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Pavel Nevski ATLAS detector performance in Heavy Ion Collisions at LHC ATLAS detector performance in Heavy Ion Collisions at LHC Pavel Nevski BNL Motivations.

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Presentation on theme: "Pavel Nevski ATLAS detector performance in Heavy Ion Collisions at LHC ATLAS detector performance in Heavy Ion Collisions at LHC Pavel Nevski BNL Motivations."— Presentation transcript:

1 Pavel Nevski ATLAS detector performance in Heavy Ion Collisions at LHC ATLAS detector performance in Heavy Ion Collisions at LHC Pavel Nevski BNL Motivations Motivations Event Characteristics Subsystems Performance

2 Pavel Nevski Heavy Ions at the LHC  Initial energy density about 5 times higher than at RHIC:  Lifetime of a quark-gluon plasma 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 : p T > 100 GeV/c (at RHIC p T  20 GeV/c) copious production of b and c quarks  deconfinement  restoration of the chiral symmetry,  physics of parton densities close to saturation Study of QCD matter at extremely high energy densities and ~vanishing baryon chemical potential: RHIC LHC 200 5500 GeV

3 Pavel Nevski ATLAS as a Heavy Ion Detector 1.High Resolution E.M. and Hadronic Calorimeters —Hermetic coverage up to |  | < 4.9 —Fine granularity (with longitudinal segmentation) 2.Large Acceptance Muon Spectrometer —Coverage up to |  | < 2.7 3.Si Tracker —Large coverage up to |  | < 2.5 —Finely segmented pixel and strip detectors —Good momentum resolution High p T probes Muons from , J/ , Z 0 decays Tracking particles with p T  1.0 GeV/c 2.+ 3. Heavy quarks(b), quarkonium suppression( ,  ’) 1.& 3.Global event characterization

4 Pavel Nevski ATLAS Detector ATLAS is an excellent detector for high p T physics and jet studies

5 Pavel Nevski Simulation DataFlow

6 Pavel Nevski Simulation Tools: Generators HIJING Event Generator: Based on PYTHIA and Lund fragmentation scheme (Soft string dynamics + hard pQCD interactions) with nuclear effects: nuclear shadowing, jet quenching However, HIJING jet quenching model does not fit the RHIC measurements quenching, no shadowing quenching, shadowing no quenching, shadowing Pb+Pb b=0 fm  s NN =5.5 TeV

7 Pavel Nevski Stable Particles after HIJING Per 10 Events All decays faster then pi0 are now done by hijing, But you can switch some of them off

8 Pavel Nevski Central Pb+Pb Collision in ATLAS  About 75,000 stable particles  ~ 40,000 particles in |  |  3.2  CPU – 6 h per central event (800MHz)  Event size 50MB (without TRT) N ch (|y|  0.5)

9 Pavel Nevski Simulated Event Samples HIJING + full GEANT3 ATLAS detector simulations Only particles within |y| < 3.2 for the moment  High Geant thresholds 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 thresholds 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

10 Pavel Nevski Global Measurements Day-one measurements: N ch, dN ch /d ,  E T, dE T /d , b  Constrain model prediction  Indispensable for all physics analyses Predictions for Pb+Pb central collisions at LHC ( dN ch /d  )  0 Model/data ~12500 HIJING:with quenching, no shadowing ~ 6500 HIJING:with quenching, with shadowing ~ 3200 HIJING:no quenching, no shadowing ~ 2300 Saturation Model (Kharzeev & Nardi) ~ 1500 Extrapolation from lower energy data

11 Pavel Nevski Measurements of N ch (|  | < 3) Based on the correlation between measurable quantity Q and the true number of charged primary particles: Q = f(N ch ) Q: N sig - all Si detectors,except PixB  E tot EM,  E tot HAD  E T EM,  E T HAD Caution: Consistency between the measured signals and the simulated ones Monte Carlo dependency

12 Pavel Nevski Estimate of the Collision Centrality Monotonic relation between measurable quantities Q and centrality parameter b (Npart,Ncoll) allows for assigning to a certain fraction of events, selected by cuts on Q, a well defined average impact parameter. Correlation improves with a larger rapidity coverage. N sig E T - EME T - HAD

13 Pavel Nevski Event Reconstruction n Most of the standard ATLAS reconstruction packages developed for PP physics are working on HI events after minimal parameter tuning: –We have successfully exercised all calorimeter reconstruction - photons, jets, missing energy. –Silicon Pixel and Strip detectors have reasonable occupancy and can provide track reconstruction already with existing PP codes. –Muon reconstruction is even simpler in HI events - provided the muon energy is above 6 GeV - provided the muon energy is above 6 GeV n Dedicated HI reconstruction packages will be developed in due time: –Jet reconstruction is a tricky issue -work is ongoing to develop an appropriate code

