Neutral meson production in pp and Pb-Pb collisions at LHC

Yuri Kharlov, for the ALICE Collaboration

Institute for High Energy Physics, Protvino 142281, Russia

Abstract

The ALICE detector at the LHC studies π0 and ηmeson production by two complementary methods, using electro-magnetic calorimeters and the central tracking system for converted photons. Production spectra of π0 and η mesonswere measured with ALICE in pp collisions at LHC energies at mid-rapidity in a wide transverse momentum range.The spectrum and the nuclear modification factor RAA of π0 measured in Pb-Pb collisions at different centralities, showa clear pattern of strong suppression in a hot QCD medium with respect to pp collisions.

Keywords: Hadron production, relativistic heavy ion collisions

1. Introduction

Nuclear matter created in heavy-ion collisions at LHC energies is characterized by an energy density which issufficiently large to achieve a state of deconfined quarks and gluons. Partons produced in initial hard processes,traverse this dense matter and loose their energy via radiative or collisional mechanisms. Measurememnt of theenergy loss by hard-scattered partons escaping the medium is a direct probe to the energy density of the medium. Themost straightforward access to the parton energy loss is a measurement of nuclear modification of inclusive identifiedhadron spectra. Light neutral pions and η mesons are among those hadrons whose production can be calculatedwithin perturbative Quantum Chromodynamics, as their fragmentation functions are well known from the bulk ofexperimental data [1, 2]. Detection of π0 and ηmesons is performed via two-photon decays and identification of thesemesons is possible in a wide transverse momentum range.

Parton energy loss in a dense QCD medium is measured via comparison of hadron production in heavy-ion col-lisions of different centralities and the relevant hadron spectra in pp collisions. Identified hadron production studiesin pp collisions are thus an essential part of a heavy-ion physics programme, however not exclusively interesting as areference for heavy-ion collisions. Hadron spectra measured in pp collisions are important to tune and validate theo-retical predictions and models of hadron production. Most of these models were proven to produce a good descriptionof particle spectra in pp and pp̄ collisions up to the energy of the Tevatron collider,

√s = 1.8 TeV. Extrapolation of the

model parameters to the LHC energy√

s = 7 TeV and comparison of new experimental data with model calculationsprovides a crucial test of predictive power of the models.

We present measurements of π0 and η mesons in pp collisions at LHC energies√

s = 0.9, 2.76, 7 TeV and π0 inPb-Pb collisions at √sNN = 2.76 TeV with the ALICE detector [3]. The measured spectra are compared with pQCDcalculations. The measurement of the nuclear modification factor RAA for π0’s in Pb-Pb collisions in four centralitybins is also presented.

Email address: Yuri.Kharlov@cern.ch (Yuri Kharlov)

Available online at www.sciencedirect.com

Nuclear Physics A 910–911 (2013) 335–338

0375-9474/ © 2013 CERN Published by Elsevier B.V. All rights reserved.

www.elsevier.com/locate/nuclphysa

http://dx.doi.org/10.1016/j.nuclphysa.2012.12.046

© CERN for the benefit of the ALICE Collaboration.

2. Detector and data sample

Neutral pions and η mesons were measured with the ALICE experiment via their two-photon decay channel. Twoalternative and complementary methods were employed in this analysis. In one method, photons from π0 and η decayswere measured with the Photon Spectrometer PHOS which is a precise high-granularity electromagnetic calorimeterbuilt of lead tungstate crystals [4]. It consists of 3 modules with 64 × 56 crystals packed into a rectangular matrix ineach module. The acceptance of the PHOS spectrometer is 60◦ in azimuthal angle and |η| < 0.13 in pseudorapidity.Another method of photon reconstruction was based on identification of photons converted to e+e− pairs in the mediumof the inner ALICE detectors. This analysis is referred below to as the photon conversion method (PCM). Conversionelectrons and positrons were reconstructed by the ALICE central tracking system which consists of the Inner TrackingSystem (ITS) [5] and the Time Projection Chamber (TPC) [6]. The central tracking system covers the full azimuthalangle and the pseudorapidity range |η| < 0.9. Photons can be reconstructed from e+e− pairs if conversion takes placebetween the beam interaction point and the half radius of the TPC, R < 180 cm with an integrated material budgetof (0.114 ± 0.005)X0. Low probability of photon conversion is compensated by the wide acceptance of the centraltracking system, making the overall neutral meson reconstruction efficiency comparable to that of the PHOS detector.See Fig.1 for a schematic view of the ALICE detectors used for photon reconstruction. Additional information can befound in [7].

Figure 1: ALICE detectors used for photon reconstruction, the Photon Spectrometerand the central tracking system consisting of ITS and TPC.

