Neutral pion and η meson production at midrapidity in Pb-Pb collisions at √sNN = 2.76 TeV

This is a self-archived version of an original article. This version may differ from the original in pagination and typographic details. Author(s): Title: Year: Version: Copyright: Rights: Rights url: Please cite the original version: CC BY 4.0 https://creativecommons.org/licenses/by/4.0/ Neutral pion and η meson production at midrapidity in Pb-Pb collisions at √sNN = 2.76 TeV © 2018 CERN, for the ALICE Collaboration Published version ALICE Collaboration ALICE Collaboration. (2018). Neutral pion and η meson production at midrapidity in Pb-Pb collisions at √sNN = 2.76 TeV. Physical Review C, 98(4), Article 044901. https://doi.org/10.1103/PhysRevC.98.044901 2018 PHYSICAL REVIEW C 98, 044901 (2018) Neutral pion and ηmeson production at midrapidity in Pb-Pb collisions at √sNN =2.76 TeV S. Acharya et al.∗ (ALICE Collaboration) (Received 21 March 2018; published 4 October 2018) Neutral pion and ηmeson production in the transverse momentum range 1 <p T<20 GeV/c have been measured at midrapidity by the ALICE experiment at the Large Hadron Collider (LHC) in central and semicentral Pb-Pb collisions at √sNN =2.76 TeV. These results were obtained using the photon conversion method as well as the Photon Spectrometer (PHOS) and Electromagnetic Calorimeter detectors. The results extend the upper pTreach of the previous ALICE π0measurements from 12 to 20 GeV/c and present the first measurement of ηmeson production in heavy-ion collisions at the LHC. The η/π0ratio is similar for the two centralities and reaches at high pTa plateau value of 0.457 ±0.013stat ±0.018syst. A suppression of similar magnitude for π0and ηmeson production is observed in Pb-Pb collisions with respect to their production in pp collisions scaled by the number of binary nucleon-nucleon collisions. We discuss the results in terms of Next to Leading Order (NLO) pQCD predictions and hydrodynamic models. The measurements show a stronger suppression than observed at lower center-of-mass energies in the pTrange 6 <p T<10 GeV/c.ForpT<3 GeV/c, hadronization models describe the π0results while for the ηsome tension is observed. DOI: 10.1103/PhysRevC.98.044901 I. INTRODUCTION Quantum chromodynamics (QCD) [1], the fundamental theory of strong interactions, predicts that, above a certain critical energy density, hadrons melt into a quark-gluon plasma (QGP) [2,3]. Such a state of matter is believed to have existed a few microseconds after the Big Bang [4]. One of the goals of lattice QCD calculations is the understanding of the properties of strongly interacting matter and the nature of the phase transition that depends on the values of the quark masses and number of flavors. For vanishing baryon chemical potential (μ) and for quark masses above a critical quark mass, a deconfinement transition associated with chiral restoration takes place through a smooth crossover [5–8]. The study and characterization of the QGP gives information on the crossover transition as well as insights on the equation of state of deconfined matter [9,10]. These transitions are expected to have occurred in the early universe and therefore their study is also of relevance to cosmology [4]. Heavy-ion collisions at relativistic energies offer the possibility of studying the QGP by creating systems of dense matter at very high temperatures. Of the many observables that probe the QGP, measurements of π0and ηmeson production over a large transverse momentum (pT) range and in different colliding systems are of particular interest. At low pT(pT<3GeV/c), light meson production in heavy-ion ∗Full author list given at the end of the article. Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. collisions gives insights about hadronization and collectivity in the evolution of the QGP. At high pT(pT>5GeV/c), it helps quantify parton energy loss mechanisms [11,12]. HighpTparticle suppression in heavy-ion collisions with respect to pp collisions may be modified by cold nuclear matter effects, such as nuclear parton distribution function (nPDF) modifications with respect to the vacuum. Measurements in pA collisions are thus needed to disentangle cold nuclear effects from the observed high-pTparticle suppression in AA collisions. Other interesting probes of the QGP that can benefit from neutral meson measurements are studies of direct photon and heavy-flavor production measurements [13,14]. The π0and ηmesons are the two most abundant sources of decay photons (and electrons); as a consequence, they generate the primary background for these rare probes. The first measurement of direct photons at the Large Hadron Collider (LHC) [15] employed mTscaling and the K0 sreference measurement to estimate the ηcontribution to decay photons. Forthcoming direct photon and heavy-flavor measurements at the LHC will be able to use the ηmeasurement directly. Measurements of pion spectra at Relativistic Heavy Ion Collider (RHIC) [16,17] at low transverse momentum were observed to be well described by thermal models that assume a hydrodynamic expansion of a system in local equilibrium [18]. The comparison of these models to data suggested the presence of a thermalized system of quarks and gluons formed in the early stages of the collision. At LHC energies, the thermal models that describe the RHIC data also describe the ALICE charged pion spectrum [19]forpT>0.5 GeV/c. Modern versions of these models fold in their calculations hydrodynamic expansion, which accounts for transverse flow effects, simultaneous chemical, and thermal freeze-out and inclusion of high mass resonance decays from the PDG [1]. 