Study of the neutral mesons in Pb-Pb collisions at s NN − − − − √ = 2.76 TeV in the ALICE experiment at LHC
SStudy of the neutral mesons in Pb-Pb collisions at √ s N N =2.76 TeV in the ALICE experiment at LHC
Lucia Leardini on behalf of the ALICE Collaboration
Physikalisches Institut, Heidelberg UniversityE-mail: [email protected]
Abstract.
The π and η meson production in Pb-Pb collisions at √ s NN =2.76 TeV is studiedwith the ALICE experiment at the LHC. The π invariant yields and nuclear modification factor R AA are presented here in six centrality classes. The results are a combined measurement usingthe Photon Conversion Method (PCM) and the PHOS detector, in the transverse momentumrange 0.4 < p T <
12 GeV/ c . The π R AA is studied in different centrality classes and comparedwith results from experiments at lower energies, both as a function of transverse momentum.The η meson production is studied using the PCM and the EMCal detector. The combinationof the individual results will make possible the measurement of the η differential invariant crosssection as a function of transverse momentum from 1 to 22 GeV/ c in different centrality classes.
1. Introduction
The ALICE experiment [1] scientific program is focused on the analysis of heavy-ion collisionsand on the study of the Quark-Gluon Plasma, a state of the matter with high temperature andlarge energy density, where gluon and quarks are deconfined, as predicted by QCD. As thismedium has high color charge density, partons produced in the earlier stage of the collision loseenergy through radiative gluon emission and multiple scattering while traversing it. The partonenergy loss affects the hadron yields in the high p T region, a modification that can be observedcomparing results on particle production in Pb-Pb collisions with those from pp collisions. Inorder to study this effect, the nuclear modification factor R AA is used: R AA ( p T ) = d N/dp T d y | AA (cid:104) T AA (cid:105) d σ/ d p T d y | pp (1)where (cid:104) T AA (cid:105) = (cid:104) N coll (cid:105) /σ ppinel is used to scale the pp yield to the Pb-Pb yield. Here, (cid:104) N coll (cid:105) is theaverage number of binary nucleon-nucleon collisions obtained from the Glauber model [2] and σ ppinel is the pp inelastic cross section [3]. The π R AA has been studied and is described in thefollowing. The ALICE experiment [4] measures neutral pions decaying in two photons [5][6].The detectors involved in the measurement of photons are a lead-scintillator electromagneticcalorimeter (EMCal) and the photon spectrometer (PHOS), made of lead tungstate crystals.The EMCal covers | η | < φ = 100 ◦ while PHOS covers | η | < φ = 60 ◦ .It is also possible to measure photons through their conversion into electrons and positrons inthe detector material. This method is called Photon Conversion Method (PCM) and relies onthe Inner Tracking System (ITS) and on the Time Projection Chamber (TPC), which have a r X i v : . [ nu c l - e x ] D ec ull azimuthal coverage. The material thickness, up to the middle of the TPC, is X/X =11.4 ± sys %. ) c (GeV/ T p ) c ( G e V / y d T p d T p N d ev N π × = 2.76 TeV NN s
0 5% Pb Pb × = 2.76 TeV NN s × = 2.76 TeV NN s
10 20% Pb Pb × = 2.76 TeV NN s
20 40% Pb Pb × = 2.76 TeV NN s
40 60% Pb Pb × = 2.76 TeV NN s
60 80% Pb Pb fits to Pb Pb = 2.76 TeV s pp Tsallis fitpower law fit ALI−PUB−81690
Figure 1. (Color online) Invariant differ-ential yields of neutral pions produced inPbPb and inelastic pp collisions. The spec-tra are the weighted average of PHOS andPCM results. Vertical lines represent statisti-cal uncertainties, boxes systematic uncertain-ties and horizontal lines indicate bin width. ) c (GeV/ T p AA R π
0 5% π
20 40% π
60 80% = 2.76 TeV NN s Pb Pb
ALI−PUB−81794 ) c (GeV/ T p AA R π = 2.76 TeV NN s PHENIX 0 10% Au Au π = 200 GeV NN s = 62.4 GeV NN s = 39 GeV NN s WA98 0 13% Pb Pb π = 17.3 GeV NN s ALI−PUB−81798
Figure 2. (Color online) Top: Neutralpion modification factor in Pb-Pb collisions.Bottom: Neutral pion modification factor inPb-Pb for centrality class 0-10% measuredby ALICE [6] is compared with the resultsfrom Au-Au collisions measured by PHENIX[10][11] and with the results from Pb-Pbcollisions at the CERN SPS [12].
