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Heavy Ion Physics with the ATLAS Detector

Heavy Ion Physics with the ATLAS Detector. Pavel Nevski Brookhaven National Laboratory On behalf of the ATLAS Collaboration. From RHIC to LHC. s NN : 200 GeV 5,500 GeV. Super-hot QCD: Will we see a weakly coupled QGP??. Initial state fully saturated (CGC)

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Heavy Ion Physics with the ATLAS Detector

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  1. Heavy Ion Physics with the ATLAS Detector Pavel Nevski Brookhaven National Laboratory On behalf of the ATLAS Collaboration SQM2006

  2. From RHIC to LHC sNN : 200 GeV 5,500 GeV Super-hot QCD: Will we see a weakly coupled QGP?? • Initial state fully saturated (CGC) • Enormous increase of high-pT processes over RHIC • Plenty of heavy quarks (b,c) • Weakly interacting probes become available (Z0, W) LHC RHIC SPS SQM2006

  3. ATLAS as a Heavy Ion Detector is: An Excellent Calorimetry A large acceptance Inner Tracker A hermetic Muon Spectrometer SQM2006

  4. ATLAS as a Heavy Ion Detector 1. Excellent Calorimetry • Hermetic coverage up to || < 4.9 • High granularity (.025x.025 electromagnetic, .1x.1 hadronic) with fine longitudinal segmentation (~7 sections) • Very good jet energy resolution (50%/sqrt(E) in pp) High pT probes (jets, jet shapes, jet correlations, 0) • Large Acceptance Muon Spectrometer • Coverage up to || < 2.7 Muons from , J/, Z0 decays • Inner Detector (Si Pixels and Strips, no TRT used) • Large coverage up to || < 2.5 • High granularity pixel and strip detectors (o~1%,10%) • Good momentum resolution (dpT/pT~3% up to 15GeV) Tracking particles with pT 0.5 GeV/c Global event characterization (dNch/dη, dET/dη, flow); Jet quenching study 1.& 3. 2.+ 3. Heavy quarks(b), quarkonium suppression(J/ ,) SQM2006

  5. And it is coming SOON ! SQM2006

  6. Studies of the Detector Performance • Constraint:No modifications to the detector, except for trigger and probably very forward region • Simulations:HIJING event generator, dNch/dη = 3200 • Full GEANT simulations of the detector response • Large event samples: • |η|< 3.2 impact parameter range: b = 0 - 15fm (27,000 events) • |η|< 5.1 impact parameter range: b = 10 - 30fm (5,000 events) SQM2006

  7. Predicted Detector Occupancies b = 0 – 1fm Calorimeters ( |η|< 3.2 ) Si detectors: Pixels < 2% SCT < 20% TRT: - High occupancy for tracking - Still visible TR signal for electrons -> Limited usage for AA collisions is under investigation Will be fully useful for pA Muon Chambers: 0.3 – 0.9 hits/chamber (<< pp at 1034 cm-2 s-1) Average ET (uncalibrated): ~ 2 GeV/Tower ~ .3 GeV/Tower SQM2006

  8. Tracking Performance Standard ATLAS reconstruction for pp is used, not optimized for PbPb. • Pixel and SCT detectors • pT threshold of 1 GeV • (used in this preliminary studies) • tracking cuts: • At least 10 hits out of 11(13) available • in the barrel (end-caps) • All three pixel hits • At most 1 shared hits • 2/dof < 4 For pT: 1 - 10 GeV/c: efficiency ~ 70 % fake rate ~ 5% Momentum resolution ~ 3% (2% - barrel, 4-5% end-caps) SQM2006

  9. In central Pb-PB collisions (3200 ch.particle per rapidity unit) factor 20 in pion rejection can be achieved by selecting a TR threshold corresponding to 50% electron efficiency Electron-pion separation in TRT Pion fraction vs electron efficiency SQM2006

  10. Day One Physics with ATLAS SQM2006

  11. Global Event Characterization Day-one measurements:Nch, dNch/d, b, ET, dET/d Single Pb+Pb event, b =0-1fm Nch(|| < 3) Histogram – true Nch Points – reconstructed Nch dNch/d|=0 3200 (HIJING, no quenching) 10% <3% No track reconstruction, only Nhits calibrated with pp SQM2006

  12. Correlation of Signals with Flow V2=0.042 -R v2Truth = 0.05 Distribution of azimuthal angle (v2) vs true reaction plane position, R SQM2006

  13. Jet Physics SQM2006

  14. Jet Rates For a 106s run with Pb+Pb at L=41026 cm-2 s-1 we expect in |η| < 2.5: And also: ~106 + jet events ~500 Z0() + jets with ET > 40 GeV SQM2006

  15. Jet reconstruction efficiency Pb-Pb collisions (b= 0 –1 fm) Angular resolution for 70 GeV jets Efficiency Fake rate ~2  resolution in pp Pb-Pb Energy resolution p-p • Two jet finder algorithms testet up to now - Sliding Window and Cone Fit • For ET > 75GeV: efficiency > 95%, fake < 5% • Good energy and angular resolution SQM2006

  16. vacuum Most of energy is radiated inside a narrow cone (Salgado & Wiedemann hep-ph/0310079) in medium Jet Quenching Studies To determine medium properties we need to measure jet shapes • 3 methods explored so far: • Fragmentation function using tracking • Core ET and jet profile using calorimeters • Neutral leading hadrons using EM calorimeters Quenching may depend on quark flavor: - Tagging of b-jets using impact parameter SQM2006