2469-9985/2018/98(4)/044901(20) 044901-1 ©2018 CERN, for the ALICE Collaboration S. ACHARYA et al. PHYSICAL REVIEW C 98, 044901 (2018) Among the many models that aim at explaining low-pTparticle production, the equilibrium and chemical nonequilibrium statistical hadronization models (EQ SHM and NEQ SHM, respectively) have had their validity tested against LHC data from pT>0.1 GeV/c. The physics picture behind the NEQ SHM is a sudden hadronization of the QGP that leads to the apperance of additional nonequlibrium chemical potentials for light and strange quarks. The low-pTpion enhancement predicted by the NEQ relative to the EQ SHM can be interpreted as the onset of pion condensation in ultrarelativistic heavy-ion collisions at the LHC energies [20–24]. Both predictions can be further tested by measuring π0and ηproduction at LHC energies. In the early RHIC program, a suppression of high-pTπ0 production was observed in heavy-ion collisions when compared to scaled pp data [25]. This suppression was interpreted as a consequence of the energy loss of the scattered partons in the QGP generated in the collisions. From these observations, it was deduced that the dense QGP medium is opaque to energetic (hard) colored probes. Regarding high-pTparticle production at the LHC, it must be considered that the energy density of the plasma is higher than measured at RHIC. This increase in energy density leads to a larger energy loss of highpTpartons with respect to those at both lower pT(<3GeV/c) and lower energy [26,27]. Moreover, it has been observed that baryons and strange mesons exhibit similar suppression as that of pions above 10 GeV/c. The measurement of another light meson, the ηmeson, provides additional information about mechanisms of particle production and energy loss, while the measurement of both mesons at higher pTwill give insight about the pTdependence of the suppression in this region. The suppression due to the QGP can also be studied with the η/π0ratio. In heavy-ion collisions, gluons are expected to experience larger energy loss in the medium than quarks, due to gluons having a larger vertex coupling factor. The energy reduction due to the presence of the medium (jet quenching effect) [28] may alter gluon and quark fragmentation differently with respect to what is observed in pp collisions. These differences between gluon and quark energy loss may introduce a modification in the suppression patterns observed for π0and ηmesons, due to a larger gluon component in the ηmeson (note that the ηmeson, unlike π0, has a two-gluon component) [29]. An intermediate pTenhancement of the η/π0ratio in AA collisions relative to pp collisions would be an indication of the plasma-induced color dependence suppression [30–32]. The magnitude of this enhancement is sensitive to the initial values of the jet transport parameters and thus could be used to quantify the suppression. In this paper, we present π0and ηmeson production measurements from the ALICE experiment in the pTrange 1<p T<20 GeV/c in Pb-Pb collisions at center-ofmass energy √sNN =2.76 TeV in two centrality classes, 0–10% and 20–50%. The results are measured at midrapidity using two complementary detection methods: the photon conversion method (PCM) and use of the Electromagnetic Calorimeter (EMCal) [33]. The π0results in the 0–10% centrality class have been combined with the previously published π0result measured with the Photon Spectrometer (PHOS) calorimeter [27]. The new π0 measurement is updated with 10 times more statistics than the previous ALICE measurement [27] and extends the pTreach from 12 GeV/c to 20 GeV/c.Theηmeasurement is the first measurement of its kind at the LHC and has a wider pTreach than what was previously measured at RHIC [34]. The paper is organized as follows: A brief description of the detectors used and of the data sample is given in Sec. II. The analysis procedure is described in Sec. III. The results and the comparison to other experimental measurements and to theoretical predictions are presented in Secs. IV and V, respectively. II. DETECTOR DESCRIPTION AND DATA SAMPLE The ALICE experiment and its performance are described in detail in Refs. [35,36]. The main detectors used for the reconstruction of π0and ηmesons are located in the central barrel, operated inside a solenoidal magnetic field of 0.5 T directed along the beam axis. The Inner Tracking System (ITS) is a high granularity and precision detector that measures the position of the primary collision vertex and the impact parameter of the tracks [37]. The ITS is composed of six cylindrical layers of silicon detectors positioned at radial distances from 4 to 43 cm. The two innermost layers of the ITS are Silicon Pixel Detectors (SPD) that cover the pseudorapidity regions |η|<2 and |η|<1.4. The next two layers are Silicon Drift Detectors (SDD) covering |η|<1, while the two outer layers are Silicon Strip Detectors (SSD) covering |η|<0.9. The Time Projection Chamber (TPC) [38]isthemain charged particle tracking and identification detector in the ALICE central barrel. It is a cylindrical drift detector filled with aNe-CO 2(90%-10%) gas mixture. This detector surrounds the ITS and is centered around the Interaction Point (IP) at a radial distance from 85 to 250 cm. The TPC has full azimuthal coverage and covers |η|<0.9 for the full track length. Particles are identified through the measurement of their specific energy loss (dE/dx) in the detector with a 6.5% resolution in the 0–5% most central Pb-Pb [36,38]. The track’s transversemomentum resolution is [σ(pT)/pT]=0.8% at 1 GeV/c and 1.7% at 10 GeV/c in central Pb-Pb collisions [36,39]. The EMCal [33] is a sampling calorimeter composed of 77 alternating layers of 1.4-mm lead and 1.7-mm polystyrene scintillators. The EMCal is a fairly high granularity detector. It has a cell area of η×φ=0.0143 ×0.0143 rad and an energy resolution of σE(GeV)/E =4.8%/E ⊕11.3/√E⊕ 1.7% [40]. In year 2011, it covered |η|<0.7 and ϕ=100◦. The main detectors used for triggering and characterization of the collision are the V0 [41] and the Zero Degree Calorimeters (ZDC) [42]. The V0 consists of two scintillator arrays located on opposite sides of the IP at 340 and 90 cm covering 2.8 <η<5.1 and −3.7<η<−1.7, respectively. The ZDC detectors are located at a distance of 114 m on both sides of the IP and detect spectator nucleons. The Pb-Pb data sample used for this analysis was collected in the 2011 LHC run. During that period, about 358 ion bunches circulated in each LHC beam, with collisions delivering a peak luminosity of 4.6 ×10−4μb−1s−1, corresponding 044901-2 NEUTRAL PION AND ηMESON PRODUCTION AT … PHYSICAL REVIEW C 98, 044901 (2018) to an average of about 10−3hadronic interactions per bunch crossing. The minimum bias (MB) trigger was defined by the coincidence of signals in the two V0 arrays synchronized with a bunch crossing. An online selection based on the measured V0 amplitudes was employed to enhance central (0–10%) and semicentral (0–50%) events [36]. The ZDC and the V0 were also used for the rejection of pile-up and beam-gas interactions. The centrality class definition was based on the V0 amplitude distributions. The number of binary collisions (Ncoll) for a given value of the centrality was extracted with the help of a Glauber model [43] as detailed in Refs. [39,44]. Only events with a reconstructed primary vertex within |zvtx|<10 cm of the nominal interaction vertex along the beam direction were accepted. The data are analyzed in two centrality classes: 0–10% and 20–50%, containing 