2. Results
The π invariant yields presented here have been measured independently by PHOS and PCMand were then combined into a weighted average. In Fig. 1, the combined PHOS-PCM π yieldsare shown for pp collisions and Pb-Pb collisions in 6 centrality classes. For PHOS, the datasample consists of 16.1 × events collected in the 2010 Pb-Pb run and of 34.7 × eventsfrom the 2011 pp run at √ s NN = 2 .
76 TeV, while for PCM it is 13.2 × and 58 × events, respectively [6]. The transverse momentum range goes from 0.6 to 12 GeV/ c . For thepp spectrum, a Tsallis function [7] for the full p T region and a power law E d N/ d p ∝ /p n T ,nly for the high p T region ( p T > c ), are shown. The power law fit extrapolation above8 GeV/ c is used as reference in the R AA calculation. The Pb-Pb spectra are fitted with theparametrization described in [6] appendix.The neutral pion nuclear modification factor is calculated with Eq. 1. The nuclear overlapfunction (cid:104) T AA (cid:105) was obtained with a Glauber Monte Carlo calculation [2][8][9], the values anduncertainties for each centrality class can be found in [6]. In Fig. 2, top panel, the combinedPCM-PHOS result for the π R AA is shown. The R AA decreases going from peripheral to centralcollisions, due to different energy loss in the medium. For all the centrality classes, the R AA has amaximum below 2 GeV/ c of transverse momentum and then decreases until, above 6 GeV/ c , thetrend of the data points is flat. In this region, particle production is expected to be dominatedby fragmentation of hard-scattered partons.The dependence of R AA on the system energy has also been studied. In the bottom panel ofFig. 2 the R AA for Pb-Pb collisions at √ s NN = 2 .
76 TeV measured with ALICE is compared toprevious results at lower energies. ALICE data points are below the lower energies points andtheir suppression is more evident for p T > c . Models attribute this behavior to the higherinitial energy densities created at larger √ s NN . This dominates over the increase of R AA fromthe harder initial parton p T spectra [13] and no increase at high p T is observed. It is also notedthat the position of the maximum seems to be shifting towards lower transverse momentum withincreasing collision energy. The neutral pion R AA has been compared with different theoreticalmodels. The GLV calculation [14][15] and WHDG [16] prediction describe the interaction ofhard-scattered parton with the medium using perturbative QCD. EPOS [17] and Nemchik etal. [18][19] calculations use a hydrodynamic description at low p T and absorption of color dipolesat high p T . Comparison plots can be found in [6]. ) (GeV/c γγ m0.4 0.45 0.5 0.55 0.6 0.65 0.7 0.75 E v en t s / M e V / c × × Signal
ALICE performance04.05.2014 =2.76 TeV NN sPb Pb, Centrality 0 10%PCM <1.5 GeV/c T ALI−PERF−68244 ) (GeV/c γγ m0.4 0.45 0.5 0.55 0.6 0.65 0.7 0.75 E v en t s / M e V / c =2.76 TeV NN sPb Pb, Centrality 0 10%EMCAL<12 GeV/c T ALICE performance04.05.2014
ALI−PERF−68307
Figure 3.
