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Heavy Ion Physics with the ATLAS Detector

Heavy Ion Physics with the ATLAS Detector. Pavel Nevski ATLAS Heavy Ion Physics working group. Motivation: High- pT Results from RHIC. c. a. b. d. Jet quenching observed in AuAu as predicted by pQCD: (unquenching in dAu). Hard processes: excellent probes to test QCD!. PRL91, (2003).

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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 ATLAS Heavy Ion Physics working group Los Alamos March 13, 2005

  2. Motivation: High-pT Results from RHIC c a b d Jet quenching observed in AuAu as predicted by pQCD: (unquenching in dAu) Hard processes: excellent probes to test QCD! PRL91, (2003) Los Alamos March 13, 2005

  3. 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 Los Alamos March 13, 2005

  4. ATLAS A Superb Detector for High-pT Studies Los Alamos March 13, 2005

  5. ATLAS as a Heavy Ion Detector 1. Excellent Calorimetry • Hermetic coverage up to || < 4.9 • Fine granularity (with longitudinal segmentation) • Very good jet resolution 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 SCT) • Large coverage up to || < 2.5 • High granularity pixel and strip detectors • Good momentum resolution Tracking particles with pT 0.5 GeV/c 2.+ 3. Heavy quarks(b), quarkonium suppression(J/ ,) 1.& 3. Global event characterization (dNch/dη, dET/dη, flow); Jet quenching Los Alamos March 13, 2005

  6. z y x Physics program • Global variable measurement dN/dη dET/dη elliptic flow azimuthal distributions • Jet measurement and jet quenching • Quarkonia suppression J/Ψ  • p-A physics • Ultra-Peripheral Collisions (UPC) Idea: take full advantage of the large calorimeter and μ-spectrometer Direct information from QGP Los Alamos March 13, 2005

  7. Studies of the Detector Performance • Constraint:No modifications to the detector, with the exception of trigger software and probably very forward region • Simulations:HIJING event generator, dNch/dη = 3200 • Full (GEANT-3) 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) Los Alamos March 13, 2005

  8. Central Pb+Pb Collision (b<1fm) • About 70,000 stable particles • ~ 40,000 particles in ||  3 • CPU – 6 h per central event (800MHz) • Event size 50MB (without TRT and with no zero-suppression) Nch(|η|0.5) Los Alamos March 13, 2005

  9. Detector Occupancies b = 0 – 1fm Calorimeters ( |η|< 3.2 ) Si detectors: Pixels < 2% SCT < 20% TRT: too high, not used for these studies (limited usage for AA collisions is under investigation) Muon Chambers: 0.3 – 0.9 hits/chamber (<< pp at 1034 cm-2 s-1) Average ET (uncalibrated): ~ 2 GeV/Tower ~ .3 GeV/Tower Los Alamos March 13, 2005

  10. Global Event Characterization Day-one measurements: Nch, dNch/d, ET, dET/d, b • Constrain model prediction; indispensable for all physics analyses Single Pb+Pb event, b =0-1fm dNch/d=0 3200 Points – reconstructed — HIJING, no quenching Histogram – true Nch Points – reconstructed Nch Nch(|| < 3) measured Reconstruction errors ~5% No track reconstruction used, only Nhits as calibrated in pp Los Alamos March 13, 2005

  11. 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. Los Alamos March 13, 2005

  12. Track Reconstruction 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) Los Alamos March 13, 2005

  13. Heavy Quark Production 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. Los Alamos March 13, 2005

  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 with ET > 50 GeV ~500 Z0() + jets with ET > 40 GeV Los Alamos March 13, 2005

  15. Jet quenching Energy loss of fast partons by excitation and gluon radiation larger in QGP • Suppression of high-z hadrons and increase of hadrons in jets. • Induced gluon radiation results in the modification of jet properties like a broader angular distribution. • Could manifest itself as an increase in the jet cone size or an effective suppression of the jet cross section within a fixed cone size. • Measuring jet profile is the most direct way to observe any change. Los Alamos March 13, 2005

  16. Jet Studies Goal is to determine medium properties. Jet quenching But, most energy is radiated INSIDE cone vacuum Salgado & Wiedemann hep-ph/0310079 in medium • 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 Los Alamos March 13, 2005

