Centrality dependence of charged particle spectra and RCP in Pb+Pb collisions at \sqrt{s_{NN}}=2.76TeV with the ATLAS detector at the LHC
CCentrality dependence of charged particle spectraand R CP in Pb+Pb collisions at √ s N N = 2 . TeVwith the ATLAS detector at the LHC
Alexander Milov, for the ATLAS Collaboration
Department of Particle Physics and Astrophysics,Weizmann Institute of Science, Rehovot 76100, IsraelE-mail: [email protected]
Abstract.
The ATLAS experiment at the LHC measures the charged particle spectraand the nuclear modification factor in Pb+Pb collisions at the √ s NN = 2 .
76 TeV in atransverse momentum range up to 30 GeV and a pseudorapidity range up to | η | < . p T of about 7 GeV. A suppression of more than a factor of2 is also measured at the upper edge of the analyzed p T range. The suppression doesnot show any strong η dependence.
1. Introduction
High p T hadrons are generally regarded to be produced through jet fragmentation, whichhas been successfully modeled using perturbative QCD. The energy loss of hard scatteredpartons traversing the hot and dense medium translates into a reduction in the yield ofhadrons, measured as a ratio of yields per nucleon-nucleon interactions in head-on heavyion collisions to those measured in p+p collisions [1, 2]. Even without equivalent proton-proton data the ratio of yields can be studied as a function of centrality in the heavyion collisions. In this case the R CP is defined as the ratio of yields measured in centralcollisions to the yield measured in peripheral collisions, both scaled by the correspondingnumbers of binary nucleon-nucleon collisions [3]. Study of the charged particle spectra athigh p T provides an independent method to understand the jet quenching independentof any particular jet reconstruction algorithm.
2. Analysis
This analysis uses 4 . × minimum bias events ( L int ≈ µ b − ) taken during the2010 LHC lead-lead run and satisfying run and event cleaning selections. The eventcentrality in data is estimated using the total transverse energy measured by the ATLASForward Calorimeter, as described in [4]. The correspondence between the cross sectionmeasured in the data after all trigger and event selection requirements and the cross a r X i v : . [ nu c l - e x ] J u l entrality dependence of charged particle spectra and R CP in Pb+Pb collisions at √ s NN = 2 . TeV with the ATLAS detector at the LHC ±
2% Using thisnumber, the data is binned in 5% and 10% centrality bins and the mean number ofparticipants and collisions, N coll , for each centrality bin is estimated with the sameGlauber calculation.The Monte Carlo studies are based on the HIJING [6] and PYTHIA [7] eventgenerators. Two samples are used, one containing 2 . × HIJING minimum biasevents and one with 1 . × PYTHIA jet events simulated at √ s NN = 2 .
76 TeV overlaidwith minimum-bias HIJING events. This sample is used to study track reconstructionefficiency at high p T . Centrality in HIJING events is determined to match the measuredoccupancy in the first layer of the ATLAS Pixel detector for each centrality bin.Charged particle tracks are reconstructed using the ATLAS Inner Detectorimmersed in 2 T field of a superconducting solenoid magnet. Tracks are measured inthe pseudorapidity region | η | < . p T cut of 500 MeV.A charged particle passing through the Inner Detector region typically traverses threelayers of silicon pixel detectors (Pixel), and four double-sided silicon strip modules of thesemiconductor tracker (SCT) [8]. To improve the purity of the ID track reconstructionin the dense environment of heavy ion collisions, track quality requirements are morestringent than those used for p+p collisions [9]. We required at least two hits on a trackin the Pixel detector and at least 8 hits in the SCT with the condition that there are nomissing hits in either detector. Somewhat more relaxed cuts were also studied to workout the systematic uncertainties. This analysis does not make use of information fromthe Transition Radiation Tracker.The contribution from combinatorial fake tracks resulting from