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Heavy Ion Physics with the ATLAS Detector

Heavy Ion Physics with the ATLAS Detector. Barbara Wosiek Barbara.Wosiek@ifj.edu.pl Institute of Nuclear Physics, Krak ów, Poland. For the ATLAS Heavy Ion Group:. S. Aronson, K. Assamagan, B. Cole, M. Dobbs, J. Dolej s i, H. Gordon, F. Gianotti, S. Kabana, M. Levine, F. Marroquin,

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Heavy Ion Physics with the ATLAS Detector

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  1. Heavy Ion Physics with the ATLAS Detector Barbara Wosiek Barbara.Wosiek@ifj.edu.pl Institute of Nuclear Physics, Kraków, Poland For the ATLAS Heavy Ion Group: S. Aronson, K. Assamagan, B. Cole, M. Dobbs, J. Dolejsi, H. Gordon, F. Gianotti, S. Kabana, M. Levine, F. Marroquin, J. Nagle, P. Nevski, A. Olszewski, L. Rosselet, P. Sawicki, H. Takai, S. Tapprogge, A. Trzupek, M.A.B. Vale, S. White, R. Witt, B. Wosiek, K. Woźniak and …… Barbara Wosiek

  2. Outline of the Talk INTRODUCTION • Why Heavy Ions at the LHC? • Why ATLAS as a Detector for Heavy Ions? ATLAS PERFORMANCE FOR HEAVY ION PHYSICS • Monte Carlo Simulations • Detector Occupancies • Global Measurements • Event Characterization • Tracking with ATLAS ID(Si) • B-tagging • Quarkonia Studies • JET PHYSICS  Ketevi’s talk Barbara Wosiek

  3. Heavy Ions at the LHC Study of QCD matter at extremely high energy densities and ~vanishing baryon chemical potential. • deconfinement • restoration of the chiral symmetry, • physics of parton densities close to saturation RHIC LHC 200 5500 GeV • Initial energy density about 5 times higher than at RHIC. • Lifetime of a hot & dense matter much longer 10-15 fm/c at LHC as compared to 1.5-4 fm/c at RHIC • Access to truly hard probes with sufficiently high rates pT > 100 GeV/c (at RHIC pT 20 GeV/c) copious production of b and c quarks Barbara Wosiek

  4. Heavy Ions at the LHC Quantitative studies of a QGP properties: • Hot/dense matter effects should dominate over initial and finalstate effects. • Studies facilitated by many hard probes. LHC – HI Phase I Pb + Pb E=2.75 TeV/beam Lmax = 1·1027 cm-2 s-1 Interaction Rate = 8 kHz Exploratory run of a few days in 2007 Extended run in 2008 (1 nb-1) LHC – HI Phase II and later • p + A collisions (benchmarking nuclear effects) • Possible lighter ion species: 115In, 84Kr, 40Ar, 16O Barbara Wosiek

  5. Heavy Ion Physics with ATLAS Detector ATLAS interest in heavy ion physics was activated by highlights fromRHIC experiments! Hot/dense Nuclear Matter Diagnostics • Suppression of high pT particles • Disappearance of back-to-back high pT jet correlations • Huge azimuthal asymmetry at high pT ATLAS is an excellent detector for high pT physics and jet studies Barbara Wosiek

  6. ATLAS as a Heavy Ion Detector • High Resolution Calorimeters • Hermetic coverage up to || < 4.9 • Fine granularity (with longitudinal segmentation) High pT probes • Large Acceptance Muon Spectrometer • Coverage up to || < 2.7 Muons from , Z0 decays • Si Tracker • Large coverage up to || < 2.5 • Finely segmented pixel and strip detectors • Good momentum resolution Tracking particles with pT 1.0 GeV/c Heavy quarks(b), quarkonium suppression(, ’) 2.+ 3. 1.& 3. Global event characterization Barbara Wosiek

  7. Simulation Tools Event Generator HIJING : Based on PYTHIA and Lund fragmentation scheme with nuclear effects:nuclear shadowing,jet quenching Simulated event samples HIJING + full GEANT3 ATLAS detector simulations Only particles within |y| < 3.2 • High Geant cuts: 1 MeV tracking/10 MeV production • 5,000 events in each of 5 impact parameter bins: • b = 0-1, 1-3, 3-6, 6-10, 10-15 fm • Standard ATLAS cuts:100 keV tracking/1 MeV production • 1,000 central events, b = 0-1fm • Initial layout – 2 pixel barrel layers • 1,000 central events, b = 0-1fm Barbara Wosiek

