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Heavy Ion Physics with the CMS Experiment at the LHC

This article discusses the capabilities of the CMS detector at the LHC for studying heavy ion physics, including high rate capability, high-level trigger, silicon tracker, calorimeters, muon reconstruction, and event characterization. It also highlights the importance of measurements such as jet reconstruction, azimuthal asymmetry, high pT suppression, quarkonia production, and Z0 production. The article concludes by emphasizing the integration of the heavy ion program into the overall CMS physics program and the extension of knowledge gained at RHIC to the new energy domain.

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Heavy Ion Physics with the CMS Experiment at the LHC

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  1. Heavy Ion Physics with the CMS Experiment at the LHC Christof Roland / MIT For the CMS Heavy Ion Group Rencontres de Moriond 2004 La Thuile

  2. CMS as a Detector for Heavy Ion Physics m chambers HCAL ECAL Si Tracker including Pixels DAQ and Trigger • High rate capability for AA, pA, pp • High-Level Trigger capable of inspecting/selecting HI events in real-time • Silicon Tracker • Good efficiency and low fake rate • for pT>1 GeV • Excellent momentum resolution • Dp/p~1% • Fine Grained High Resolution Calorimeter (E-cal+H-cal) • Hermetic coverage up to |h|<5 • (|h|<7 proposed using CASTOR) • Zero Degree Calorimeter (proposed) • Muon Reconstruction • Tracking m from Z0, J/, • Wide rapidity range |h|<2.4 • σm ~50 MeV at  Fully functional at highest expected multiplicities Detailed studies at dN/dy~3000-5000 and cross-checks at 7000-8000

  3. Heavy Ion Physics at sNN =5.5TeV p0 p0 p0 Large Cross section for Hard Probes • Copious production of high pT particles • Nuclear modification factors RAA at very high pT • Large cross section for J/ψ and family production • Different “melting” for members of  family • Large jet cross section • Jets directly identifiable • Study in medium modifications

  4. Global Event Characterization ? dNch/dh/(<Npart>/2) Energy of Collision • Hit counting in the first pixel layer • Needs few events O(1000) • Few seconds of data taking dN/dh One Event • Reconstructed • + MC Input • -- Systematic Error -2 -1 0 1 2 h • Charged Particle Multiplicities • Predictions vary by a factor of 4! • dN/dy ~ 2000 – 8000 • (RHIC extrapolation vs. HIJING)

  5. Azimuthal Asymmetry Energy Flow in Calorimeters: (Pb+Pb with b=6 fm) Charged Tracks vs. pT: (STAR nucl-ex/0206006) s=0.1 rad Measure azimuthal anisotropy of • Charged Particle Production (vs. pT) • Energy flow in the Calorimeters

  6. Energy dependence of high pT suppression • Inclusive pT spectra vs. collision centrality • Determine nuclear modification factors RAA • Hard spectrum and high data rate will allow to perform this measurement very early in the LHC program and out to high pT 1000 Pb Pb Events @ LHC: I. Vitev and M. Gyulassy, Phys.Rev.Lett. 89 (2002)

  7. Quarkonia  family J/ background subtracted Pb+Pb, 1 month at L=1027, 50% Eff. • Excellent mass resolution (σm~50 MeV)and high statistics • for the J/y and U family • full simulation of di-muon channel. • Level 1 trigger simulations in the barrel

  8. High Mass Dimuon, Z0 Production • Z0->mm can be reconstructed with high efficiency • A probe to study nuclear shadowing • Z0 alsoproposed as reference for  production. • High statistics (1 month):

  9. Jet Reconstruction in CMS using Calorimeters 100GeV Jet in a Pb+Pb event (after background subtraction)

  10. Balancing g or Z0 vs. Jets: Calibrated jet energy ! g, Z0 <E>=0 GeV <E>=4 GeV Background <E>=8 GeV # Events/4 GeV Isol. p0+jet ETg/p0-ETJet (GeV) Jet+Z0 ETjet, g>120 GeV in the barrel 1 month at 1027 cm-2s-1 Pb+Pb Direct Measurement of parton energy Loss! Trigger capabilities + large acceptance are essential

  11. Jet Shapes and Fragmentation • Jet Reconstruction in Calorimeters plus high-precision tracking allow for detailed jet shape analysis to study the energy loss mechanism • Use tagged b Jets to measure charged-particle fragmentation function and jet-shapes of heavy quark jets and compare properties of light quark jets. Longitudinal momentum fraction z along the thrust axis of a jet: pT relative to thrust axis:

  12. Conclusions • LHC will extend energy range and in particular high pT reach of heavy ion physics • CMS is preparing to take advantage of its capabilities • Excellent coverage and resolution • Quarkonia • Jets • Centrality, Multiplicity, Energy Flow reaching very low pT • Essentially no modification to the detector hardware • New High-Level Trigger algorithms • Zero Degree Calorimeter, CASTOR and TOTEM proposed to extend forward coverage • Heavy Ion program is well integrated into overall CMS Physics Program • The knowledge gained at RHIC will be extended to the new energy domain

