Charged particle yields and spectra in p+p and Heavy Ion Collisions with ATLAS at the LHC Jiří Dolejší (Charles University Prague) for the ATLAS collaboration Hard Probes 2010, Eilat

„Take-off“ Delivered and recorded integrated luminosity grows exponentially and ATLAS works well 2

The ATLAS experiment has extensive charged particle tracking over full azimuth and within |  |<2.5.  =2.5 3

Illustration of tracking and vertexing performance, vertex resolution better than ~200 μm 5 cm 4

HI  10% at worse The Inner detector consists of three pixel layers, four double-layer strip layers (SCT), giving together 11 space points, and transition radiation tracker (TRT) in 2 T magnetic field. TRT channels 97,1% operational Momentum resolution:  /p T ~ 3.4x10 -4 p T (GeV)  SCT 6,3 mil. channels, 99,2% operational HI less than 1% Pixels 80 mil. channels, 97,3% operational 5

The Inner detector: number of Pixel and SCT hits /ATLAS-CONF /fig_03b.png p T > 500 MeV 6

Current results from tracking (ATLAS-CONF , 20 July 2010) Event selection requirement s:  to have all Inner Detector sub- systems at nominal conditions, stable beam and defined beam spot values,  to have passed the Level 1 Minimum Bias Trigger Scintillator single-arm trigger,  to have a reconstructed primary vertex,  to not have a second reconstructed primary vertex with four or more tracks in the same bunch crossing (to remove pile-up),  to have at least two good tracks in the event. A good track should satisfy :  p T > 100 MeV,  a hit in the first layer of the Pixel detector (layer-0) if one is expected,  a minimum of one Pixel hit in any of the 3 layers,  at least two (p T > 100 MeV), four (p T > 200 MeV) or six (p T > 300 MeV) SemiConductor Tracker (SCT) hits,  transverse and longitudinal impact parameters calculated with respect to the event primary vertex |d 0 | < 1.5 mm and | z 0. sin  | < 1.5 mm, respectively,   2 probability > 0,01 for reconstructed tracks with p T > 10 GeV, to remove mis-measured tracks Data:  First 190  b -1 of data recorded by the ATLAS experiment at 7 TeV: 10,066,072 events passed event selection, containing a total of 209,809,430 selected tracks.  0.9 TeV data sample (7  b -1 ) contains 357,523 events with 4,532,663 selected tracks. 7

Illustration of vertexing performance Data: A zoomed-in view of the Y vs. X distribution of secondary reconstructed vertices due to hadronic interactions in minimum-bias events (after K 0 S, , and  vetoes, and  Z  < 300 mm cut). 1 mm bins in X and Y. Vertex reconstruction efficiency as a function of n sel BS. The coloured error bands show the total uncertainty, the black vertical lines the statistical uncertainty. 8

Illustration of tracking performance (ATLAS-CONF , 20 July 2010) All uncertainties are quoted relative to the track reconstruction effciency. 9

Charged-particles multiplicities (ATLAS-CONF , 20 July 2010) Charged-particle multiplicities for events with n ch  2 within the kinematic range p T  100 MeV and |  | < 2.5 at  s = 0.9 TeV and at  s = 7 TeV 10

Charged-particles multiplicities (ATLAS-CONF , 20 July 2010) Charged-particle multiplicities for events with n ch  2 within the kinematic range p T  100 MeV and |  | < 2.5 at  s = 0.9 TeV. 11

Charged-particles multiplicities (ATLAS-CONF , 20 July 2010) Charged-particle multiplicities for events with n ch  2 within the kinematic range p T  100 MeV and |  | < 2.5 at  s = 0.9 TeV. 12

Charged-particles multiplicities (ATLAS-CONF , 20 July 2010) Charged-particle multiplicities for events with n ch  2 within the kinematic range p T  100 MeV and |  | < 2.5 at  s = 0.9 TeV. The average charged particle multiplicity per unit of rapidity for  = 0 as a function of the centre of mass energy. ATLAS Preliminary 13

A jump from pp to PbPb … RHIC  LHC 0.1 TeV/A  2.76 TeV/A 197 Au  207 Pb ? LHCPPPbPb Max. energy7+7 TeV TeV Pile-up230 dN/d  ? Rate40 MHz8 kHz … at nominal conditions 14

Tracking is expected to work well in heavy-ion collisions, here the results from previous ATLAS tracking algorithm. Commisioning the current default ATLAS tracking (NewT) for heavy ion collisions is progressing. Tracking efficiency and fake rate in |  | < 1 extracted from a sample of central (b=2 fm) HIJING events produced with quenching effects turned off (and with embedded   ) in the ATLAS tracking system. Acceptable efficiency of about 70%, negligible fake rate above 1 GeV/c Top: Tracking efficiency as a function of pseudorapidity for tracks with 3 < p T < 8 GeV extracted from the same central sample of events. Bottom: Fake rate as function of pseudo- rapidity for the same tracks as above. Negligible dependence on  ATLAS Preliminary 15

There are several „alternative“ algorithms developed parallel to ATLAS default options to cope with heavy ion collisions, e.g. tracklet method. These alternatives give us some redundancy, better control over systematic errors, flexibility to cope with new phenomena… ATLAS Preliminary Distribution of dN ch /d  from HIJING events (histogram) and reconstructed using the tracklet method event-by event, but without efficiency corrections (points with error bars). Tracklet reconstruction efficiency at |  | < 1. Tracklets = three point tracks (vertex + two pixel hits) 16

Many further ongoing efforts, e.g. to use the dE/dx measurement via time over threshold available in pixels and TRT for particle identification at low momenta Time over Threshold (TOT) versus injected charge for all pixels. Note, that the overwhelming majority (in red area) of the pixels shows a near linear relationship. There is a small fraction of pixels that contribute to the tail (in blue area) Energy loss in Pixel The track dE/dx is calculated starting from the charge collected in the pixel clusters associated to the track itself. 17

But ATLAS has also Calorimeters with fine granularity and full azimuthal coverage Pb-Liquid Argon EM calorimeter with resolution  /E ~ 10%/  E, channels, 98.1% oper. Hadronic calorimeter with Fe/scintillator Tiles (central), Cu/W-Liquid Argon (forward), resolution:  /E ~ 50%/  E  0.03 Tile: channels, 96.9% operational, Endcap LAr channels, 99.9% oper., Forward LAr channels, 100% oper. 18

ATLAS heavy ion group elaborated the methods to find jets and to measure their energy in heavy ion collisions and also elaborated methods to analyze particles within jets E. g. fragmentation function: D(z) Reliable reconstruction of D(z): Reconstructed tracks with p T > 2 GeV matching calorimeter jets The scale of possible modifications of fragmentation function – comparison of PYTHIA and PYQUEN ATLAS Preliminary We can measure jet quenching of the size simulated by PYQUEN 19

Summary ATLAS is fully operational, effectively uses the luminosity delivered by LHC. First physics results show good agreement with MC simulation of physics and the detector, but they also provide hints for further tuning of models, especially of the soft physics. Extensive preparations for Pb+Pb program show a promissing performance of ATLAS for heavy ion beams. Heavy ion beams expected in November/December. 20

Sources of further information ATLAS Public Results 21