The data analyzed for the neutral meson measurements in pp collisions were recorded in 2010–2011 with theminimum bias trigger [8] which selected most of pp inelastic events with efficiencies 0.91+0.03

−0.01, 0.88+0.06−0.04 and 0.85+0.06

−0.04for the three collision energies

√s = 0.9, 2.76 and 7 TeV, respectively. Integrated luminosities of the analyzed pp data

samples are compiled in Table 1. Pb-Pb collision results presented here were obtained from data collected by ALICE

√s (TeV)

Method 0.9 2.76 7PCM 0.14 1.05 5.6PHOS 0.14 0.63 5.7

Table 1: Integrated luminosities Lint in nb−1 of the analyzed pp data samples.

in 2010. The minimum bias trigger in Pb-Pb collisions was fully efficient, and the integrated luminosity used in theanalysis was 2 μb−1.

3. π0 and η meson spectra in pp collisions

Transverse momentum spectra of π0 and η mesons in pp collisions were measured by ALICE at all three collisionenergies [7], [9] and are shown in Fig.2. Wide pT ranges and fairly good accuracy of the measured differential crosssections were considered as a valuable experimental input to test pQCD calculations at the LHC energies. Next-to-Leading Order (NLO) pQCD calculations of π0 and η meson production with different sets of parton distributionfunctions CTEQ6M5 and DSS [10] and different fragmentation functions BKK [11] and AESSS [2] were performedand compared with the ALICE measurements. The pQCD calculations describe the data at

√s = 0.9 TeV well,

however larger center-of-mass energies are overestimated by the calculations by a factor of 2 − 3. Additionally, the

Y. Kharlov / Nuclear Physics A 910–911 (2013) 335–338336

Figure 2: Differential cross sections of π0 and ηmeson production in pp collisions at√

s = 0.9, 2.76 and 7 TeV compared to NLO pQCD calculationswith different QCD scales, shown by curves. Statistical and systematic uncertainties are shown by solid lines and shaded area respectively.

calculated pT spectra show a harder slope compared to the measured results. This discrepancy of pQCD and datacan be explained by uncertainties in fragmentation functions, especially in the gluon fragmentation function, becausepion production is considered to be dominated by gluon fragmentation at LHC energies in a wide pT range, up to100 GeV/c [1, 12].

The ALICE results of the π0 and η spectra in pp at√

s = 0.9, 2.76 and 7 TeV allowed to measure the ratio η/π0

vs pT. The comparison of the ALICE results with the compilation of η/π0 ratio measured by many lower-energyexperiments, presented in Fig.3, shows an universal behavior at any collision energy. The ratio η/π0 grows up topT = 2− 3 GeV/c and then saturates at the same value π0/η ≈ 0.5. As it was shown in [7], NLO pQCD predictions ofthis ratio is consistent with data in spite of the discrepancy of individual π0 and η spectra.

Figure 3: World compilation of the η to π0 production spectra ra-tio in pp collisions at low energies, along with the ALICE results at√

s = 0.9, 2.76 and 7 TeV, shown with the combined statistical andsystematic uncertainties.

Figure 4: Nuclear modification factor RAA for π0 production in Pb-Pbcollisions at √sNN = 2.76 TeV in four centrality classes. Statisticaland systematic uncertainties are shown by solid lines and boxes re-spectively.

4. π0 spectrum in Pb-Pb collisions

The available statistics of Pb-Pb collisions taken by the ALICE experiment in 2010 allowed to split data to 4collision centrality classes, 0 − 20%, 20 − 40%, 40 − 60% and 60 − 80% [13]. Both π0 reconstruction methods, PCM

Y. Kharlov / Nuclear Physics A 910–911 (2013) 335–338 337

and PHOS, were employed in Pb-Pb collisions. The high-multiplicity environment in heavy-ion collisions affectssignificantly the performance of both methods to extract the π0 signal. In PHOS, the overall detector occupancy inthe most central collisions reaches up to 20% of the total number of cells which makes the efficiency calculationchallenging. In PCM, an increase of the overall track multiplicity in central Pb-Pb collisions by a factor of 400 [14] incomparison with that in pp collisions leads also to a high combinatorial background of the converted photon candidatepairs. Here we present the π0 spectrum measurement by the photon conversion method only [15].

The nuclear modification factor of the π0 production RAA was defined as a ratio of the π0 spectrum measured inPb-Pb collision of a given centrality and normalized to the number of binary nucleon-nucleon collisions Nbc [13], tothe spectrum in pp collisions derived to inelastic events. The result shown in Fig.4 demonstrates a strong dependenceof the π0 suppression on collision centrality. It is instructive to note that the pT dependence of RAA in the most centralcollisions reveals the same behavior for most hadron spectra measured at the LHC energy, see e.g. a review [16] inthese proceedings.