  8. Central Pb+Pb Collision Nch(|y|0.5) • About 75,000 stable particles • ~ 40,000 particles in ||  3 • CPU – 6 h per central event (800MHz) • Event size 50MB (without TRT) Barbara Wosiek

  9. Average Occupancies Central Collision Events b=0-1 fm • Occupancies still reasonable in all Si Detectors: below 2% in Pixels and below 20% in Strips (after accounting for local fluctuations in the data with low GEANT cuts) • TRT unusable – too high occupancy Barbara Wosiek

  10. Global Measurements DAY-ONE MEASUREMENTS! Nch, dNch/d, ET, dET/d, b • Constrain model prediction • Indispensable for all physics analyses Predictions for Pb+Pb central collisions at LHC (dNch/d)0 Model/data ~ 6500 HIJING:with quenching, with shadowing ~ 3200 HIJING:no quenching, with shadowing ~ 2300 Saturation Model (Kharzeev & Nardi) ~ 1500 Extrapolation from lower energy data Barbara Wosiek

  11. Measurements of Nch(|| < 3) Based on the correlation between measurable quantity Q and the true number of charged primary particles: Q = f(Nch) Q: Nsig (all Si detectors,except PixB-B) EtotEM, EtotHAD ETEM , ETHAD • Caution: • Consistency between the measured signals • and the simulated ones • Monte Carlo dependency Barbara Wosiek

  12. Measurements of Nch(|| < 3) Relative reconstruction errors: |Nrec-Nch|/Nch Reconstructed multiplicity distribution (Nsig) Histogram – true Nch Points – reconstructed Nch Uncertainty up to 10% at low Nch, less than 3% at high Nch Barbara Wosiek

  13. Reconstruction of dNch/d Motivation: shape of the dNch/d distribution is sensitive to dynamical effects like e.g. quenching and shadowing. • Analysis is based on signals only from Pixel barrel layers • (done separately for each layer). • Clusterization procedure i.e. merging of hits in neighbor pixels is applied (particle traverses more than one pixel when   0). • Correction factors need to be applied to account for the • excess of clusters at large ||: • double hits from overlapping sensors • magnetic field effects (low pT particles bending back) • production of secondary particles Barbara Wosiek

  14. Reconstructed dNch/d • Comparison of the reconstructed dNch/d distributions with the true one of charged primary particles. • One single correction function C() calculated from a sample of central events is used. Single Pb+Pb event, b =0-1fm 5 peripheral collision, b =10-15fm Reconstruction errors ~5% Reconstruction errors ~13% Correction factors are ~ centrality independent! Barbara Wosiek

  15. Reconstructed dNch/d Single Pb+Pb HIJING event with jet quenching, b =0-1fm 100 p+p events at s=200 GeV Different shape and higher density are correctly reproduced!! Correction factors are ~insensitive to the detailed properties of generated particles! Barbara Wosiek

  16. Estimate of the Collision Centrality Based on the correlation between the measurable quantity Q and the centrality parameter: b, (Npart, Ncoll) Monotonic relation between Q and b allows for assigning to a certain fraction of events selected by cuts on Q, a well defined average impact parameter. Nsig ET - EM ET - HAD Barbara Wosiek

  17. Estimate of the Collision Centrality Resolution of the estimated impact parameter Remark: A better approach would be to use a quantity measured outside the mid-rapidity region,e.g. energy in forward calorimeters, which is less sensitive to dynamical effects. Barbara Wosiek

  18. Track Reconstruction Pixel and SCT detectors – ATLAS xKalman algorithm Starting with software release 6.1.0 all modifications specific to Heavy Ion track reconstruction are included in the official xKalman code. • pT threshold for reconstructable • tracks is 1 GeV (reduce CPU). • Tracking cuts are optimized to • get a decent efficiency and • low rate of fake tracks. • At least 10 measurements per track • Maximum two shared measurements • 2/ndf  4 • Tracking in the || < 2.5 • For pT: 1 - 15 GeV/c: efficiency ~ 70; fake rate<10% • Fake rate at high pT can be reduced by matching with Calo data Barbara Wosiek