Photon pair invariant mass distribution in the η mass region in selected p T slicesfor PCM (left) and EMCal (right) for 0-10% Pb-Pb collisions. The black histogram and thered points show the data before and after background subtraction, respectively. For PCM, theinvariant mass spectra after background subtraction is scaled by a factor 50. The blue line showsthe fit to the invariant mass spectra after background subtraction.The η meson can be measured in ALICE in Pb-Pb collisions at √ s NN = 2.76 TeV withthe 2011 data sample. It has an integrated luminosity one order of magnitude larger than thecorresponding one for 2010, making possible a reliable reconstruction of the η meson spectrum.We show here the first steps towards a combined PCM-EMCal η meson measurement. The datasample consists of about 17 million events in the central class 0-10% for both PCM and EMCal.he transverse momentum range for this combined measurement can go from 1 up to 22 GeV/ c with an overlap between the two approaches in the intermediate transverse momentum region(4 < p T <
10 GeV/ c ).In Fig. 3, the photon pair invariant mass distribution is shown for two sample transversemomentum bins. The η meson peak is clearly visible. For PCM (left panel), the lowest p T bin is shown. The general fit function used by PCM for all p T is a Gaussian function combinedwith an exponential tail on left side of the peak (to account for the electron bremsstrahlung)and with a linear fit to describe the remaining combinatorial background under the peak afterthe combinatorial background subtraction. The fit is used only to extract the η peak positionand width. The η yields are then extracted by integrating the peak area and subtracting theintegral of the linear fit. In the right panel of Fig. 3, a bin in the intermediate range of theEMCal p T reach is shown. The fit function used here is a Crystal-Ball function while a linearfit or a parabola, depending on the p T and centrality, is adopted for the residual backgroundhypothesis.
3. Conclusions
Measurements of the neutral pion production in Pb-Pb collisions at √ s NN = 2.76 TeV have beenshown combining two independent methods, the measurement of the photons with the PHOSdetector and the reconstruction of the converted photons with the tracking system. These twoindependent measurements are consistent and were combined in a weighted average for the π spectra. The neutral pion nuclear suppression factor R AA was calculated from the measuredneutral pion spectra separately for the two methods and then combined, in order to reduce thesystematic errors. The π R AA was studied for different centrality classes and compared to low-energy results. The suppression is stronger for higher collision energy when comparing Pb-Pbcollisions at √ s NN = 2.76 TeV to results from RHIC and CERN SPS.The η meson invariant mass distribution for two transverse momentum bins from the 2011 Pb-Pbcollisions at √ s NN = 2.76 TeV is shown. Also here two independent methods are used, photonmeasurement with the EMCal detector and photon conversion. The transverse momentum rangefor this combined measurement will go from 1 up to 22 GeV/ c with an overlap in the intermediatetransverse momentum region (4 < p T <
10 GeV/ c ). This analysis will allow for a study of the η meson suppression in Pb-Pb collisions. References [1] B. Abelev et al. [ALICE Collaboration],
JINST S08002.[2] B. Abelev et al. [ALICE Collaboration],
Phys. Rev. C (2013) 044909.[3] B. Abelev et al. [ALICE Collaboration], Eur. Phys. J. C (2013) 2456.[4] B. Abelev et al. [ALICE Collaboration], Int. J. Mod. Phys. A (2014) 1430044.[5] B. Abelev et al. [ALICE Collaboration], Phys. Lett. B (2012) 162.[6] B. Abelev et al. [ALICE Collaboration], Eur. Phys. J. C (2014) 3108.[7] C. Tsallis, J. Statist. Phys. Ann. Rev. Nucl. Sci. (2007) 205.[9] B. Alver, M. Baker, C. Loizides and P.Steinberg, (2008) arXiv:0805.4411.[10] A. Adare et al. [PHENIX Collaboration], Phys. Rev. Lett. (2012) 152301.[11] A. Adare et al. [PHENIX Collaboration],
Phys. Rev. Lett. (2008) 232301.[12] M. Aggarwal et al. [WA89 Collaboration],
Phys. Rev. Lett. (2008) 242301.[13] K. C. Zapp, F. Krauss and U. A. Wiedemann, JHEP , 080 (2013), arXiv:1212.1599.[14] R. Sharma, I. Vitev and B. -W. Zhang,
Phys. Rev. C (2009) 054902.[15] R. Neufeld, I. Vitev and B. -W. Zhang, Phys. Lett. B (2011) 590.[16] W. A. Horowitz, Int. J. Mod. Phys. E (2007) 2193.[17] K. Werner, I. Karpenko, M. Bleicher, T. Pierog and S. Porteboeuf-Houssais, Phys. Rev. C (2012) 064907.[18] B. Kopeliovich, J. Nemchik, I. Potashnikova adn I. Schmidt, Phys. Rev. C86