1.9(1.6) ×107and 1.3(1.1) ×107events for PCM (EMCal), respectively. The minimum bias trigger cross section, σPbPb MB = [7.64 ±0.22(syst.)] b [44], was determined using van der Meer scans [45]. The integrated luminosity, corresponding to the number of analyzed events normalized by σPbPb MB in each centrality percentile, is 20.1 μb−1and 4.8 μb−1for 0–10% and for 20–50%, respectively. III. ANALYSIS METHODS The π0and ηmesons are reconstructed using the two-photon decay channel, π0→γγ and η→γγ, with a branching ratio of (98.823 ±0.034)% and (39.41 ±0.20)% [1], respectively. With the photon conversion method, photons that convert in the detector material are measured by reconstructing the electron-positron pairs in the central rapidity detectors using a secondary vertex (V0) finding algorithm [36]. This method produces a V0candidate sample on which the analysis quality selection criteria were applied, as done in Refs. [27,46]. Electrons, positrons, and photons are required to have |η|<0.9. To ensure track quality, a minimum track momentum of 50 MeV/c and a fraction of TPC clusters over findable clusters (the number of geometrically possible clusters which can be assigned to a track) above 0.6 have been required. Moreover, a maximum conversion radius of 180 cm delimits the TPC fiducial volume for good track reconstruction, while a minimum of 5 cm rejects Dalitz decays of the type π0(η)→e+e−γ. The specific energy loss dE/dx should be within the interval [−3σdE/dx,+5σdE/dx]fromthe expected electron Bethe-Bloch parametrization value, where σis the standard deviation of the energy loss measurement. Pions are rejected by a selection of 3σabove the pion hypothesis in the range 0.4 <p< 2GeV/c and of 1σfor p> 2GeV/c. The smaller rejection with respect to the previous Pb-Pb measurement translates into a larger efficiency at high pTfor the π0and ηmesons. To further reject K0 s,, and from the V0candidates, a selection is applied on the components of the momenta relative to the V0, using the asymmetry of the longitudinal momentum of the V0daughters [αV0=(pe+ L−pe− L)/(pe+ L+pe− L)], and on the transverse momentum of the electron with respect to the V0momentum (qT=pe×sin θV0,e). V0candidates are selected with a two-dimensional elliptic selection criterion of (αV0/αV0 max )2+(qT/qT,max)2<1, with αV0 max =0.95 and qT,max =0.05 GeV/c, in order to increase the purity while optimizing efficiency of the photon sample. As conversion electrons have a preferred decay orientation, a selection on ψpair, the angle between the plane perpendicular to the magnetic field and the plane containing the electron and positron tracks, together with a cut on the photon χ2of the Kalman filter [47], further suppresses the contamination from nonphotonic V0candidates. This cut, described in Ref. [48], is applied requiring χ2 γ,max =20 and ψpair,max =0.1. To improve the signal significance, a pT-dependent cut on the energy asymmetry of the photons |α|<0.65tanh[1.8(GeV/c)−1pT] [where α=(Eγ1−Eγ2)/(Eγ1+Eγ2), pTin GeV/c]is applied. For the measurement with the EMCal, photons stemming from meson decays are measured directly. Photonlike hits in the detector are identified by energy deposits in the neighboring cells, which are grouped into clusters with a minimum size of two cells. A minimum energy per cell of 50 MeV is required. The cluster finding algorithm employs a seed energy of Eseed =0.3 GeV, which is slightly above the minimum ionizing particle threshold [36]. EMCal clusters that coincide within a window of |η|<0.025 and |φ|<0.05 radians of a charged particle reconstructed in the TPC and projected to the EMCal surface are rejected. Each selected EMCal cluster is then required to have a total energy of at least 1.5 GeV to remove low-energy pairs consisting of predominantly combinatorial background and particle conversions in the material. A loose photonlike electromagnetic shower shape selection is applied to the clusters by looking at the eccentricity of the cluster via the weighted RMS of the shower energy along the major ellipse axis according to σ2 long =sηη +sϕϕ 2+(sηη −sϕϕ)2 4+s2 ηϕ,(1) where sij =ij −ijare the covariance matrix elements; i, j are cell indices in ηor ϕaxes; and ij and i,jare the second and the first moments of the cluster cells weighted with the cell energy logarithm [36,49–51]. The purpose of this loose shower shape selection 0.1 <σ 2 long <0.5 (photons sit in a narrow peak centered at 0.25) is to remove noisy and very deformed or asymmetric cluster shapes which result from the merging of different particle showers produced nearby in the calorimeter. For the PCM and EMCal analyses, the reconstructed twophoton invariant mass is measured in bins of pTin the rapidity range |y|<0.85 and |y|<0.7, respectively. The pTranges in which the separate methods contribute are reported in Table I. In addition, a minimum photon pair opening angle of 5 mrad is used to reject background in the PCM analysis. The background under the neutral meson signal contains combinatorial and correlated contributions. The combinatorial background is estimated with the event mixing method by mixing photons from different events but with similar photon multiplicity and topological (vertex location on the zaxis and in the particular case of the PCM analysis the event plane angle) characteristics. The mixed event background is normalized to the reconstructed two-photon invariant mass in a region at higher mass with respect to the meson peak and 044901-3 S. ACHARYA et al. PHYSICAL REVIEW C 98, 044901 (2018) TABLE I. Transverse momentum ranges for the π0and ηmeson measurements. For the ηmeson in both centralities and for the π0in 20–50% centrality class the combination is between PCM and EMCal. For π0in the 0–10%, the final results are obtained combining PCM, EMCal, as well as previously published results using the PHOS detector [27]. π0η PCM EMCal PHOS PCM EMCal 0–10% 1–14 GeV/c 4–20 GeV/c 1–12 GeV/c 1–10 GeV/c 4–20 GeV/c 20–50% 1–14 GeV/c 4–20 GeV/c – 1–10 GeV/c 4–20 GeV/c subtracted. Additionally, various fitting functions for the total background are also used in order to obtain the number of mesons and to evaluate the corresponding systematic uncertainty (EMCal). The resulting invariant mass distributions are fit with either a Gaussian combined with a low-mass exponential tail [52] (PCM, to account for electron bremsstrahlung) on top of a linear function (PCM, to account for residual background) or with a Crystal Ball distribution [53] (EMCal) in order to obtain the position and width of the peak [36]. After subtracting the total background, the yields