5. Conclusion

The π0 and η spectra and the nuclear modification factor RAA measurements have been performed with the ALICEdetector. The measured spectra in pp collisions at

√s = 0.9 TeV agree with pQCD calculations, but show disagree-

ment with that at√

s = 2.76 and 7 TeV. The nuclear suppression measured by ALICE has a strong dependence on thecollision centrality and it increases with collision energy in comparison with RHIC [17], indicating an increase of theparton energy loss with energy.

This work was partially supported by the grants RFBR 10-02-91052 and RFBR 12-02-91527.

References

[1] R. Sassot, P. Zurita, M. Stratmann, Inclusive Hadron Production in the CERN-LHC Era, Phys.Rev. D82 (2010) 074011. arXiv:1008.0540,doi:10.1103/PhysRevD.82.074011.

[2] C. A. Aidala, F. Ellinghaus, R. Sassot, J. P. Seele, M. Stratmann, Global Analysis of Fragmentation Functions for Eta Mesons, Phys.Rev. D83(2011) 034002. arXiv:1009.6145, doi:10.1103/PhysRevD.83.034002.

[3] K. Aamodt, et al., The ALICE experiment at the CERN LHC, JINST 3 (2008) S08002. doi:10.1088/1748-0221/3/08/S08002.[4] G. Dellacasa, et al., ALICE technical design report of the photon spectrometer (PHOS), CERN-LHCC-99-04 (1999).[5] K. Aamodt, et al., Alignment of the ALICE Inner Tracking System with cosmic-ray tracks, JINST 5 (2010) P03003. arXiv:1001.0502,

doi:10.1088/1748-0221/5/03/P03003.[6] J. Alme, Y. Andres, H. Appelshauser, S. Bablok, N. Bialas, et al., The ALICE TPC, a large 3-dimensional tracking device with fast readout

for ultra-high multiplicity events, Nucl.Instrum.Meth. A622 (2010) 316–367. arXiv:1001.1950, doi:10.1016/j.nima.2010.04.042.[7] B. Abelev, et al., Neutral pion and η meson production in proton-proton collisions at

√s = 0.9 TeV and

√s = 7 TeV, Phys.Lett.B 717 (2012)

162–172. arXiv:1205.5724.[8] K. Aamodt, et al., Charged-particle multiplicity measurement in proton-proton collisions at sqrt(s) = 0.9 and 2.36 TeV with ALICE at LHC,

Eur.Phys.J. C68 (2010) 89–108. arXiv:1004.3034, doi:10.1140/epjc/s10052-010-1339-x.[9] K. Reygers, Production of Neutral Pions and Eta-mesons in pp Collisions Measured with ALICE, J.Phys.G G38 (2011) 124076. arXiv:

1106.5932, doi:10.1088/0954-3899/38/12/124076.[10] D. de Florian, R. Sassot, M. Stratmann, Global analysis of fragmentation functions for pions and kaons and their uncertainties, Phys.Rev.

D75 (2007) 114010. arXiv:hep-ph/0703242, doi:10.1103/PhysRevD.75.114010.[11] J. Binnewies, B. A. Kniehl, G. Kramer, Next-to-leading order fragmentation functions for pions and kaons, Z.Phys. C65 (1995) 471–480.

arXiv:hep-ph/9407347, doi:10.1007/BF01556135.[12] P. Chiappetta, M. Greco, J. Guillet, S. Rolli, M. Werlen, Next-to-leading order determination of pion fragmentation functions, Nucl.Phys.

B412 (1994) 3–38. arXiv:hep-ph/9301254, doi:10.1016/0550-3213(94)90492-8.[13] A. Toia, Bulk Properties of Pb-Pb collisions at sqrt(sNN) = 2.76 TeV measured by ALICE, J.Phys.G G38 (2011) 124007. arXiv:1107.1973,

doi:10.1088/0954-3899/38/12/124007.[14] K. Aamodt, et al., Centrality dependence of the charged-particle multiplicity density at mid-rapidity in Pb-Pb collisions at sqrt(sNN) = 2.76

TeV, Phys.Rev.Lett. 106 (2011) 032301. arXiv:1012.1657, doi:10.1103/PhysRevLett.106.032301.[15] G. Conesa Balbastre, Suppression of neutral pion production at large transverse momentum measured with the ALICE experiment in Pb-Pb

collisions at√

sNN = 2.76 TeV, J.Phys.G G38 (2011) 124117. arXiv:1109.4929, doi:10.1088/0954-3899/38/12/124117.[16] P. Christiansen, et al., High pT identified particle production in ALICE, these proceedings (2012).[17] A. Adare, et al., Suppression pattern of neutral pions at high transverse momentum in Au + Au collisions at s(NN)**(1/2) = 200-GeV and

constraints on medium transport coefficients, Phys.Rev.Lett. 101 (2008) 232301. arXiv:0801.4020, doi:10.1103/PhysRevLett.101.232301.

Y. Kharlov / Nuclear Physics A 910–911 (2013) 335–338338