  13. To be continued…

  14. The Algorithm Adapted from default p+p reconstruction. Based on Kalman Filter Modifications to the p+p Algorithm: • Trajectory Seed Generation Three pixel hit combinations compatible with primary event vertex • Trajectory Building Special error assignment to merged hits • Trajectory cleaning Allow only one track per trajectory seed • Trajectory Smoothing Final fit with split stereo layers Running and tested in ORCA_6_3_0 and ORCA_7_2_4

  15. Occupancy • Occupancy in central Pb+Pb Event: • 1-3% in Pixel Layers • Up to 70% in Strip Layers @ dNdy 7000

  16. Pixel Triplet Seeds • Pixel triplets provide precise initial estimate of track parameters. See M.Konecki CPT Week 5/03 • Generate only seeds by consistent with primary event vertex (constraint: dr 100mm dz 350mm) • Minimal number of seeds is crucial for runtime performance and low number of fake tracks • HitTriplets are copied to TrajectorySeeds • => CombinatorialSeedGeneratorFromPixelTriplets.cc

  17. Trajectory Building • Study pattern recognition with standard tools (e.g AnalysingTrajectoryBuilder) • Number of candidates per layer drops much faster than occupancy.

  18. Geometrical Acceptance • Require the track to cross more than 8 (~12 hits) • detector layers and hits in three pixel layers. • Geometrical acceptance ~80% • Defines cutoff at low pT (~1GeV)

  19. Algorithmic Efficiency and Fake Rate • Require more than 12 hits on Track and Fit Probability > 0.01 to reject Fake Tracks • High efficiency and low fake rate even at very high track density |h| < 0.7

  20. Performance of the Track Reconstruction • Match Reconstructed tracks to MC input on a hit by hit basis. (Event sample: dn/dy ~3000 + one 100GeV Jet/Event) |h| < 0.7 dpT/pT < 1%

  21. Tracking in a Jet Cone • The increased local track density in a jet-cone leads to a decrease in reconstruction efficiency of ~5-10% • Can be corrected for since jets will be reconstructed by the calorimetry

  22. Background Subtraction Algorithm • Thanks to I. Vardanyan, A. Oulianov, O. Kodolova • Event-by-event background subtraction: • Calculate <ETTower(h)> and DTower (h) for each h ring • Recalculate all ETTower tower energies: • ETTower = ETTower – Etpile-up • Etpile-up= <ETTower(h)> + DTower (h) • Negative tower energies are replaced by zero • Find Jets with ETjet > Etcut using standard iterative cone algorithm using new tower energies • Recalculate pile-up energy with towers outside of the jet cone • Recalculate tower energy with new pile up energy • Final jets are found with the same iterative • cone algorithm ETJet = ETcone – Etpile-up new

  23. Transverse Energy Flow central Pb-Pb background dNch/dy ~ 5000 (HIJING) Transverse Tower energy Dispersion of Tower energy ECAL + HCAL BARREL : <ETTower> = 1.7+- 0.9 GeV ENDCAP : <ETTower> = 4.8+- 5.0 GeV Most of the energy is deposited in the ECAL

  24. Jet reco. in pure HIJING Events central Pb-Pb background dNch/dy ~ 5000 (HIJING) FAKE JETS With pile-up subtraction With pile-up subtraction After background subtraction max. ET of fake jets is less than 30 GeV

  25. QCD dijet events with initial parton energy of 50-300 GeV Reconstruct without background with Pb-Pb background: dNch/dy ~ 5000 Barrel and Endcap Only one jet with maximal jet energy was used for further analysis Threshold on reconstructed jet energy is 30 GeV Observe linear response of the reconstructed energy to the MC input Jet Energy Jet Reconstruction

  26. Find jet without background Find same jet with background and calculate the number of common cells Define the true reconstructed jet as a jet with more than 60% overlapping cells. This corresponds to dRmin < 0.25 Jet Purity Find the fraction of true jets in all reconstructed jets

  27. Efficiency, Purity vs. Jet Energy Reconstructing 50-300 GeV Jets in Pb-Pb background • EFFICIENCY • Number of events with true reco. Jets/Number of all generated events • PURITY • Number of events with true reco. QCD Jets/ Number of all reco. Jet events (true+fake). • Threshold of jet reco. ET >30 GeV. • Above 75(100) GeV we achieve • 100% efficiency and purity • in the barrel (endcap)

  28. Jet Spatial Resolution Dh Df Endcap: h,f resolution of jets slightly better • The h,f resolution is degraded in Pb Pb collisions • Still better than h,f size of calorimeter tower (0.087x0.087)

  29. Jet Reconstruction Summary • The event by event pile-up subtraction method will allow for jet reconstruction in heavy ion events with high efficiency and purity • Jet direction can be reconstructed with good accuracy. The resolution in h and f is smaller than the size of one calorimeter tower (0.087x0.087)

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