  17. Jet Studies with Tracks • Jets with ET = 100 GeV • Cone radius of 0.4 • Track pT > 3 GeV Momentum component perpendicular to jet axis Fragmentation function dN/djT broader in PbPb than in pp (background fluctuations) PbPb  HIJING-unquenched  pp SQM2006

  18. b-quark Jet Tagging Motivation: Heavy quarks may radiate less energy in the dense medium (dead-cone effect) than light quarks. b-tagging capabilities offer additional tool to understand quenching. • To evaluate b-tagging performance: • ppWHlbb events overlayed • on HIJING background have been used. • A displaced vertex in the Inner Detector • has been searched for. Rejection factor against u-jets ~ 100 for b-tagging efficiency of 25% Should be improved by optimized algorithms and with soft muon tagging in the Muon Spec. SQM2006

  19. Physics with Muon Spectrometer SQM2006

  20. +-reconstruction   +– Muons momenta measured by ID tracks tagged by coincidence with track segment in -spectrometer || 2 For |η| < 2 (12.5% acc+eff)we expect 15K /month of 106s at L=41026 cm-2 s-1 SQM2006

  21. J/+-reconstruction J/  +– ||2.5, pT1.5 GeV We expect 8K to 100K J/+- per month of 106s at L=41026 cm-2 s-1 If a trigger is possible forward with a muon pT >1.5 GeV, we gain a factor 4 in statistics…A solution might be to reduce the toroidal field for HI runs SQM2006

  22. Other issues SQM2006

  23. Ultra-Peripheral Nuclear Collisions • High-energy - and -nucleus collisions • Measurements of hadron structure at high energies (above HERA) • Di-jet and heavy quark production • Tagging of UPC requires a Zero Degree Calorimeter • Ongoing work on ZDC design and integration • with the accelerator instrumentation: Proton-Nucleus Collisions • Link between pp and AA physics • Study of the nuclear modification of the gluon distribution at low xF. • Study of the jet fragmentation function modification • Full detector capabilities (including TRT) will be available. • L~1030 translates to about 1MHz interaction rate (compare to 40 MHz in pp) SQM2006

  24. z y x CONCLUSION ATLAS has a very good potential for making a valuable and significant contribution to the LHC’s heavy-ion physics programme: • Global variable measurement on Day1 dN/dη, dET/dη, elliptic flow. • Jet measurement and jet quenching • Quarkonia suppression (J/Ψ, ) • p-A physics • Ultra-Peripheral Collisions (UPC) • More will come Direct information from QGP SQM2006

  25. SQM2006

  26. Back-up material SQM2006

  27. Collision Centrality Use 3 detector systems to obtain impact parameter: Pixel&SCT, EM-Cal HAD-Cal Resolution of the estimated impact parameter ~1fm for all three systems. SQM2006

  28. ATLAS Calorimeters SQM2006

  29. Jet ETcore Measurements Energy deposited in a narrow cone around the jet axis: R < 0.11 (HADCal), R < 0.07 (EMCal) pp PbPb <ETcore> sensitive to ~10% change in ETjet More work is needed to minimize effect of background fluctuations. The background has not been subtracted: <ETcore>PbPb  <ETcore>pp SQM2006

  30. Isolated Neutral Hadrons Select jets that have an isolated neutral cluster along the axis (~1% of the total jet sample) Jets with ET >75GeV embedded into central PbPb events: pp Gauss =4.5% Relative error on the measurement of the neutral cluster ET. Cluster ET sensitive to small change in ETjet SQM2006

  31. Jet studies PYTHIA jets embedded with central Pb-Pb HIJING events Reconstruction: - sliding window algorithm ΔΦxΔη =0.4x0.4 with splitting/merging after background energy subtraction (average and local) - ET fitting with Gaussian+constant background Average background 50 ±11 GeV (Pb-Pb Hijing, b=0-1 fm) => threshold for jet reconstruction ~30-40 GeV in calorimeter cf p-p ~ 15 GeV Studies of algorithm optimization is underway SQM2006

  32. Quarkonia suppression Color screening prevents various ψ, , χ states to be formed when T→Ttrans to QGP (color screening length < size of resonance) Modification of the potential can be studied by a systematic measurement of heavy quarkonia states characterized by different binding energies and dissociation temperatures ~thermometer for the plasma Upsilon family (1s) (2s) (3s) Binding energies (GeV) 1.1 0.54 0.2 Dissociation at the temperature ~2.5Ttrans ~0.9Ttrans ~0.7Ttrans =>Important to separate (1s) and (2s) SQM2006

  33. Upsilon Reconstruction   +– • Overlay  decays on top of HIJING events. • Use combined info from ID and μ-Spectrometer • Single Upsilons • HIJING background • (Half ’s from c, b decays, half from π, K decays for pT>3 GeV) • Background rejection: • 2 cut • geometrical    cut • pT cut. • Momentum resolution is worse at higher rapidity due to material ,: differences between ID and µ-spectrometer tracks after back-extrapolation to the vertex for the best 2 association. SQM2006

  34. Summary • Global observables, including elliptic flow, should be accessible fromday-one, even with a very low luminosity (early scheme) • Jet physics (jet quenching) is very promising,jet reconstruction is possible despite the additional background,possibility to study separately light and heavy q-jets • Heavy-quarkonia physics (suppression in dense matter) well accessible,capabilityto measure and separate and ’, to measure the J/ using a specially developed  tagging method • Ongoing studies: • - Optimization of algorithms for high-multiplicity environment • - The flow effects and its impact on the jet reconstruction • - pA collisions ATLAS has a very good potential for making a valuable and significant contribution to the LHC’s heavy-ion physics programme SQM2006

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