are extracted for each pTbin by integrating the invariant mass distributions over a range that depends on the peak position and resolution. Figure 1shows the invariant mass distribution for the π0and ηmesons reconstructed with PCM and EMCal. Corrections for geometrical acceptance, reconstruction efficiency, secondary π0from weak decays (the measured spectra of the relevant particles [54] are taken as input) and hadronic interactions and occupancy effects due to cluster overlaps (for EMCal) were estimated with a Monte Carlo simulation using HIJING [55] as the event generator. The simulated particles are propagated through the apparatus via GEANT3 [56], where a realistic detector response based on experimental conditions is applied in order to reproduce the performance of the ALICE detector during data taking. The simulated events are then analyzed with the same reconstruction and analysis selection criteria applied to the experimental data. It was verified that the detector resolutions were well reproduced by the Monte Carlo simulations [36]. The mass peak positions and widths measured in the data for each centrality interval for the PCM (EMCal) analysis were reproduced within 0.5% (1.5%) or better, and the remaining discrepancies have been taken into account in the systematic uncertainties associated with the difference of the energy scale and position of the calorimeter between data and Monte Carlo. In the PCM analysis, the pile-up contribution is estimated by analyzing the distance of closest approach distribution for the photon candidates, as done in Ref. [27]. The effect of pileup in the EMCal analysis was verified to be negligible since the EMCal cell timing resolution is an order of magnitude better than the bunch crossing spacing of 200 ns used in the 2011 Pb-Pb run. For both methods, the systematic uncertainties were studied by varying the selection criteria used in the two analyses and by studying the resulting variations of the fully corrected spectra in individual pTbins. The largest contribution to the systematic uncertainties for the PCM analysis comes from the uncertainty in the material budget [36] and amounts to 9%. Other sources of systematic uncertainties include the yield extraction, track reconstruction, electron identification, and photon reconstruction (mainly for the ηmeson). The details of the PCM systematic uncertainties are listed in Table II. The main source of systematic uncertainties for the neutral meson detection with the EMCal is associated with the particle identification criteria used to select photon pairs (PID). The uncertainties due to the signal extraction in a given pT interval are taken as the mean of the uncertainties obtained in all signal and background parametrizations. Variations on the values used for the meson identification selection criteria are also included and the RMS of these values is used as a systematic uncertainty. The EMCal detector energy response was determined by analyzing test beam data [40]. Comparisons of the mass peak position and the energy-to-momentum ratios of electron tracks [57] in data and Monte Carlo simulations quantify the overall systematic uncertainty due to the Monte Carlo description of the energy response and position of the calorimeter. This uncertainty amounts to 8.6% of the invariant yield measurements. Other sources of systematic uncertainties are the material budget, the pTdistribution of the simulations used for the extraction of efficiencies and the contribution from higher mass decays. The details of the EMCal systematic uncertainties are listed in Table II. When computing the η/π0ratio and the nuclear modification factor, fully and partially correlated errors, such as material budget and energy scale (EMCal only), are taken into account. IV. RESULTS A. Invariant yields of the π0and ηmeson The invariant differential yields for π0and ηmesons have been calculated employing Ed3N dp3=1 2πNevt 1 BRatio Aε Nraw pTpTy,(2) where Nevt is the number of events in the centrality class considered, BRatio is the branching ratio [1] for the process π0(η)→γγ,Aε are the corresponding acceptance and efficiency corrections, and Nraw corresponds to the reconstructed π0(η) raw yield within the rapidity range yand the transverse momentum bin pT. The horizontal location of the data points is shifted towards lower pTfrom the bin center by a few MeV and illustrates the pTvalue where the differential 044901-4 NEUTRAL PION AND ηMESON PRODUCTION AT … PHYSICAL REVIEW C 98, 044901 (2018) ) 2 c (GeV/ γγ→ 0 π M 0.1 0.12 0.14 0.16 0.18 0.2 2 cCounts per 2 MeV/ 0.0 0.2 0.4 0.6 0.8 1.0 1.2 6 10× (a) ALICE = 2.76 TeV sPb,−Pb 0-10% PCM c < 1.2 GeV/ T p < c: 1.0 GeV/ 0 π Raw real events Mixed event BG Signal after BG subtraction scaled by 10 Fit ) 2 c (GeV/ γγ→η M 0.4 0.45 0.5 0.55 0.6 0.65 2 cCounts per 5 MeV/ 0.0 0.2 0.4 0.6 0.8 6 10× (b) ALICE = 2.76 TeV sPb,−Pb 0-10% PCM c < 3.0 GeV/ T p < c: 2.0 GeV/η Raw real events Mixed event BG Signal after BG subtraction scaled by 40 Fit ) 2 c (GeV/ γγ→ 0 π M 0.1 0.12 0.14 0.16 0.18 0.2 2 cCounts per 10 MeV/ 0 100 200 300 400 500 (c) ALICE = 2.76 TeV sPb,−Pb 0-10% EMCal c < 14.0 GeV/ T p < c: 12.0 GeV/ 0 π Raw real events Mixed event BG Signal after BG subtraction Fit ) 2 c (GeV/ γγ→η M 0.4 0.45 0.5 0.55 0.6 0.65 2 cCounts per 10 MeV/ 0 20 40 60 80 100 120 140 (d) ALICE = 2.76 TeV sPb,−Pb 0-10% EMCal c < 14.0 GeV/ T p < c: 12.0 GeV/η Raw real events Mixed event BG Signal after BG subtraction Fit FIG. 1. Invariant mass distribution of reconstructed photon pairs Mγγ for the π0and ηmesons measured with PCM [(a) and (b)] and EMCal [(c) and (d)] in the centrality class 0–10%. The black histograms show the signal before background subtraction while the red bullets show the signal after subtraction. The estimated background is indicated by the gray dashed lines. The blue lines are the fit to the invariant mass peak after the combinatorial and residual background subtraction (see text for description). cross section is equal to the measured integral of the cross section over the corresponding bin [58]. For the η/π 0ratio and RAA the bin-shift correction is done in ycoordinates. The pTranges in which the measurements were performed are reported in Table I. In the overlap region a weighted average of the two results (or three when applicable) is performed using the inverse of the quadratic sum of the uncertainties (statistical and systematic) that are uncorrelated between the methods as weights [59–61]. Figure 2shows the invariant differential yields of (a) π0 and (b) ηmeson measured in pp [51] and Pb-Pb collisions in the