwork); Stichting voor Fundamenteel Onderzoek der Materie (FOM) and the Nederlandse Organisatievoor Wetenschappelijk Onderzoek (NWO), Netherlands; Research Council of Norway (NFR); NationalScience Centre, Poland; Ministry of National Education/Institute for Atomic Physics and National Coun-cil of Scientific Research in Higher Education (CNCSI-UEFISCDI), Romania; Ministry of Educationand Science of Russian Federation, Russian Academy of Sciences, Russian Federal Agency of AtomicEnergy, Russian Federal Agency for Science and Innovations and The Russian Foundation for Basic Re-search; Ministry of Education of Slovakia; Department of Science and Technology, South Africa; Centrode Investigaciones Energeticas, Medioambientales y Tecnologicas (CIEMAT), E-Infrastructure sharedbetween Europe and Latin America (EELA), Ministerio de Econom´ıa y Competitividad (MINECO) ofSpain, Xunta de Galicia (Conseller´ıa de Educaci´on), Centro de Aplicaciones Tecnolgicas y DesarrolloNuclear (CEADEN), Cubaenerg´ıa, Cuba, and IAEA (International Atomic Energy Agency); SwedishResearch Council (VR) and Knut & Alice Wallenberg Foundation (KAW); Ukraine Ministry of Edu-cation and Science; United Kingdom Science and Technology Facilities Council (STFC); The UnitedStates Department of Energy, the United States National Science Foundation, the State of Texas, and theState of Ohio; Ministry of Science, Education and Sports of Croatia and Unity through Knowledge Fund,Croatia; Council of Scientific and Industrial Research (CSIR), New Delhi, India; Pontificia Universidad12easurement of electrons from heavy-flavour hadron decays in p–Pb collisions ALICE CollaborationCat´olica del Per´u.
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J. Adam , D. Adamov´a , M.M. Aggarwal , G. Aglieri Rinella , M. Agnello , N. Agrawal ,Z. Ahammed , S.U. Ahn , S. Aiola , A. Akindinov , S.N. Alam , D. Aleksandrov , B. Alessandro ,D. Alexandre , R. Alfaro Molina , A. Alici
12 ,104 , A. Alkin , J.R.M. Almaraz , J. Alme , T. Alt ,S. Altinpinar , I. Altsybeev , C. Alves Garcia Prado , C. Andrei , A. Andronic , V. Anguelov ,J. Anielski , T. Antiˇci´c , F. Antinori , P. Antonioli , L. Aphecetche , H. Appelsh¨auser , S. Arcelli ,R. Arnaldi , O.W. Arnold
37 ,92 , I.C. Arsene , M. Arslandok , B. Audurier , A. Augustinus ,R. Averbeck , M.D. Azmi , A. Badal`a , Y.W. Baek , S. Bagnasco , R. Bailhache , R. Bala ,A. Baldisseri , R.C. Baral , A.M. Barbano , R. Barbera , F. Barile , G.G. Barnaf¨oldi , L.S. Barnby ,V. Barret , P. Bartalini , K. Barth , J. Bartke , E. Bartsch , M. Basile , N. Bastid , S. Basu ,B. Bathen , G. Batigne , A. Batista Camejo , B. Batyunya , P.C. Batzing , I.G. Bearden , H. Beck ,C. Bedda , N.K. Behera , I. Belikov , F. Bellini , H. Bello Martinez , R. Bellwied , R. Belmont ,E. Belmont-Moreno , V. Belyaev , G. Bencedi , S. Beole , I. Berceanu , A. Bercuci , Y. Berdnikov ,D. Berenyi , R.A. Bertens , D. Berzano , L. Betev , A. Bhasin , I.R. Bhat , A.K. Bhati ,B. Bhattacharjee , J. Bhom , L. Bianchi , N. Bianchi , C. Bianchin
57 ,134 , J. Bielˇc´ık , J. Bielˇc´ıkov´a ,A. Bilandzic , R. Biswas , S. Biswas , S. Bjelogrlic , J.T. Blair , D. Blau , C. Blume , F. Bock
