Charged Particle Production in Pb+Pb Collisions at snn=2.76 TeV with the ATLAS detector at the LHC

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1 Charged Particle Production in Pb+Pb Collisions at snn=2.76 TeV with the ATLAS detector at the LHC Dominik Derendarz for the ATLAS Collaboration Institute of Nuclear Physics PAN, Kraków, Poland

2 Introduction ATLAS detector Data collected by ATLAS during 2010 HI run at LHC Centrality determination Multiplicity of charged particles Mechanism of particle production in nuclear collisions Entropy production on the early stage of collision Properties of the initial state (energy/gluon density) Charged particle spectra and RCP Hot and dense medium dynamics Energy loss in the QGP (jet quenching) Conclusions 2/19

3 The ATLAS detector 3/19

4 ATLAS in HI environment 4/19

5 Data collected during 2010 HI run ~10 μb-1 delivered by the LHC machine ~9 μb-1 recorded and ~8 μb-1 with B field-on Minimum Bias events were selected by requiring: - coincidence of MBTS and ZDC based triggers - reconstructed primary vertex 5/19

6 Centrality determination Energy deposited in Forward Calorimeters (3.2 < η < 4.9) Compared with MC Glauber (used for N part, Ncoll determination) calculations convoluted with p+p at s=2.76 TeV Sampled fraction of σ inel is f = 100+/-2% 6/19

7 Charged particle multiplicity measurement The measurement has been made with the ATALS pixel detector (barrel part) using B-filed off data (480mb-1, events) to access low pt regime Various methods were used: Pixel tracks standard tracking restricted to the Pixel detector Two-point tracklets simplified tracking which uses signals from two inner layers of the Pixel detector (B Layer, Layer 1) PIXEL D E TECTO R Layer 2 Layer 1 B Layer η <2 l e r r Ba 7/19

8 Two-point tracklets Tracklets are built from primary vertex and clusters from the first two layers of the Pixel detector Tracklet is accepted for analysis if 8/19

9 Correction factors Final multiplicity is obtained with Monte Carlo corrections, C(O,η) depending on centrality (expressed by detector occupancy, O) and pseudorapidity, applied to the number of reconstructed: Pixel tracks C pt (O, η)= 1 (1 b pt (O, η)) ε pt (O, η) N match (O, η) ε pt (O, η)= pr N pr (O, η) N backg (O, η) b pt (O, η)= pt N pt (O, η) Two-point tracklets C 2ptr (O, η )= N pt N pr N 2ptr N pr (O, η ) N 2ptr (O, η ) - number of pixel tracks - number of primary particles - number of two-point tracklets Systematic uncertainties Summary of centrality dependent systematic uncertainty estimates Only uncertainty based due to the choice of HYDJET is η dependent 9/19

10 Energy and Npart dependence of dnch/dη η <0.5 ATL-COM-PHYS Charged particle multiplicity in Pb+Pb collisions at sqrt{s_nn}=2.76 TeV by the ATLAS expriment Significant increase of multiplicity at the LHC energy relative to the log extrapolation from lower energies ATLAS and ALICE measurements agree within errors (ALICE point shifted along x-axis for clarity) Preliminary ALICE Collaboration, K. Aamodt et al., Phys. Rev. Lett. 106 (2011) RHIC results has been scaled by a factor of 2.15 to match LHC data A rise by a factor of 2 is observed within full centrality range ATLAS and ALICE measurements agree within errors 10/19

11 Charged particle pseudorapidity distribution Preliminary Measurement covers 4 units in η over a broad centrality range Multiplicity extends across 2 orders of magnitude with centrality Systematic uncertainty is shown as a grey band Pb+Pb snn=2.76 TeV 11/19

12 Charged particle pseudorapidity distribution dnch/dη increases by 7% from η=0 to η >1 for the centrality 70-80% Preliminary For the centrality range 0-70% we don't observe significant variation of the shape of dnch/dη 12/19

13 Charged particle pt spectra measurement Analysis: All data with B-filed on ~7μb-1 Standard tracking which uses information from the Pixel detector and semiconductor tracker (SCT) Track quality cuts Transverse and longitudinal impact parameter cuts: d0/σ(d0) < 3 z0sin(θ)/σ(z0sin(θ)) < 3 More than 1 pixel hit, and 1 hit in the innermost layer if it is expected More than 7 hits in SCT Pixel η < 2.5 pt > 500 MeV SCT 13/19

14 Yields are corrected in each centrality, pt and η bin using MC simulations : background Efficiency and systematics Background contamination (secondaries, fakes) Efficiency Tracking cuts Vertex pointing Truth association Efficiency Systematicuncertainties uncertainties Systematic 4% 1-4% Material budget Momentum resolution 2.5-5% 1-3% 1% 3% Feed-down Mis-measured tracks 1.25% % 14/19

15 Charged particle spectra ATLAS-CONF Measurement of the centrality dependence of charged particle spectra and RCP in lead-lead collisions at snn = 2.76 TeV with the ATLAS detector at the LHC Fully corrected transverse momentum spectra for charged particles for four centrality classes, three η ranges ( η <2.5) and pt up to 30 GeV Interesting structure in p convex up to 2 GeV T observed: concave within 2-10 GeV for central collisions 15/19

16 Spectra comparison K. Aamodt et al. [ALICE Collaboration], Phys. Lett. B696 (2011) S. S. Adler et al. [PHENIX Collaboration], Phys. Rev. C 69 (2004) Significant increase in particle production at the LHC energy Good agreement with the ALICE measurement 16/19

17 Charged particle RCP Suppression of high-pt particles strongest for the most central collisions within pt range 7-8 GeV R CP rises for pt > 8 GeV No substantial pseudorapidity dependence 17/19

18 Charged particle RCP ATLAS-CONF Centrality dependence of Jet Yields and Jet Fragmentation in Lead-Lead Collisions Decrease of RCP with rise of centrality up to value ~0.4 is observed Similar centrality dependence of R CP for hadrons and jets 18/19

19 Conclusions dnch/dη in Pb+Pb collisions at snn=2.76 TeV was measured in ATLAS detector over a broad pseudorapidity ( η <2.0) and centrality range Significant increase of multiplicity at the LHC energy relative to the log extrapolation from lower energies is observed dnch/dη/(npart/2) rises by a factor of 2 with Npart in centrality range 0-80% Charged particle pseudorapidity distribution for centrality interval 70-80% increase by 7% from η=0 to η >1 Charged particle spectra and RCP were measured in wide pseudorapidity range ( η <2.5), pt (up to 30 GeV) and centrality range The RCP ratio shows strong suppression of high-pt particles in the most central collisions similar to suppression found in measurement of fully reconstructed jets Measurements are consistent with results obtained by the ALICE experiment 19/19