two centrality bins under study. The π0meson measurements are in agreement with the previously published ALICE π0spectra [27] and extend the transverse momentum reach from 12 to 20 GeV/c.Fortheηmeson, the results presented here are the first measurement of its kind in heavy-ion collisions at the LHC and the first measurement of this meson to reach down to pTof 1 GeV/c in a collider experiment [34,62]. Both meson spectra have been parametrized over the full pTrange by the function proposed in Refs. [63,64] that combines a Boltzmann factor at low pTwith a power law at 044901-5 S. ACHARYA et al. PHYSICAL REVIEW C 98, 044901 (2018) TABLE II. Summary of the systematic uncertainties in percentages for selected pTregions for the PCM and EMCal analyses. PCM 0–10% 20–50% π0ηπ 0η 1.1 GeV/c 5.5 GeV/c 2.5 GeV/c 5.0 GeV/c 1.1 GeV/c 5.5 GeV/c 2.5 GeV/c 5.0 GeV/c Material budget 9.0 9.0 9.0 9.0 9.0 9.0 9.0 9.0 Track reconstruction 2.3 2.6 6.0 6.2 1.4 2.3 7.0 9.0 Yield extraction 1.5 2.1 6.4 7.0 2.5 2.8 10.0 11.0 e+/e−identification 1.7 2.5 6.0 6.1 1.4 2.4 5.5 9.3 Photon reconstruction 3.7 2.1 13.7 13.6 2.1 2.2 8.0 8.6 EMCal 0–10% 20–50% π0ηπ 0η 7.0 GeV/c 18.5 GeV/c 7.0 GeV/c 18.5 GeV/c 7.0 GeV/c 18.5 GeV/c 7.0 GeV/c 18.5 GeV/c Signal extraction 2.9 5.1 4.2 5.5 7.5 5.8 6.0 7.1 Photon identification 9.5 8.0 4.6 6.0 7.5 4.5 14.1 5.0 Energy response 8.6 8.6 8.6 8.6 8.6 8.6 8.6 8.6 Material budget 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 Hijing simulation 8.6 10.0 8.6 10.0 2.0 5.3 2.0 5.3 Monte Carlo input 2.0 3.0 <1 1.5 <1<1<1<1 Higher mass decays 4.0 2.0 – – 3.2 2.0 – – )c (GeV/ T p 1− 10×312 3 4 5 67 10 20 30 ] -2 )c [(GeV/ yd T pd T p 0 π N 2 d ev Nπ2 1 11− 10 10− 10 9− 10 8− 10 7− 10 6− 10 5− 10 4− 10 3− 10 2− 10 1− 10 1 10 2 10 3 10 = 2.76 TeVspp, EPJC 77 (2017) 339 = 2.76 TeVsPb,−Pb 10%− 0 50%−20 Pb−fits to Pb γγ→ 0 π 4× (a) )c (GeV/ T p 1− 10×312 3 4 5 67 10 20 30 ] -2 )c [(GeV/ yd T pd T p η N 2 d ev Nπ2 1 9− 10 8− 10 7− 10 6− 10 5− 10 4− 10 3− 10 2− 10 1− 10 1 10 2 10 = 2.76 TeVspp, EPJC 77 (2017) 339 = 2.76 TeVsPb,−Pb 10%− 0 50%−20 Pb−fits to Pb γγ→η 4× (b) FIG. 2. Invariant yields of the (a) π0and (b) ηmeson in the centrality classes 0–10% (circles) and 20–50% (squares). The vertical error bars represent the statistical uncertainties while the boxes represent the systematic uncertainties. The Pb-Pb measurements are compared with the corresponding pp invariant cross sections (stars) measured at the same center-of-mass energy [27,51]. The dashed black lines correspond to the fits to the data with the two-component function. See Table III and corresponding text for details. 044901-6 NEUTRAL PION AND ηMESON PRODUCTION AT … PHYSICAL REVIEW C 98, 044901 (2018) TABLE III. Parameters of the fits to the differential invariant yields of π0and ηmeson using the two-component function of Bylinkin and Rostovtsev [63,64]. The total uncertainties, i.e., quadratic sum of statistical and systematic uncertainties, are used for the fits. π0η 0–10% 20–50% 0–10% 20–50% Ae(GeV/c)−2162 ±20 30 ±715±6 4.2 ±2.5 Te(GeV/c) 0.37 ±0.01 0.38 ±0.02 0.44 ±0.03 0.42 ±0.06 A(GeV/c)−2840 80 100 2 T(GeV/c) 0.34 ±0.01 0.50 ±0.02 0.38 ±0.03 0.76 ±0.05 n3.00 ±0.05 3.00 ±0.05 3.0 ±0.1 3.0 ±0.1 χ2/ndf 0.18 0.20 0.22 0.14 high pT, Ed3N dp3=Aeexp −p2 T+M2−M Te+A 1+p2 T T2nn,(3) where Mis the meson mass (in GeV/c2) and Ae,A,Te,T, and nare free parameters of the fit. The parameters resulting from the fits to the meson invariant yields in both centrality classes are reported in Table III. All parameters are free except for the amplitude A. The values are chosen after a systematic study of the two separate components of the Bylinkin-Rostovtsev function and of the parameter limits variation. B. Particle ratios The η/π0ratio measured in the two centrality classes is shown in Fig. 3(a).InFig.3(b), the measurement in the 0–10% centrality class is compared to the same ratio measured in pp collisions at √s=2.76 TeV [51], as well as to the K±/π±ratio in the same centrality class and in the same collision system and energy [19], measured by ALICE. The K±/π±ratio is of interest as the relative mass differences between these particles is similar to the one for the ηand π0mesons. At pT<2GeV/c,theη/π0and the K±/π± ratios in Pb-Pb are in agreement within uncertainties. At 2 < pT<4GeV/c, due to the large uncertainties in the η/π0ratio in Pb-Pb, no conclusion can be made on the significance of the difference between the η/π0ratio in pp or the K±/π±ratio in Pb-Pb. At pT>4GeV/c, the value for all ratios is of similar magnitude. Moreover, a constant fit from 3 to 20 GeV/c gives a plateau value for the ratio of 0.457 ±0.013stat ±0.018syst, in agreement with the value quoted in lower center-of-mass energy measurements [34]. C. The nuclear modification factor RAA The nuclear modification factor can be used to quantify particle production suppression in heavy-ion collisions with respect to pp collisions. It is defined as RAA (pT)=d2N/ddy|AA TAA×d2σ/dpTdy|pp .(4) where the nuclear overlap function TAAis related to the average number of inelastic collisions by TAA=Ncoll/σ pp inel )c (GeV/ T p 1− 10×412 3 4 5 6 7 8 910 20 30 0 π/η 0.2 0.4 0.6 0.8 1.0 = 2.76 TeVsPb,−Pb 10%− 0 50%−20 (a) )c (GeV/ T p 1− 10×412 3 4 5 6 7 8 910 20 30 Particle ratio 0.2 0.4 0.6 0.8 1.0 = 2.76 TeVsPb,−10% Pb−0 0 π/η ± π/ ± K PRC 93 (2016) 034913 = 2.76 TeVspp, 0 π/η EPJC 77 (2017) 339 (b) FIG. 3. (a) η/π0ratio in the two centrality classes measured, 0–10% (circles) and 20–50% (squares). (b) Comparison of the η/π0 measurement in the 0–10% centrality class (full circles) to the corresponding ratio in pp collisions [51] (stars) and to the K±/π±measurement in the same centrality class, system, and collision energy [19] (open circles). 