93 ,74 ,A. Bogdanov , H. Bøggild , L. Boldizs´ar , M. Bombara , J. Book , H. Borel , A. Borissov ,M. Borri
82 ,124 , F. Boss´u , E. Botta , S. B¨ottger , C. Bourjau , P. Braun-Munzinger , M. Bregant ,T. Breitner , T.A. Broker , T.A. Browning , M. Broz , E.J. Brucken , E. Bruna , G.E. Bruno ,D. Budnikov , H. Buesching , S. Bufalino
27 ,36 , P. Buncic , O. Busch
93 ,128 , Z. Buthelezi , J.B. Butt ,J.T. Buxton , D. Caffarri , X. Cai , H. Caines , L. Calero Diaz , A. Caliva , E. Calvo Villar ,P. Camerini , F. Carena , W. Carena , F. Carnesecchi , J. Castillo Castellanos , A.J. Castro ,E.A.R. Casula , C. Ceballos Sanchez , J. Cepila , P. Cerello , J. Cerkala , B. Chang , S. Chapeland ,M. Chartier , J.L. Charvet , S. Chattopadhyay , S. Chattopadhyay , V. Chelnokov , M. Cherney ,C. Cheshkov , B. Cheynis , V. Chibante Barroso , D.D. Chinellato , S. Cho , P. Chochula ,K. Choi , M. Chojnacki , S. Choudhury , P. Christakoglou , C.H. Christensen , P. Christiansen ,T. Chujo , S.U. Chung , C. Cicalo , L. Cifarelli
12 ,28 , F. Cindolo , J. Cleymans , F. Colamaria ,D. Colella
59 ,33 ,36 , A. Collu
74 ,25 , M. Colocci , G. Conesa Balbastre , Z. Conesa del Valle ,M.E. Connors ,ii,136 , J.G. Contreras , T.M. Cormier , Y. Corrales Morales , I. Cort´es Maldonado ,P. Cortese , M.R. Cosentino , F. Costa , P. Crochet , R. Cruz Albino , E. Cuautle , L. Cunqueiro ,T. Dahms
92 ,37 , A. Dainese , A. Danu , D. Das , I. Das
51 ,100 , S. Das , A. Dash
121 ,79 , S. Dash ,S. De , A. De Caro
31 ,12 , G. de Cataldo , C. de Conti , J. de Cuveland , A. De Falco , D. DeGruttola
12 ,31 , N. De Marco , S. De Pasquale , A. Deisting
96 ,93 , A. Deloff , E. D´enes
135 ,i , C. Deplano ,P. Dhankher , D. Di Bari , A. Di Mauro , P. Di Nezza , M.A. Diaz Corchero , T. Dietel ,P. Dillenseger , R. Divi`a , Ø. Djuvsland , A. Dobrin
57 ,81 , D. Domenicis Gimenez , B. D¨onigus ,O. Dordic , T. Drozhzhova , A.K. Dubey , A. Dubla , L. Ducroux , P. Dupieux , R.J. Ehlers ,D. Elia , H. Engel , E. Epple , B. Erazmus , I. Erdemir , F. Erhardt , B. Espagnon ,M. Estienne , S. Esumi , J. Eum , D. Evans , S. Evdokimov , G. Eyyubova , L. Fabbietti
92 ,37 ,D. Fabris , J. Faivre , A. Fantoni , M. Fasel , L. Feldkamp , A. Feliciello , G. Feofilov ,J. Ferencei , A. Fern´andez T´ellez , E.G. Ferreiro , A. Ferretti , A. Festanti , V.J.G. Feuillard
15 ,70 ,J. Figiel , M.A.S. Figueredo
124 ,120 , S. Filchagin , D. Finogeev , F.M. Fionda , E.M. Fiore ,M.G. Fleck , M. Floris , S. Foertsch , P. Foka , S. Fokin , E. Fragiacomo , A. Francescon
30 ,36 ,U. Frankenfeld , U. Fuchs , C. Furget , A. Furs , M. Fusco Girard , J.J. Gaardhøje , M. Gagliardi ,A.M. Gago , M. Gallio , D.R. Gangadharan , P. Ganoti
88 ,36 , C. Gao , C. Garabatos , E. Garcia-Solis ,C. Gargiulo , P. Gasik
37 ,92 , E.F. Gauger , M. Germain , A. Gheata , M. Gheata
62 ,36 , P. Ghosh ,S.K. Ghosh , P. Gianotti , P. Giubellino