  17. Reconstruction – 280 GeV jet HIJING jet (counted as fake) HIJING jet Los Alamos March 13, 2005

  18. 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 • For ET > 75GeV: efficiency > 95%, fake < 5% • Good energy and angular resolution • Next: use tracking information to lower the threshold and reduce the fakes Los Alamos March 13, 2005

  19. 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 Promising, but a lot of additional work is needed! Los Alamos March 13, 2005

  20. 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 Los Alamos March 13, 2005

  21. 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% Cluster ET sensitive to small change in ETjet Relative error on the measurement of the neutral cluster ET. Further studies of large samples of PbPb events with jets are needed. Los Alamos March 13, 2005

  22. 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) Los Alamos March 13, 2005

  23. 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. ,: differences between ID and µ-spectrometer tracks after back-extrapolation to the vertex for the best 2 association. Los Alamos March 13, 2005

  24. Upsilon Reconstruction Mass resolution vs |η| of decay ’s. Deterioration of the mass resolution at larger |η| is due to material effects. Integrated Acceptance & efficiency as a function of |η|. Drop in the reconstruction efficiency for  +PbPb events is now understood (software technical problem) and signal and background are being re-evaluated. A compromise has to be found between acceptance (overall rate of events) and mass resolution to clearly separate upsilon states. Los Alamos March 13, 2005

  25. Upsilon Reconstruction Barrel only (|| <1) || <1 || <2.5 Acceptance 4.9% 14.1% +efficiency Resolution 123 MeV147 MeV S/B 1.30.5 Purity 94-99%91-95% For a 106s run with Pb+Pb at L=41026 cm-2 s-1 we expect 104 events in |η| < 1.2 (6% acc+eff). J/  +– - a study is under way. Los Alamos March 13, 2005

  26. J/ Reconstruction J/  +– - a study is under way • Large cross-section • σmass=53 MeV =>easy separation of J/ψ and ψ’ • good for J/ψ with pT > 4-5 GeV Full pT analysis is only possible in forward andbackward regions, but there thebackgroundis largest. Los Alamos March 13, 2005

  27. Trigger and DAQ • Interaction rate of 8 kHz for L = 1027 cm-2 s –1 • Event size ~ 5 MB for central PbPb (a conservative estimate) • Data rate to storage the same as in pp: ~300 Hz  MB • Output rate (after HLT) ~ 50 Hz for central events • Unbiased interaction trigger (LVL1) – use forward calorimeters (FCAL) • - Triggering on the total ET in FCAL with a Trigger Tower threshold • of 0.5 GeV selects 95% of the inelastic cross-section. Los Alamos March 13, 2005

  28. Trigger and DAQ • Triggering on events with b < 10 fm - use full ATLAS Calorimetry • High Level Triggers (ATLAS T/DAQ) - jet trigger, di-muon trigger,... • Jet rate ~ 40 Hz for ET threshold = 50 GeV • ~ 0.1 Hz for ET threshold = 100 GeV Selection signatures Los Alamos March 13, 2005

  29. Ultra-Peripheral Nuclear Collisions • High-energy - and -nucleon 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) Los Alamos March 13, 2005

  30. Summary • The Letter of Intent represents a first look into heavy-ion physics with the ATLAS detector. • The high granularity and large coverage of the calorimeter system, the acceptance of the muon spectrometer and the tracking capabilities of the inner detector allow for a comprehensive study of high-pT phenomena and heavy quark production in heavy-ion collisions. • Studies of the detector and physics performance are ongoing: • Optimization of algorithms for high-multiplicity environment • The flow effects and its impact on the jet reconstruction • pA collisions These first results already demonstrate a very good potential of the ATLAS experiment for heavy-ion studies and ensure its valuable and significant contribution to the LHC heavy-ion physics programme. Los Alamos March 13, 2005

  31. Back-up slides Los Alamos March 13, 2005

  32. ATLAS Calorimeters Los Alamos March 13, 2005

  33. <L>/L0 1 0.8 0.6 0.4 0.2 0 0 5 10 15 20 ALICE PPR CERN/LHCC 2003-049 Luminosity Issues Yves Schutz, QM’2004 b* = 2 - 0.5 m IO = 108 ions/bunch 1 2 Experiments Tuning time 3 Time (h) 20% reduction for 3 experiments as opposed to 2 experiments Los Alamos March 13, 2005