spurious hitcombinations and from secondaries is further reduced by requiring the significance ofthe longitudinal and transverse impact parameters of each track to be less than 3. Thesignificance is the track impact parameter d and z sin( θ ) divided by the track fit errorsarising from the reconstruction, which accounts for the p T and η dependence of thetracking performance.The track reconstruction efficiency was evaluated using Monte Carlo samples. Theefficiencies are found to systematically decrease with | η | from 0.7 to 0.5 in the range ofmeasurement and also show p T dependence below 1 GeV. At the lowest measured p T theefficiency decreases by about 10%. The decrease of the efficiency with centrality (up to10% at | η | >
2) is predominantly due to the increased occupancy in the SCT. Residualsecondaries and some fake tracks, contributing up to 5% at the lowest measured p T ,are subtracted based on the HIJING sample. The correction due to finite momentumresolution, reaching 3% at p T =30 GeV, has been applied based on the detector responsestudies made with the HIJING+jet sample.Systematic uncertainties can be attributed to several different sources. Trackingcuts and vertex pointing contribute up to 4% each. The Monte-Carlo relateduncertainties such as tracking efficiency, truth particle association, backgroundsubtraction at low p T and the detector material contribute up to 5% depending on p T . Many of these contributions completely or partially cancel when using R CP . The entrality dependence of charged particle spectra and R CP in Pb+Pb collisions at √ s NN = 2 . TeV with the ATLAS detector at the LHC [GeV] T p1 10 ] - [ G e V T / dp h / d c h ) d N T p p / ( -8 -7 -6 -5 -4 -3 -2 -1
10 ATLAS Preliminary=2.76 TeV NN sPb+Pb -1 b m =7 int L |<0.35 h | (0-5)%(30-40)%(50-60)%(60-80)% [GeV] T p1 10 ] - [ G e V T / dp h / d c h ) d N T p p / ( -8 -7 -6 -5 -4 -3 -2 -1 |<0.80 h [GeV] T p1 10 ] - [ G e V T / dp h / d c h ) d N T p p / ( -8 -7 -6 -5 -4 -3 -2 -1 |<1.50 h [GeV] T p1 10 ] - [ G e V T / dp h / d c h ) d N T p p / ( -8 -7 -6 -5 -4 -3 -2 -1 |<2.00 h [GeV] T p1 10 ] - [ G e V T / dp h / d c h ) d N T p p / ( -8 -7 -6 -5 -4 -3 -2 -1 |<2.50 h Figure 1.
Fully corrected transverse momentum spectra for charged particles for fourcentrality classes and five η ranges. main contribution to the systematic uncertainty comes from the differences in the trackimpact parameter errors of 20-50% between data and Monte-Carlo. It contributes up to30% uncertainty in the most central collisions. The uncertainty on N coll , coming mainlyfrom the uncertainty in the correspondence of the measured cross section and Glaubermodel cross section, contributes from 3.8% in peripheral to 11.7% in the most central R CP presented in these proceeding.
3. Results and Summary
The corrected spectra are shown in Fig. 1. Invariant yields have been measured in five η bins for different collision centrality as a function of particle p T . The ALICE [10]and ATLAS central (0-5%) yields of charged particles are consistent to within 6-7%and the peripheral yields differ by more than 20%. However, this difference can be dueto differences in the definition of centrality classes which affect the number of N coll .Including this factor the results are consistent within the stated uncertainties.Figure 2 shows the R CP measured for different centralities. The spectra measuredin the (60-80)% centrality interval, shown in Fig. 1, is used as the denominator in R CP .The nuclear modification factor R CP drops to values comparable to the ones measuredat RHIC and then increases up to values close to 0.5 at the upper end of p T range at 30GeV for the most central events. The R CP does not show any strong η -dependence.The R CP averaged over the entire η range and integrated over p T from 20 to 30 GeVare shown in Fig. 3. The behavior of the integrated R CP is very similar to R CP of thefully reconstructed jets measured by ATLAS [11]. entrality dependence of charged particle spectra and R CP in Pb+Pb collisions at √ s NN = 2 . TeV with the ATLAS detector at the LHC [GeV] T p1 10 C P R -1