36 ,110 , P. Giubilato , E. Gladysz-Dziadus , P. Gl¨assel ,D.M. Gom´ez Coral , A. Gomez Ramirez , V. Gonzalez , P. Gonz´alez-Zamora , S. Gorbunov ,L. G¨orlich , S. Gotovac , V. Grabski , O.A. Grachov , L.K. Graczykowski , K.L. Graham ,A. Grelli , A. Grigoras , C. Grigoras , V. Grigoriev , A. Grigoryan , S. Grigoryan , B. Grinyov ,N. Grion , J.M. Gronefeld , J.F. Grosse-Oetringhaus , J.-Y. Grossiord , R. Grosso , F. Guber ,R. Guernane , B. Guerzoni , K. Gulbrandsen , T. Gunji , A. Gupta , R. Gupta , R. Haake ,Ø. Haaland , C. Hadjidakis , M. Haiduc , H. Hamagaki , G. Hamar , J.W. Harris , A. Harton ,D. Hatzifotiadou , S. Hayashi , S.T. Heckel , M. Heide , H. Helstrup , A. Herghelegiu , G. HerreraCorral , B.A. Hess , K.F. Hetland , H. Hillemanns , B. Hippolyte , R. Hosokawa , P. Hristov ,M. Huang , T.J. Humanic , N. Hussain , T. Hussain , D. Hutter , D.S. Hwang , R. Ilkaev ,M. Inaba , M. Ippolitov
75 ,99 , M. Irfan , M. Ivanov , V. Ivanov , V. Izucheev , P.M. Jacobs , M.B. Jadhav , S. Jadlovska , J. Jadlovsky
115 ,59 , C. Jahnke , M.J. Jakubowska , H.J. Jang ,M.A. Janik , P.H.S.Y. Jayarathna , C. Jena , S. Jena , R.T. Jimenez Bustamante , P.G. Jones ,H. Jung , A. Jusko , P. Kalinak , A. Kalweit , J. Kamin , J.H. Kang , V. Kaplin , S. Kar ,A. Karasu Uysal , O. Karavichev , T. Karavicheva , L. Karayan
93 ,96 , E. Karpechev , U. Kebschull ,R. Keidel , D.L.D. Keijdener , M. Keil , M. Mohisin Khan , P. Khan , S.A. Khan , A. Khanzadeev ,Y. Kharlov , B. Kileng , D.W. Kim , D.J. Kim , D. Kim , H. Kim , J.S. Kim , M. Kim ,M. Kim , S. Kim , T. Kim , S. Kirsch , I. Kisel , S. Kiselev , A. Kisiel , G. Kiss , J.L. Klay ,C. Klein , J. Klein
93 ,36 , C. Klein-B¨osing , S. Klewin , A. Kluge , M.L. Knichel , A.G. Knospe ,T. Kobayashi , C. Kobdaj , M. Kofarago , T. Kollegger
43 ,96 , A. Kolojvari , V. Kondratiev ,N. Kondratyeva , E. Kondratyuk , A. Konevskikh , M. Kopcik , M. Kour , C. Kouzinopoulos ,O. Kovalenko , V. Kovalenko , M. Kowalski , G. Koyithatta Meethaleveedu , I. Kr´alik ,A. Kravˇc´akov´a , M. Kretz , M. Krivda
59 ,101 , F. Krizek , E. Kryshen , M. Krzewicki , A.M. Kubera ,V. Kuˇcera , C. Kuhn , P.G. Kuijer , A. Kumar , J. Kumar , L. Kumar , S. Kumar , P. Kurashvili ,A. Kurepin , A.B. Kurepin , A. Kuryakin , M.J. Kweon , Y. Kwon , S.L. La Pointe , P. La Rocca ,P. Ladron de Guevara , C. Lagana Fernandes , I. Lakomov , R. Langoy , C. Lara , A. Lardeux ,A. Lattuca , E. Laudi , R. Lea , L. Leardini , G.R. Lee , S. Lee , F. Lehas , R.C. Lemmon ,V. Lenti , E. Leogrande , I. Le´on Monz´on , H. Le´on Vargas , M. Leoncino , P. L´evai , S. Li
70 ,7 ,X. Li , J. Lien , R. Lietava , S. Lindal , V. Lindenstruth , C. Lippmann , M.A. Lisa ,H.M. Ljunggren , D.F. Lodato , P.I. Loenne , V. Loginov , C. Loizides , X. Lopez , E. L´opez Torres ,A. Lowe , P. Luettig , M. Lunardon , G. Luparello , A. Maevskaya , M. Mager , S. Mahajan ,S.M. Mahmood , A. Maire , R.D. Majka , M. Malaev , I. Maldonado Cervantes , L. Malinina ,iii,66 ,D. Mal’Kevich , P. Malzacher , A. Mamonov , V. Manko , F. Manso , V. Manzari
36 ,103 ,M. Marchisone
27 ,65 ,126 , J. Mareˇs , G.V. Margagliotti , A. Margotti , J. Margutti , A. Mar´ın ,C. Markert , M. Marquard , N.A. Martin , J. Martin Blanco , P. Martinengo , M.I. Mart´ınez ,G. Mart´ınez Garc´ıa , M. Martinez Pedreira , A. Mas , S. Masciocchi , M. Masera , A. Masoni ,L. Massacrier , A. Mastroserio , A. Matyja , C. Mayer , J. Mazer , M.A. Mazzoni ,D. Mcdonald , F. Meddi , Y. Melikyan , A. Menchaca-Rocha , E. Meninno , J. Mercado P´erez ,M. Meres , Y. Miake , M.M. Mieskolainen , K. Mikhaylov