14 Pavel Nevski Inner Detector Occupancy Pixel DetectorSilicon Tracker Impact parameter b=0-1 fm, HIJING event generator. TRT is excluded from analysis

15 Pavel Nevski Track Reconstruction Track reconstruction performed with ATLAS pp tracking code using the Pixel and SCT detectors (xKalman++). —p T threshold for reconstructed tracks is set to 1 GeV. —Tracking cuts are optimized to get a decent efficiency and low rate of fake tracks. — Further high p T fake rejection can be achieved using calorimeter For p T 1 to 15 GeV/c: efficiency ~ 70 % fake rate ~5 % Much better in |y|<1 May very with cuts: Eff. ~80%, fake rate 15-20% Eff. ~65%, fake rate ~2% - Subject to further optimization

16 Pavel Nevski Track Reconstruction Momentum resolution Efficiency versus rapidity Flat dependency for |y| < 2~3% for p T up to 20 GeV/c ~2% for |y|<1

17 Pavel NevskiCalorimetry Energy Per Cell: n 0.10 x 0.10 cell in e.m. calorimeter n 0.10 x 0.10 cell in hadron calorimeter

18 Pavel Nevski Jets and Clusters n Reconstructed e.m. clusters – exotic processes can be observed with cluster energy more than ~15 GeV (?) n Reconstructed hadronic jets – jet signature can be used with Pt above 50 GeV (?)

19 Pavel Nevski Modified Jet Reconstruction - Pythia jets embedded in Hijing events - Local energy level is evaluated and subtracted - Reconstructed Jet parameters are compared to MC truth for embedded jets

20 Pavel Nevski Heavy Quark Production Heavy quarks live through the thermalization of QGP  can be affected by the presence of QGP Their radiative energy loss is different than for light quarks. 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 Promising, should be improved when combined with muon tagging! Rejection factors against light quarks versus b-tagging efficiency p-p Pb-Pb

21 Pavel NevskiMuons n On average muons loose 5 GeV in calorimeter and have strong multiple scattering angles - Use combined info from ID+muon spectrometer to increase accuracy - Use combined info from ID+muon spectrometer to increase accuracy n Association based on geometrical cuts:  φ x  η after back extrapolation at vertex  φ x  η after back extrapolation at vertex + global fit of all possible combinations, ordered in decreasing χ2 + χ2 cut + global fit of all possible combinations, ordered in decreasing χ2 + χ2 cut n Loose cuts at the beginning: 96.2% of μ from  are kept 96.2% of μ from  are kept n Compare 2 samples: pure  and Hijing events: 5000   μ+μ- generated with T=240 MeV 5000   μ+μ- generated with T=240 MeV 5000 Pb-Pb Hijing events with b=0 5000 Pb-Pb Hijing events with b=0 ( after full Geant 3 + reconstruction) ( after full Geant 3 + reconstruction) n n Invariant mass is calculated using the overall fit

22 Pavel Nevski Quarkonium Suppression Upsilon family  (1s)  (2s)  (3s) Binding energies (GeV) 1.1 0.54 0.2 Dissociation at the temperature ~2.5T c ~0.9T c ~0.7T c  μ+ μ- Signal – 5000 generated  μ+ μ- 0.18 μ, 0.008 μ+ μ- pairs reconstructed per event Hijing(b=0): 0.18 μ, 0.008 μ+ μ- pairs reconstructed per event 0.0009 expected μ+ μ-/ev, 387,640 mixed pairs For min.bias -> 0.0009 expected μ+ μ-/ev, 387,640 mixed pairs 8b/410μb  >  8b/410μb  >-> 18 background pairs per one   +  – Background estimate (HIJING+G3)  S/B ~ 0.6:

23 Pavel Nevski Trigger DAQ For Pb+Pb collisions the interaction rate is 8kHz, a factor of 10 smaller than LVL1 bandwidth. We expect further reduction to 1kHz by requiring central collisions and pre-scaled minimum bias events (or high p T 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.

24 Pavel NevskiConclusion n n ATLAS detector will be capable of measuring many aspects of High pT Heavy Ion physics n n Simulation, Reconstruction and Analysis tools exist to evaluate the detector performance n n Work is in progress to understand the detector performance for studying the truly high pT phenomena


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