044901-7 S. ACHARYA et al. PHYSICAL REVIEW C 98, 044901 (2018) )c (GeV/ T p 0 2 4 6 8 101214161820 AA R 0.2 0.4 0.6 0.8 1.0 1.2 10%−0 0 πη PRC 93 (2016) 034913 ± π± K = 2.76 TeV sPb,−Pb (a) )c (GeV/ T p 0 2 4 6 8 101214161820 AA R 0.2 0.4 0.6 0.8 1.0 1.2 50%−20 0 πη 40%, PRC 93 (2016) 034913−20 ± π± K = 2.76 TeV sPb,−Pb (b) FIG. 4. Measured nuclear modification factor for the π0(empty symbols) and ηmeson (full symbols) in the (a) 0–10% and (b) 20–50% centrality classes, compared to ALICE π±and K±[68,69] (open and full diamonds) in the same centrality classes. The boxes around unity represent quadratic sum of the uncertainty on TAAand on the pp spectrum normalization uncertainty. and σpp inel is the total inelastic cross section determined using van der Meer scans [65]. The mean number of collisions is 1501 ±165 for the centrality class 0–10% and 349 ±34 for the centrality class 20–50% [44]. The π0and ηmeson spectra measured in pp collisions at the same center-of-mass energy are obtained from Ref. [51]. The measured RAA is presented in Fig. 4for the π0and the ηmesons. A pTand centrality-dependent suppression is clearly observed. For the most central collisions, the RAA has a maximum around pT≈1.5 GeV/c and a minimum for pT≈7GeV/c, after which it increases. The increase at high pTcould be due to the variation of the relative gluon and quark contributions to meson production as a function of pT, with gluons being expected to suffer a stronger suppression than quarks due to a larger Casimir factor [66]. The suppression observed at high pTis consistent with recent ATLAS results [67] and may indicate a larger quark than gluon relative contribution for high-pTjet production in heavy-ion collisions at the LHC. A similar behavior is observed for semicentral events, though with a smaller suppression over the full transverse momentum range. The magnitude and pattern of the suppression is the same for the π0and ηmesons for pT>4GeV/c despite the difference in mass. At lower pT, the present accuracy is not enough to determine if the suppression is different for the two mesons. The RAA values for both centrality classes are also compared to the ALICE charged kaon RAA [68] measured at the same center-of-mass energy and collision system (Fig. 4) and is of interest given the similar masses of kaons and ηmesons. This comparison indicates similar suppression patterns for η and K±across the whole pTrange and similar suppression between all particles for pT>4GeV/c. This result is consistent with previous baryon and strange meson RAA results [68,69], indicating that the energy loss in the medium is likely a purely partonic effect. D. Comparisons to lower energy measurements The nuclear modification factor in the 0–10% centrality class is compared to previous π0measurements reported by the WA98 [70] and PHENIX collaborations [25,71] [Fig. 5, (a)] for center-of-mass energies per binary collision √sNN ranging from 17.3 GeV (WA89) to 200 GeV (PHENIX). Our results confirm a dependence of the suppression on the center-of-mass energy and indicate a larger suppression for increasing collision energy. At pT>11 GeV/c, the relative difference in suppression between the PHENIX and ALICE data is inconclusive due to the large uncertainties. The ηmeson RAA is compared to the corresponding PHENIX measurement [34]at√sNN =200 GeV [Fig. 5(b)]. Similarly to the π0case, the ALICE measurement shows a larger suppression compared to the PHENIX data in the region 5<p T<14 GeV/c. V. COMPARISONS TO MODELS The π0and ηinvariant pT-differential yields are compared to predictions using a statistical hadronization model (SHM) [18,20] and the EPOS2 [72] event generator. Results from two versions of the SHM are presented here, an EQ and NEQ prediction. In the NEQ SHM, the mean particle multiplicities are described with the use of four thermodynamic parameters: temperature T, volume V, and two parameters to account for the nonequilibrium conditions—γsand γq.The EQ SHM can be treated as a particular case of the NEQ when γs=γq=1. The parameters of the model are determined by fits to the measured charged pion and kaon spectra [20]. While only these two particles are considered in the fits, the 044901-8 NEUTRAL PION AND ηMESON PRODUCTION AT … PHYSICAL REVIEW C 98, 044901 (2018) [79] S. Wicks, W. Horowitz, M. Djordjevic, and M. 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Pagano,136 G. Pai´ c,70 P. Palni,7J. Pan,140 A. K. Pandey,48 S. Panebianco,134 V. Papikyan,1 P. Pareek,49 J. Park,60 S. Parmar,98 A. Passfeld,141 S. P. Pathak,123 R. N. Patra,138 B. Paul,58 H. Pei,7T. Peitzmann,63 X. Peng,7 L. G. Pereira,71 H. Pereira Da Costa,134 D. Peresunko,92,88 E. Perez Lezama,69 V. Peskov,69 Y. Pestov,5V. Petrᡠcek,38 M. Petrovici,47 C. Petta,30 R. P. Pezzi,71 S. Piano,59 M. Pikna,15 P. Pillot,111 L. O. D. L. Pimentel,89 O. Pinazza,53,36 L. Pinsky,123 S. Pisano,51 D. B. Piyarathna,123 M. Płosko´ n,80 M. Planinic,97 F. Pliquett,69 J. Pluta,139 S. Pochybova,142 P. L. M. Podesta-Lerma,117 M. G. Poghosyan,95 B. Polichtchouk,91 N. Poljak,97 W. Poonsawat,112 A. Pop,47 H. Poppenborg,141 S. Porteboeuf-Houssais,131 V. Pozdniakov,75 S. K. Prasad,4R. Preghenella,53 F. Prino,58 C. A. Pruneau,140 I. Pshenichnov,62 M. Puccio,28 V. Punin,106 J. Putschke,140 S. Raha,4S. Rajput,99 J. Rak,124 A. Rakotozafindrabe,134 L. Ramello,34 F. Rami,133 D. B. Rana,123 R. Raniwala,100 S. Raniwala,100 S. S. Räsänen,44 B. T. Rascanu,69 D. Rathee,98 V. Ratza,43 I. Ravasenga,33 K. F. Read,127,95 K. Redlich,85,eA. Rehman,24 P. Reichelt,69 F. Reidt,36 X. Ren,7R. Renfordt,69 A. Reshetin,62 K. Reygers,102 V. Riabov,96 T. Richert,63,81 M. Richter,23 P. Riedler,36 W. Riegler,36 F. Riggi,30 C. Ristea,68 M. Rodríguez Cahuantzi,2 044901-16 NEUTRAL PION AND ηMESON PRODUCTION AT … PHYSICAL REVIEW C 98, 044901 (2018) K. Røed,23 R. Rogalev,91 E. Rogochaya,75 D. Rohr,36,40 D. Röhrich,24 P. S. Rokita,139 F. Ronchetti,51 E. D. Rosas,70 K. Roslon,139 P. Rosnet,131 A. Rossi,31,56 A. Rotondi,135 F. Roukoutakis,84 C. Roy,133 P. Roy,107 O. V. Rueda,70 R. Rui,27 B. Rumyantsev,75 A. Rustamov,87 E. Ryabinkin,88 Y. Ryabov,96 A. Rybicki,115 S. Saarinen,44 S. Sadhu,138 S. Sadovsky,91 K. Šafaˇ rík,36 S. K. Saha,138 B. Sahoo,48 P. Sahoo,49 R. Sahoo,49 S. Sahoo,66 P. K. Sahu,66 J. Saini,138 S. Sakai,130 M. A. Saleh,140 J. Salzwedel,19 S. Sambyal,99 V. Samsonov,96,92 A. Sandoval,72 A. Sarkar,73 D. Sarkar,138 N. Sarkar,138 P. Sarma,42 M. H. P. Sas,63 E. Scapparone,53 F. Scarlassara,31 B. Schaefer,95 H. S. Scheid,69 C. Schiaua,47 R. Schicker,102 C. Schmidt,104 H. R. Schmidt,101 M. O. Schmidt,102 M. Schmidt,101 N. V. Schmidt,69,95 J. Schukraft,36 Y. Schutz,36,133 K. Schwarz,104 K. Schweda,104 G. Scioli,29 E. Scomparin,58 M. Šefˇ cík,39 J. E. Seger,17 Y. Sekiguchi,129 D. Sekihata,45 I. Selyuzhenkov,92,104 K. Senosi,73 S. Senyukov,133 E. Serradilla,72 P. Sett,48 A. Sevcenco,68 A. Shabanov,62 A. Shabetai,111 R. Shahoyan,36 W. Shaikh,107 A. Shangaraev,91 A. Sharma,98 A. Sharma,99 N. Sharma,98 A. I. Sheikh,138 K. Shigaki,45 M. Shimomura,83 S. Shirinkin,64 Q. Shou,7K. Shtejer,9,28 Y. Sibiriak,88 S. Siddhanta,54 K. M. Sielewicz,36 T. Siemiarczuk,85 S. Silaeva,88 D. Silvermyr,81 G. Simatovic,90,97 G. Simonetti,36,103 R. Singaraju,138 R. Singh,86 V. Singhal,138 T. Sinha,107 B. Sitar,15 M. Sitta,34 T. B. Skaali,23 M. Slupecki,124 N. Smirnov,143 R. J. M. Snellings,63 T. W. Snellman,124 J. Song,20 F. Soramel,31 S. Sorensen,127 F. Sozzi,104 I. Sputowska,115 J. Stachel,102 I. Stan,68 P. Stankus,95 E. Stenlund,81 