66 ,58 , L. Milano , J. Milosevic ,L.M. Minervini
103 ,23 , A. Mischke , A.N. Mishra , D. Mi´skowiec , J. Mitra , C.M. Mitu ,N. Mohammadi , B. Mohanty
79 ,132 , L. Molnar
55 ,113 , L. Monta˜no Zetina , E. Montes , D.A. Moreira DeGodoy
54 ,113 , L.A.P. Moreno , S. Moretto , A. Morreale , A. Morsch , V. Muccifora , E. Mudnic ,D. M¨uhlheim , S. Muhuri , M. Mukherjee , J.D. Mulligan , M.G. Munhoz , R.H. Munzer
92 ,37 ,S. Murray , L. Musa , J. Musinsky , B. Naik , R. Nair , B.K. Nandi , R. Nania , E. Nappi ,M.U. Naru , H. Natal da Luz , C. Nattrass , K. Nayak , T.K. Nayak , S. Nazarenko , A. Nedosekin ,L. Nellen , F. Ng , M. Nicassio , M. Niculescu , J. Niedziela , B.S. Nielsen , S. Nikolaev ,S. Nikulin , V. Nikulin , F. Noferini
12 ,104 , P. Nomokonov , G. Nooren , J.C.C. Noris , J. Norman ,A. Nyanin , J. Nystrand , H. Oeschler , S. Oh , S.K. Oh , A. Ohlson , A. Okatan , T. Okubo ,L. Olah , J. Oleniacz , A.C. Oliveira Da Silva , M.H. Oliver , J. Onderwaater , C. Oppedisano ,R. Orava , A. Ortiz Velasquez , A. Oskarsson , J. Otwinowski , K. Oyama
93 ,76 , M. Ozdemir ,Y. Pachmayer , P. Pagano , G. Pai´c , S.K. Pal , J. Pan , A.K. Pandey , P. Papcun , V. Papikyan ,G.S. Pappalardo , P. Pareek , W.J. Park , S. Parmar , A. Passfeld , V. Paticchio , R.N. Patra ,B. Paul , H. Pei , T. Peitzmann , H. Pereira Da Costa , E. Pereira De Oliveira Filho , D. Peresunko
99 ,75 ,C.E. P´erez Lara , E. Perez Lezama , V. Peskov , Y. Pestov , V. Petr´aˇcek , V. Petrov , M. Petrovici ,C. Petta , S. Piano , M. Pikna , P. Pillot , O. Pinazza
104 ,36 , L. Pinsky , D.B. Piyarathna ,M. Płosko´n , M. Planinic , J. Pluta , S. Pochybova , P.L.M. Podesta-Lerma , M.G. Poghosyan
84 ,86 ,B. Polichtchouk , N. Poljak , W. Poonsawat , A. Pop , S. Porteboeuf-Houssais , J. Porter ,J. Pospisil , S.K. Prasad , R. Preghenella
104 ,36 , F. Prino , C.A. Pruneau , I. Pshenichnov , M. Puccio ,G. Puddu , P. Pujahari , V. Punin , J. Putschke , H. Qvigstad , A. Rachevski , S. Raha , S. Rajput ,J. Rak , A. Rakotozafindrabe , L. Ramello , F. Rami , R. Raniwala , S. Raniwala , S.S. R¨as¨anen ,B.T. Rascanu , D. Rathee , K.F. Read
125 ,84 , K. Redlich , R.J. Reed , A. Rehman , P. Reichelt ,F. Reidt
93 ,36 , X. Ren , R. Renfordt , A.R. Reolon , A. Reshetin , J.-P. Revol , K. Reygers , V. Riabov ,R.A. Ricci , T. Richert , M. Richter , P. Riedler , W. Riegler , F. Riggi , C. Ristea , E. Rocco ,M. Rodr´ıguez Cahuantzi , A. Rodriguez Manso , K. Røed , E. Rogochaya , D. Rohr , D. R¨ohrich ,R. Romita , F. Ronchetti
72 ,36 , L. Ronflette , P. Rosnet , A. Rossi
30 ,36 , F. Roukoutakis , A. Roy ,C. Roy , P. Roy , A.J. Rubio Montero , R. Rui , R. Russo , E. Ryabinkin , Y. Ryabov ,A. Rybicki , S. Sadovsky , K. ˇSafaˇr´ık , B. Sahlmuller , P. Sahoo , R. Sahoo , S. Sahoo , P.K. Sahu , J. Saini , S. Sakai , M.A. Saleh , J. Salzwedel , S. Sambyal , V. Samsonov , L. ˇS´andor ,A. Sandoval , M. Sano , D. Sarkar , E. Scapparone , F. Scarlassara , C. Schiaua , R. Schicker ,C. Schmidt , H.R. Schmidt , S. Schuchmann , J. Schukraft , M. Schulc , T. Schuster , Y. Schutz