  19. Track Reconstruction Momentum resolution Efficiency versus rapidity ~3% for pT up to 20 GeV/c (for || < 2.5) Flat dependency for |y| < 2 Higher in EC (more layers) Tracking in HI events looks promising, still can be optimized! Barbara Wosiek

  20. Heavy-Quark Production • Heavy quarks live through the thermalization of QGP can be affected by the presence of QGP • Their radiative energy loss is qualitatively different than for light quarks. Open Beauty via semi-leptonic decays • Tagging of the B-jets: • the high pT in the MS • displaced vertex in the ID Barbara Wosiek

  21. B-jet Tagging • Preliminary study: • Standard ATLAS algorithm for pp • Higgs events embedded into pp or Pb-Pb event • Cuts on the vertex impact parameter in the Pixel and SCT Rejection factors against light quarks versus b-tagging efficiency p-p Pb-Pb Promising, should be improved when combined with muon tagging! Barbara Wosiek

  22. Quarkonium Suppression Direct probe of the QGP: Color screening of the binding potential leads to the dissociation of the quarkonium states. Upsilon family(1s) (2s) (3s) Binding energies (GeV) 1.1 0.54 0.2 Dissociation at the temperature ~2.5Tc ~0.9Tc ~0.7Tc Important to separate (1s) and (2s)   +– Upsilon mass reconstruction using the Muon Spectrometer, Silicon Tracker and the Pixel Detector (barrel sections only). Barbara Wosiek

  23. Quarkonium Suppression • GEANT3 simulations of pure (1s) and (2s) states  +– • Muons with pT > 3GeV are tracked backwards to the ID • Invariant mass is calculated from the overall fit.  = 130 MeV • Background estimate (HIJING+G3)  S/B ~ 0.6 • Acceptance 10-15% providing 100% efficient dimuon trigger • Overlay with HIJING Event is under study! Barbara Wosiek

  24. Summary • ATLAS detector will be capable of measuring many aspects of relatively low pT heavy-ion physics • Let’s see the detector performance for studying the truly high pT phenomena … Barbara Wosiek

  25. BACKUPS Barbara Wosiek

  26. Detector Occupancies Examples of occupancy versus z and Nch (high GEANT thresholds) Occ Occ z z Nch Nch Pix1 SCT1 Barbara Wosiek

  27. Detector occupancies Central collisions b=0-1 fm, low GEANT thresholds Pixel Detector Silicon Tracker Barbara Wosiek

  28. Trigger DAQ For Pb+Pb collisions the interaction rate is 8kHz, a factor of 10 smaller than LVL 1 bandwidth. We expect further reduction to 1kHz by requiring central collisions and pre-scaled minimum bias events (or high pT jets or muons). The event size for a central collision is ~ 5 Mbytes. Similar bandwidth to storage as pp at design L implies that we can afford ~ 50 Hz data recording. ~200 Hz Barbara Wosiek

  29. Correction Factors Correction factors are defined as: C() calculated from the sample of 50 central (b=0-1fm) Pb+Pb events, and then parameterized. Correction function for the inner most barrel layer. Barbara Wosiek

  30. Maximal Cluster Size Define the expected maximal size of the cluster: • In Z-direction the number of pixels to be merged • depends on the Z-coordinate of the hit: e.g. for R=5cm, Zhit=40cm Npixel  6 -7 • In -direction the number of traversed pixels depends on pT. • For a track with a curvature r, an angle at which particle • enters the sensor is cos()=R/2r (assuming that sensors form • an ideal tube). • Taking r = 15cm (corresponding to pT=90 MeV/c): Npixels = 4 – 6 for R=12cm Npixels = 3 – 4 for R= 5cm Barbara Wosiek

  31. Cluster Formation • Choose seeds  large signals > 10,000 electrons • Start with the seed with the largest signal • Attached to it a signal in the adjacent pixel as long as: • There is a signal in a pixel • One of the closest neighbor pixels already belongs to the cluster • The distance from the seed to the pixel is not larger than the expected maximal size of the cluster (in both Z and  directions) up to 6pixelsin Z (depending on Zhit) and 3 pixels in  (depending on R) Barbara Wosiek

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