D. Stocco,111 M. M. Storetvedt,37 P. Strmen,15 A. A. P. Suaide,118 T. Sugitate,45 C. Suire,61 M. Suleymanov,16 M. Suljic,27 R. Sultanov,64 M. Šumbera,94 S. Sumowidagdo,50 K. Suzuki,110 S. Swain,66 A. Szabo,15 I. Szarka,15 U. Tabassam,16 J. Takahashi,119 G. J. Tambave,24 N. Tanaka,130 M. Tarhini,111,61 M. Tariq,18 M. G. Tarzila,47 A. Tauro,36 G. Tejeda Muñoz,2A. Telesca,36 K. Terasaki,129 C. Terrevoli,31 B. Teyssier,132 D. Thakur,49 S. Thakur,138 D. Thomas,116 F. Thoresen,89 R. Tieulent,132 A. Tikhonov,62 A. R. Timmins,123 A. Toia,69 M. Toppi,51 S. R. Torres,117 S. Tripathy,49 S. Trogolo,28 G. Trombetta,35 L. Tropp,39 V. Trubnikov,3W. H. Trzaska,124 T. P. Trzcinski,139 B. A. Trzeciak,63 T. Tsuji,129 A. Tumkin,106 R. Turrisi,56 T. S. Tveter,23 K. Ullaland,24 E. N. Umaka,123 A. Uras,132 G. L. Usai,26 A. Utrobicic,97 M. Vala,113 J. Van Der Maarel,63 J. W. Van Hoorne,36 M. van Leeuwen,63 T. Vanat,94 P. Vande Vyvre,36 D. Varga,142 A. Vargas,2M. Vargyas,124 R. Varma,48 M. Vasileiou,84 A. Vasiliev,88 A. Vauthier,79 O. Vázquez Doce,103,114 V. Vechernin,137 A. M. Veen,63 A. Velure,24 E. Vercellin,28 S. Vergara Limón,2L. Vermunt,63 R. Vernet,8R. Vértesi,142 L. Vickovic,126 J. Viinikainen,124 Z. Vilakazi,128 O. Villalobos Baillie,108 A. Villatoro Tello,2A. Vinogradov,88 L. Vinogradov,137 T. Virgili,32 V. Vislavicius,81 A. Vodopyanov,75 M. A. Völkl,101 K. Voloshin,64 S. A. Voloshin,140 G. Volpe,35 B. von Haller,36 I. Vorobyev,103,114 D. Voscek,113 D. Vranic,36,104 J. Vrláková,39 B. Wagner,24 H. Wang,63 M. Wang,7Y. Watanabe,129,130 M. Weber,110 S. G. Weber,104 A. Wegrzynek,36 D. F. Weiser,102 S. C. Wenzel,36 J. P. Wessels,141 U. Westerhoff,141 A. M. Whitehead,122 J. Wiechula,69 J. Wikne,23 G. Wilk,85 J. Wilkinson,53 G. A. Willems,36,141 M. C. S. Williams,53 E. Willsher,108 B. Windelband,102 W. E. Witt,127 R. Xu,7S. Yalcin,78 K. Yamakawa,45 P. Yang,7S. Yano,45 Z. Yin,7H. Yokoyama,79,130 I.-K. Yoo,20 J. H. Yoon,60 E. Yun,20 V. Yurchenko,3V. Zaccolo,58 A. Zaman,16 C. Zampolli,36 H. J. C. Zanoli,118 N. Zardoshti,108 A. Zarochentsev,137 P. Závada,67 N. Zaviyalov,106 H. Zbroszczyk,139 M. Zhalov,96 H. Zhang,24,7X. Zhang,7 Y. Zhang,7C. Zhang,63 Z. Zhang,7,131 C. Zhao,23 N. Zhigareva,64 D. Zhou,7Y. Zhou,89 Z. Zhou,24 H. Zhu,7,24 J. Zhu,7Y. Zhu,7 A. Zichichi,29,11 M. B. Zimmermann,36 G. Zinovjev,3J. Zmeskal,110 and S. Zou7 (ALICE Collaboration) 1A. I. Alikhanyan National Science Laboratory (Yerevan Physics Institute) Foundation, Yerevan, Armenia 2Benemérita Universidad Autónoma de Puebla, Puebla, Mexico 3Bogolyubov Institute for Theoretical Physics, National Academy of Sciences of Ukraine, Kiev, Ukraine 4Bose Institute, Department of Physics and Centre for Astroparticle Physics and Space Science (CAPSS), Kolkata, India 5Budker Institute for Nuclear Physics, Novosibirsk, Russia 6California Polytechnic State University, San Luis Obispo, California, USA 7Central China Normal University, Wuhan, China 8Centre de Calcul de l’IN2P3, Villeurbanne, Lyon, France 9Centro de Aplicaciones Tecnológicas y Desarrollo Nuclear (CEADEN), Havana, Cuba 10Centro de Investigación y de Estudios Avanzados (CINVESTAV), Mexico City and Mérida, Mexico 11Centro Fermi - Museo Storico della Fisica e Centro Studi e Ricerche “Enrico Fermi”, Rome, Italy 12Chicago State University, Chicago, Illinois, United States 13China Institute of Atomic Energy, Beijing, China 14Chonbuk National University, Jeonju, Republic of Korea 15Comenius University Bratislava, Faculty of Mathematics, Physics and Informatics, Bratislava, Slovakia 16COMSATS Institute of Information Technology (CIIT), Islamabad, Pakistan 17Creighton University, Omaha, Nebraska, United States 18Department of Physics, Aligarh Muslim University, Aligarh, India 19Department of Physics, Ohio State University, Columbus, Ohio, United States 20Department of Physics, Pusan National University, Pusan, Republic of Korea 21Department of Physics, Sejong University, Seoul, Republic of Korea 044901-17 S. ACHARYA et al. PHYSICAL REVIEW C 98, 044901 (2018) 22Department of Physics, University of California, Berkeley, California, United States 23Department of Physics, University of Oslo, Oslo, Norway 24Department of Physics and Technology, University of Bergen, Bergen, Norway 25Dipartimento di Fisica dell’Università “La Sapienza” and Sezione INFN, Rome, Italy 26Dipartimento di Fisica dell’Università and Sezione INFN, Cagliari, Italy 27Dipartimento di Fisica dell’Università and Sezione INFN, Trieste, Italy 28Dipartimento di Fisica dell’Università and Sezione INFN, Turin, Italy 29Dipartimento di Fisica e Astronomia dell’Università and Sezione INFN, Bologna, Italy 30Dipartimento di Fisica e Astronomia dell’Università and Sezione INFN, Catania, Italy 31Dipartimento di Fisica e Astronomia dell’Università and Sezione INFN, Padova, Italy 32Dipartimento di Fisica “E. R. Caianiello” dell’Università and Gruppo Collegato INFN, Salerno, Italy 33Dipartimento DISAT del Politecnico and Sezione INFN, Turin, Italy 34Dipartimento di Scienze e Innovazione Tecnologica dell’Università del Piemonte Orientale and INFN Sezione di Torino, Alessandria, Italy 35Dipartimento Interateneo di Fisica ‘M. Merlin’ and Sezione INFN, Bari, Italy 36European Organization for Nuclear Research (CERN), Geneva, Switzerland 37Faculty of Engineering and Business Administration, Western Norway University of Applied Sciences, Bergen, Norway 38Faculty of Nuclear Sciences and Physical Engineering, Czech Technical University in Prague, Prague, Czech Republic 39Faculty of Science, P. J. Šafárik University, Košice, Slovakia 40Frankfurt Institute for Advanced Studies, Johann Wolfgang Goethe-Universität Frankfurt, Frankfurt, Germany 41Gangneung-Wonju National University, Gangneung, Republic of Korea 42Gauhati University, Department of Physics, Guwahati, India 43Helmholtz-Institut für Strahlenund Kernphysik, Rheinische Friedrich-Wilhelms-Universität Bonn, Bonn, Germany 44Helsinki Institute of Physics (HIP), Helsinki, Finland 45Hiroshima University, Hiroshima, Japan 46Hochschule Worms, Zentrum für Technologietransfer und Telekommunikation (ZTT), Worms, Germany 47Horia Hulubei National Institute of Physics and Nuclear Engineering, Bucharest, Romania 48Indian Institute of Technology Bombay (IIT), Mumbai, India 49Indian Institute of Technology Indore, Indore, India 50Indonesian Institute of Sciences, Jakarta, Indonesia 51INFN, Laboratori Nazionali di Frascati, Frascati, Italy 52INFN, Sezione di Bari, Bari, Italy 53INFN, Sezione di Bologna, Bologna, Italy 54INFN, Sezione di Cagliari, Cagliari, Italy 55INFN, Sezione di Catania, Catania, Italy 56INFN, Sezione di Padova, Padova, Italy 