113 ,36 ,K. Schwarz , K. Schweda , G. Scioli , E. Scomparin , R. Scott , M. ˇSefˇc´ık , J.E. Seger ,Y. Sekiguchi , D. Sekihata , I. Selyuzhenkov , K. Senosi , S. Senyukov , E. Serradilla
10 ,64 ,A. Sevcenco , A. Shabanov , A. Shabetai , O. Shadura , R. Shahoyan , A. Shangaraev , A. Sharma ,M. Sharma , M. Sharma , N. Sharma , K. Shigaki , K. Shtejer , Y. Sibiriak , S. Siddhanta ,K.M. Sielewicz , T. Siemiarczuk , D. Silvermyr
84 ,34 , C. Silvestre , G. Simatovic , G. Simonetti ,R. Singaraju , R. Singh , S. Singha
132 ,79 , V. Singhal , B.C. Sinha , T. Sinha , B. Sitar , M. Sitta ,T.B. Skaali , M. Slupecki , N. Smirnov , R.J.M. Snellings , T.W. Snellman , C. Søgaard , J. Song ,M. Song , Z. Song , F. Soramel , S. Sorensen , F. Sozzi , M. Spacek , E. Spiriti , I. Sputowska ,M. Spyropoulou-Stassinaki , J. Stachel , I. Stan , G. Stefanek , E. Stenlund , G. Steyn , J.H. Stiller ,D. Stocco , P. Strmen , A.A.P. Suaide , T. Sugitate , C. Suire , M. Suleymanov , M. Suljic
26 ,i ,R. Sultanov , M. ˇSumbera , A. Szabo , A. Szanto de Toledo
120 ,i , I. Szarka , A. Szczepankiewicz ,M. Szymanski , U. Tabassam , J. Takahashi , G.J. Tambave , N. Tanaka , M.A. Tangaro ,M. Tarhini , M. Tariq , M.G. Tarzila , A. Tauro , G. Tejeda Mu˜noz , A. Telesca , K. Terasaki ,C. Terrevoli , B. Teyssier , J. Th¨ader , D. Thomas , R. Tieulent , A.R. Timmins , A. Toia ,S. Trogolo , G. Trombetta , V. Trubnikov , W.H. Trzaska , T. Tsuji , A. Tumkin , R. Turrisi ,T.S. Tveter , K. Ullaland , A. Uras , G.L. Usai , A. Utrobicic , M. Vajzer , M. Vala , L. ValenciaPalomo , S. Vallero , J. Van Der Maarel , J.W. Van Hoorne , M. van Leeuwen , T. Vanat , P. VandeVyvre , D. Varga , A. Vargas , M. Vargyas , R. Varma , M. Vasileiou , A. Vasiliev , A. Vauthier ,V. Vechernin , A.M. Veen , M. Veldhoen , A. Velure , M. Venaruzzo , E. Vercellin , S. VergaraLim´on , R. Vernet , M. Verweij , L. Vickovic , G. Viesti
30 ,i , J. Viinikainen , Z. Vilakazi ,O. Villalobos Baillie , A. Villatoro Tello , A. Vinogradov , L. Vinogradov , Y. Vinogradov
98 ,i ,T. Virgili , V. Vislavicius , Y.P. Viyogi , A. Vodopyanov , M.A. V¨olkl , K. Voloshin ,S.A. Voloshin , G. Volpe , B. von Haller , I. Vorobyev
37 ,92 , D. Vranic
96 ,36 , J. Vrl´akov´a ,B. Vulpescu , A. Vyushin , B. Wagner , J. Wagner , H. Wang , M. Wang , D. Watanabe ,Y. Watanabe , M. Weber
112 ,36 , S.G. Weber , D.F. Weiser , J.P. Wessels , U. Westerhoff ,A.M. Whitehead , J. Wiechula , J. Wikne , M. Wilde , G. Wilk , J. Wilkinson , M.C.S. Williams ,B. Windelband , M. Winn , C.G. Yaldo , H. Yang , P. Yang , S. Yano , C. Yasar , Z. Yin ,H. Yokoyama , I.-K. Yoo , J.H. Yoon , V. Yurchenko , I. Yushmanov , A. Zaborowska , V. Zaccolo ,A. Zaman , C. Zampolli , H.J.C. Zanoli , S. Zaporozhets , N. Zardoshti , A. Zarochentsev ,P. Z´avada , N. Zaviyalov , H. Zbroszczyk , I.S. Zgura , M. Zhalov , H. Zhang , X. Zhang ,Y. Zhang , C. Zhang , Z. Zhang , C. Zhao , N. Zhigareva , D. Zhou , Y. Zhou , Z. Zhou , H. Zhu ,J. Zhu
113 ,7 , A. Zichichi
28 ,12 , A. Zimmermann , M.B. Zimmermann
54 ,36 , G. Zinovjev , M. Zyzak Affiliation notes i Deceased ii Also at: Georgia State University, Atlanta, Georgia, United States iii