  34. Single Pb+Pb event, b =0-1fm dNch/d=0 3200 Points – reconstructed — HIJING, no quenching dNch/d=0 6000 Points – reconstructed — HIJING, with quenching Reconstruction errors ~5% Global Event Characterization ATLAS CMS Bolek Wyslouch, QM’2004 Comparable performance Los Alamos March 13, 2005

  35. Tracking Efficiency & Fake Rate CMS Bolek Wyslouch, QM’2004 ATLAS Slightly lower efficiency as compared to CMS, but reconstruction algorithm is not yet optimized for PbPb. Los Alamos March 13, 2005

  36. Momentum Resolution CMS Bolek Wyslouch, QM’2004 ATLAS Slightly worse resolution as compared to CMS, but reconstruction algorithm is not optimized for PbPb and current study is limited to pT < 20 GeV. ALICE Los Alamos March 13, 2005

  37. Jet Reconstruction • Sliding window algorithm,    = 0.4  0.4, • after subtracting the pedestal (the average pedestal is 50 11 GeV) • Accepted, if ET(window) > 40 GeV • The used algorithm is not optimal, studies are ongoing. Di-jet reconstructed in PbPb Di-jet event from PYTHIA in pp: pThard=55GeV Di-jet embedded in PbPb before pedestal subtraction Di-jet embedded in PbPb after pedestal subtraction Los Alamos March 13, 2005

  38. Pb-Pb collisions (b= 0 –1 fm) Efficiency Fraction of fake jets Jet energy resolution Pb-Pb p-p Jet Reconstruction ATLAS CMS Bolek Wyslouch, QM’2004 ALICE Sarah Blyth, QM’2004 Using US EMCal (under discussion) ATLAS and CMS comparable (ATLAS has better efficiency at 40-50 GeV), both better than ALICE with the proposed EMCal Los Alamos March 13, 2005

  39. Jet Pointing Resolution ATLAS ALICE Sarah Blyth, QM’2004 Using US EMCal (under discussion) ATLAS better performance than in ALICE with the proposed EMCal. Los Alamos March 13, 2005

  40. Fragmentation Function CMS Bolek Wyslouch, QM’2004 ALICE Sarah Blyth, QM’2004 ATLAS Using US EMCal (under discussion) Comparable performance for ATLAS, CMS and ALICE with the proposed EMCal Los Alamos March 13, 2005

  41. Fragmentation Function CMS Bolek Wyslouch, QM’2004 ATLAS ALICE Sarah Blyth, QM’2004 Using US EMCal (under discussion) Los Alamos March 13, 2005

  42. Upsilon Reconstruction Comparison between experiments for +- ALICE can also measure 2600 e+e- in the central spectrometerper 106 s if B is increased from 0.2 to 0.4 T Los Alamos March 13, 2005

  43. 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) 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 algorithm is not fully optimized yet Los Alamos March 13, 2005

  44. → μ+ μ- using combined info from ID and μ-spectrometer: Single Upsilons Δη, ΔΦ=difference between ID and μ-spectrometer tracks after back-extrapolation to the vertex for the best χ2 association. Los Alamos March 13, 2005

  45. → μ+ μ- using combined info from ID and μ-spectrometer: Single Upsilons HIJING background Half μ’s from c, b decays, half from π, K decays for pT>3 GeV. Background rejection based on χ2cut, geometrical Δη x ΔΦ cut and pT cut. Δη, ΔΦ=difference between ID and μ-spectrometer tracks after back-extrapolation to the vertex for the best χ2 association. Los Alamos March 13, 2005

  46. +-reconstruction Barrel only (|| <1) || <1 || <2.5 Acceptance 4.9% 14.3% +efficiency Resolution 126 MeV152 MeV S/B 1.30.5 Purity 94-99%91-95% A compromise has to be found between acceptance and mass resolution to clearly separate  states with maximum statistics. E.g. for |η| < 1.2 (6% acc+eff)we expect 104/month of 106s at L=41026 cm-2 s-1 - a study is under way (mass =53 MeV). J/  +– Los Alamos March 13, 2005

  47. Ultra-Peripheral Collisions (UPC) b > 2R only electromagnetic interactions γ–γ γ–N γ–Ρ with/without nucleus diffraction γ–W σ(γ–γ)~Z4 W γ–γ < 2γħc/RA=200 GeV for Pb σ(γ–γ)~Z4 W γ–γ < 2γħc/RA=200 GeV for Pb Φ η Φ η Los Alamos March 13, 2005

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