57INFN, Sezione di Roma, Rome, Italy 58INFN, Sezione di Torino, Turin, Italy 59INFN, Sezione di Trieste, Trieste, Italy 60Inha University, Incheon, Republic of Korea 61Institut de Physique Nucléaire d’Orsay (IPNO), Institut National de Physique Nucléaire et de Physique des Particules (IN2P3/CNRS), Université de Paris-Sud, Université Paris-Saclay, Orsay, France 62Institute for Nuclear Research, Academy of Sciences, Moscow, Russia 63Institute for Subatomic Physics of Utrecht University, Utrecht, Netherlands 64Institute for Theoretical and Experimental Physics, Moscow, Russia 65Institute of Experimental Physics, Slovak Academy of Sciences, Košice, Slovakia 66Institute of Physics, Bhubaneswar, India 67Institute of Physics of the Czech Academy of Sciences, Prague, Czech Republic 68Institute of Space Science (ISS), Bucharest, Romania 69Institut für Kernphysik, Johann Wolfgang Goethe-Universität Frankfurt, Frankfurt, Germany 70Instituto de Ciencias Nucleares, Universidad Nacional Autónoma de México, Mexico City, Mexico 71Instituto de Física, Universidade Federal do Rio Grande do Sul (UFRGS), Porto Alegre, Brazil 72Instituto de Física, Universidad Nacional Autónoma de México, Mexico City, Mexico 73iThemba LABS, National Research Foundation, Somerset West, South Africa 74Johann-Wolfgang-Goethe Universität Frankfurt Institut für Informatik, Fachbereich Informatik und Mathematik, Frankfurt, Germany 75Joint Institute for Nuclear Research (JINR), Dubna, Russia 76Konkuk University, Seoul, Republic of Korea 77Korea Institute of Science and Technology Information, Daejeon, Republic of Korea 78KTO Karatay University, Konya, Turkey 79Laboratoire de Physique Subatomique et de Cosmologie, Université Grenoble-Alpes, CNRS-IN2P3, Grenoble, France 044901-18 NEUTRAL PION AND ηMESON PRODUCTION AT … PHYSICAL REVIEW C 98, 044901 (2018) 80Lawrence Berkeley National Laboratory, Berkeley, California, USA 81Lund University Department of Physics, Division of Particle Physics, Lund, Sweden 82Nagasaki Institute of Applied Science, Nagasaki, Japan 83Nara Women’s University (NWU), Nara, Japan 84National and Kapodistrian University of Athens, School of Science, Department of Physics, Athens, Greece 85National Centre for Nuclear Research, Warsaw, Poland 86National Institute of Science Education and Research, HBNI, Jatni, India 87National Nuclear Research Center, Baku, Azerbaijan 88National Research Centre Kurchatov Institute, Moscow, Russia 89Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark 90Nikhef, National institute for subatomic physics, Amsterdam, Netherlands 91NRC Kurchatov Institute IHEP, Protvino, Russia 92NRNU Moscow Engineering Physics Institute, Moscow, Russia 93Nuclear Physics Group, STFC Daresbury Laboratory, Daresbury, United Kingdom 94Nuclear Physics Institute of the Czech Academy of Sciences, ˇ Rež u Prahy, Czech Republic 95Oak Ridge National Laboratory, Oak Ridge, Tennessee, USA 96Petersburg Nuclear Physics Institute, Gatchina, Russia 97Physics department, Faculty of science, University of Zagreb, Zagreb, Croatia 98Physics Department, Panjab University, Chandigarh, India 99Physics Department, University of Jammu, Jammu, India 100Physics Department, University of Rajasthan, Jaipur, India 101Physikalisches Institut, Eberhard-Karls-Universität Tübingen, Tübingen, Germany 102Physikalisches Institut, Ruprecht-Karls-Universität Heidelberg, Heidelberg, Germany 103Physik Department, Technische Universität München, Munich, Germany 104Research Division and ExtreMe Matter Institute EMMI, GSI Helmholtzzentrum für Schwerionenforschung GmbH, Darmstadt, Germany 105Rudjer Boškovi´c Institute, Zagreb, Croatia 106Russian Federal Nuclear Center (VNIIEF), Sarov, Russia 107Saha Institute of Nuclear Physics, Kolkata, India 108School of Physics and Astronomy, University of Birmingham, Birmingham, United Kingdom 109Sección Física, Departamento de Ciencias, Pontificia Universidad Católica del Perú, Lima, Peru 110Stefan Meyer Institut für Subatomare Physik (SMI), Vienna, Austria 111SUBATECH, IMT Atlantique, Université de Nantes, CNRS-IN2P3, Nantes, France 112Suranaree University of Technology, Nakhon Ratchasima, Thailand 113Technical University of Košice, Košice, Slovakia 114Technische Universität München, Excellence Cluster “Universe”, Munich, Germany 115The Henryk Niewodniczanski Institute of Nuclear Physics, Polish Academy of Sciences, Cracow, Poland 116The University of Texas at Austin, Austin, Texas, USA 117Universidad Autónoma de Sinaloa, Culiacán, Mexico 118Universidade de São Paulo (USP), São Paulo, Brazil 119Universidade Estadual de Campinas (UNICAMP), Campinas, Brazil 120Universidade Federal do ABC, Santo Andre, Brazil 121University College of Southeast Norway, Tonsberg, Norway 122University of Cape Town, Cape Town, South Africa 123University of Houston, Houston, Texas, USA 124University of Jyväskylä, Jyväskylä, Finland 125University of Liverpool, Liverpool, United Kingdom 126University of Split, Faculty of Electrical Engineering, Mechanical Engineering and Naval Architecture, Split, Croatia 127University of Tennessee, Knoxville, Tennessee, USA 128University of the Witwatersrand, Johannesburg, South Africa 129University of Tokyo, Tokyo, Japan 130University of Tsukuba, Tsukuba, Japan 131Université Clermont Auvergne, CNRS/IN2P3, LPC, Clermont-Ferrand, France 132Université de Lyon, Université Lyon 1, CNRS/IN2P3, IPN-Lyon, Villeurbanne, Lyon, France 133Université de Strasbourg, CNRS, IPHC UMR 7178, F-67000 Strasbourg, France, Strasbourg, France 134Université Paris-Saclay Centre dÉtudes de Saclay (CEA), IRFU, Department de Physique Nucléaire (DPhN), Saclay, France 135Università degli Studi di Pavia, Pavia, Italy 136Università di Brescia, Brescia, Italy 137V. Fock Institute for Physics, St. Petersburg State University, St. Petersburg, Russia 138Variable Energy Cyclotron Centre, Kolkata, India 044901-19 S. ACHARYA et al. PHYSICAL REVIEW C 98, 044901 (2018) 139Warsaw University of Technology, Warsaw, Poland 140Wayne State University, Detroit, Michigan, USA 141Westfälische Wilhelms-Universität Münster, Institut für Kernphysik, Münster, Germany 142Wigner Research Centre for Physics, Hungarian Academy of Sciences, Budapest, Hungary 143Yale University, New Haven, Connecticut, USA 144Yonsei University, Seoul, Republic of Korea aDipartimento DET del Politecnico di Torino, Turin, Italy. bM. V. Lomonosov Moscow State University, D. V. Skobeltsyn Institute of Nuclear, Physics, Moscow, Russia. cDepartment of Applied Physics, Aligarh Muslim University, Aligarh, India. dDeceased. eInstitute of Theoretical Physics, University of Wroclaw, Poland. 044901-20