Also at: M.V. Lomonosov Moscow State University, D.V. Skobeltsyn Institute of Nuclear, Physics,Moscow, Russia
Collaboration Institutes A.I. Alikhanyan National Science Laboratory (Yerevan Physics Institute) Foundation, Yerevan, Armenia Benem´erita Universidad Aut´onoma de Puebla, Puebla, Mexico Bogolyubov Institute for Theoretical Physics, Kiev, Ukraine Bose Institute, Department of Physics and Centre for Astroparticle Physics and Space Science (CAPSS),Kolkata, India Budker Institute for Nuclear Physics, Novosibirsk, Russia California Polytechnic State University, San Luis Obispo, California, United States Central China Normal University, Wuhan, China Centre de Calcul de l’IN2P3, Villeurbanne, France Centro de Aplicaciones Tecnol´ogicas y Desarrollo Nuclear (CEADEN), Havana, Cuba Centro de Investigaciones Energ´eticas Medioambientales y Tecnol´ogicas (CIEMAT), Madrid, Spain Centro de Investigaci´on y de Estudios Avanzados (CINVESTAV), Mexico City and M´erida, Mexico Centro Fermi - Museo Storico della Fisica e Centro Studi e Ricerche “Enrico Fermi”, Rome, Italy Chicago State University, Chicago, Illinois, USA China Institute of Atomic Energy, Beijing, China Commissariat `a l’Energie Atomique, IRFU, Saclay, France COMSATS Institute of Information Technology (CIIT), Islamabad, Pakistan Departamento de F´ısica de Part´ıculas and IGFAE, Universidad de Santiago de Compostela, Santiago deCompostela, Spain Department of Physics and Technology, University of Bergen, Bergen, Norway Department of Physics, Aligarh Muslim University, Aligarh, India Department of Physics, Ohio State University, Columbus, Ohio, United States Department of Physics, Sejong University, Seoul, South Korea Department of Physics, University of Oslo, Oslo, Norway Dipartimento di Elettrotecnica ed Elettronica del Politecnico, Bari, Italy Dipartimento di Fisica dell’Universit`a ’La Sapienza’ and Sezione INFN Rome, Italy Dipartimento di Fisica dell’Universit`a and Sezione INFN, Cagliari, Italy Dipartimento di Fisica dell’Universit`a and Sezione INFN, Trieste, Italy Dipartimento di Fisica dell’Universit`a and Sezione INFN, Turin, Italy Dipartimento di Fisica e Astronomia dell’Universit`a and Sezione INFN, Bologna, Italy Dipartimento di Fisica e Astronomia dell’Universit`a and Sezione INFN, Catania, Italy Dipartimento di Fisica e Astronomia dell’Universit`a and Sezione INFN, Padova, Italy Dipartimento di Fisica ‘E.R. Caianiello’ dell’Universit`a and Gruppo Collegato INFN, Salerno, Italy Dipartimento di Scienze e Innovazione Tecnologica dell’Universit`a del Piemonte Orientale and GruppoCollegato INFN, Alessandria, Italy Dipartimento Interateneo di Fisica ‘M. Merlin’ and Sezione INFN, Bari, Italy Division of Experimental High Energy Physics, University of Lund, Lund, Sweden Eberhard Karls Universit¨at T¨ubingen, T¨ubingen, Germany European Organization for Nuclear Research (CERN), Geneva, Switzerland Excellence Cluster Universe, Technische Universit¨at M¨unchen, Munich, Germany Faculty of Engineering, Bergen University College, Bergen, Norway Faculty of Mathematics, Physics and Informatics, Comenius University, Bratislava, Slovakia Faculty of Nuclear Sciences and Physical Engineering, Czech Technical University in Prague, Prague,Czech Republic Faculty of Science, P.J. ˇSaf´arik University, Koˇsice, Slovakia Faculty of Technology, Buskerud and Vestfold University College, Vestfold, Norway Frankfurt Institute for Advanced Studies, Johann Wolfgang Goethe-Universit¨at Frankfurt, Frankfurt,Germany Gangneung-Wonju National University, Gangneung, South Korea Gauhati University, Department of Physics, Guwahati, India Helsinki Institute of Physics (HIP), Helsinki, Finland Hiroshima University, Hiroshima, Japan Indian Institute of Technology Bombay (IIT), Mumbai, India Indian Institute of Technology Indore, Indore (IITI), India Inha University, Incheon, South Korea Institut de Physique Nucl´eaire d’Orsay (IPNO), Universit´e Paris-Sud, CNRS-IN2P3, Orsay, France Institut f¨ur Informatik, Johann Wolfgang Goethe-Universit¨at Frankfurt, Frankfurt, Germany Institut f¨ur Kernphysik, Johann Wolfgang Goethe-Universit¨at Frankfurt, Frankfurt, Germany Institut f¨ur Kernphysik, Westf¨alische Wilhelms-Universit¨at M¨unster, M¨unster, Germany Institut Pluridisciplinaire Hubert Curien (IPHC), Universit´e de Strasbourg, CNRS-IN2P3, Strasbourg,France Institute for Nuclear Research, Academy of Sciences, Moscow, Russia Institute for Subatomic Physics of Utrecht University, Utrecht, Netherlands Institute for Theoretical and Experimental Physics, Moscow, Russia Institute of Experimental Physics, Slovak Academy of Sciences, Koˇsice, Slovakia Institute of Physics, Academy of Sciences of the Czech Republic, Prague, Czech Republic Institute of Physics, Bhubaneswar, India Institute of Space Science (ISS), Bucharest, Romania Instituto de Ciencias Nucleares, Universidad Nacional Aut´onoma de M´exico, Mexico City, Mexico Instituto de F´ısica, Universidad Nacional Aut´onoma de M´exico, Mexico City, Mexico iThemba LABS, National Research Foundation, Somerset West, South Africa Joint Institute for Nuclear Research (JINR), Dubna, Russia Konkuk University, Seoul, South Korea Korea Institute of Science and Technology Information, Daejeon, South Korea KTO Karatay University, Konya, Turkey Laboratoire de Physique Corpusculaire (LPC), Clermont Universit´e, Universit´e Blaise Pascal,CNRS–IN2P3, Clermont-Ferrand, France Laboratoire de Physique Subatomique et de Cosmologie, Universit´e Grenoble-Alpes, CNRS-IN2P3,Grenoble, France Laboratori Nazionali di Frascati, INFN, Frascati, Italy Laboratori Nazionali di Legnaro, INFN, Legnaro, Italy Lawrence Berkeley National Laboratory, Berkeley, California, United States Moscow Engineering Physics Institute, Moscow, Russia Nagasaki Institute of Applied Science, Nagasaki, Japan National Centre for Nuclear Studies, Warsaw, Poland National Institute for Physics and Nuclear Engineering, Bucharest, Romania National Institute of Science Education and Research, Bhubaneswar, India Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark Nikhef, Nationaal instituut voor subatomaire fysica, Amsterdam, Netherlands Nuclear Physics Group, STFC Daresbury Laboratory, Daresbury, United Kingdom Nuclear Physics Institute, Academy of Sciences of the Czech Republic, ˇReˇz u Prahy, Czech Republic Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States Petersburg Nuclear Physics Institute, Gatchina, Russia Physics Department, Creighton University, Omaha, Nebraska, United States Physics Department, Panjab University, Chandigarh, India Physics Department, University of Athens, Athens, Greece Physics Department, University of Cape Town, Cape Town, South Africa Physics Department, University of Jammu, Jammu, India Physics Department, University of Rajasthan, Jaipur, India Physik Department, Technische Universit¨at M¨unchen, Munich, Germany Physikalisches Institut, Ruprecht-Karls-Universit¨at Heidelberg, Heidelberg, Germany Purdue University, West Lafayette, Indiana, United States Pusan National University, Pusan, South Korea Research Division and ExtreMe Matter Institute EMMI, GSI Helmholtzzentrum f¨urSchwerionenforschung, Darmstadt, Germany Rudjer Boˇskovi´c Institute, Zagreb, Croatia Russian Federal Nuclear Center (VNIIEF), Sarov, Russia Russian Research Centre Kurchatov Institute, Moscow, Russia
Saha Institute of Nuclear Physics, Kolkata, India
School of Physics and Astronomy, University of Birmingham, Birmingham, United Kingdom
Secci´on F´ısica, Departamento de Ciencias, Pontificia Universidad Cat´olica del Per´u, Lima, Peru
Sezione INFN, Bari, Italy
Sezione INFN, Bologna, Italy
Sezione INFN, Cagliari, Italy
Sezione INFN, Catania, Italy
Sezione INFN, Padova, Italy
Sezione INFN, Rome, Italy
Sezione INFN, Trieste, Italy
Sezione INFN, Turin, Italy
SSC IHEP of NRC Kurchatov institute, Protvino, Russia
Stefan Meyer Institut f¨ur Subatomare Physik (SMI), Vienna, Austria
SUBATECH, Ecole des Mines de Nantes, Universit´e de Nantes, CNRS-IN2P3, Nantes, France
Suranaree University of Technology, Nakhon Ratchasima, Thailand
Technical University of Koˇsice, Koˇsice, Slovakia
Technical University of Split FESB, Split, Croatia
The Henryk Niewodniczanski Institute of Nuclear Physics, Polish Academy of Sciences, Cracow, Poland
The University of Texas at Austin, Physics Department, Austin, Texas, USA
Universidad Aut´onoma de Sinaloa, Culiac´an, Mexico
Universidade de S˜ao Paulo (USP), S˜ao Paulo, Brazil
Universidade Estadual de Campinas (UNICAMP), Campinas, Brazil
University of Houston, Houston, Texas, United States
University of Jyv¨askyl¨a, Jyv¨askyl¨a, Finland
University of Liverpool, Liverpool, United Kingdom
University of Tennessee, Knoxville, Tennessee, United States
University of the Witwatersrand, Johannesburg, South Africa
University of Tokyo, Tokyo, Japan
University of Tsukuba, Tsukuba, Japan
University of Zagreb, Zagreb, Croatia
Universit´e de Lyon, Universit´e Lyon 1, CNRS/IN2P3, IPN-Lyon, Villeurbanne, France
V. Fock Institute for Physics, St. Petersburg State University, St. Petersburg, Russia
Variable Energy Cyclotron Centre, Kolkata, India
Warsaw University of Technology, Warsaw, Poland
Wayne State University, Detroit, Michigan, United States
Wigner Research Centre for Physics, Hungarian Academy of Sciences, Budapest, Hungary
Yale University, New Haven, Connecticut, United States
Yonsei University, Seoul, South Korea