HeavyIon Physics with CMS

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Heavy-Ion Physics with CMS

• Aneta Iordanova
• University of Illinois at Chicago

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CMS Heavy-Ion program
J. Phys. G Nucl. Part. Phys. 34 (2007) 2307-2455

• Study of QCD matter under extreme conditions
• PbPb _at_ vsNN5.5 TeV
• Bulk observables
• (soft physics)
• Hard probes
• Ultra peripheral collisions
• Proton-proton program
• First measurements of bulk observables
• Analysis exercise

dET/dh ? ?Bj J.D.Bjorken, Phys.Rev.D27(1983) 140
presents the capabilities of the CMS experiment
to explore the rich heavy-ion physics programme
offered by the CERN Large Hadron Collider (LHC) .
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The CMS detector
central detectors transverse slice

• Global Event Characterization
• Silicon tracker (p, K, p) , L, K0 (via
  displaced vertices)
• Infer energy density, freeze-out temperatures and
  chemical potential...

• Specific Probes
• Calorimetry e , g and hadronic jets
• probe of early times and jet-medium interactions,
  energy loss
• Muon Chambers µ (from J/?, , Z)
• (heavy) quark energy loss and sensitivity to QGP
  temperature

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The CMS detector
detector h -f coverage

• Silicon tracker hlt2.5
• Momentum resolution lt2 for pTlt100GeV and hlt0.5.

• Calorimetry ECal hlt3, HB,HE,HF hlt5,
• Castor 5lthlt7, ZDC hgt8
• Wide energy-space range measure of jets and MET
• Muon Chambers hlt2.5
• Precise measure of position/momentum and fast L1
  response

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Soft physicsGlobal Event Characterization

• Low-momentum tracking
• dE/dx measurement using the inner silicon layers
• PID for p, K (plt0.8 GeV/c) and protons (plt1.5
  GeV/c)
• Good efficiency and resolution
• Central PbPb collisions occupancy of pixel layers
  2

p-p _at_ 14 TeV (Pythia)
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Soft physicsGlobal Event Characterization
Particle identification

• Particle identification
• Charged hadrons from dE/dx
• Neutral hadrons from decay topology (L, K0)
• Multistrange baryons (X-,W-)
• Freeze-out parameters
• Chemical potential (mB) and temperature
• From identified particles
• Kinetic freeze-out temperature and radial flow
• From particle spectra
• Baryon transport and strangeness production

p-p _at_ 14 TeV (Pythia)
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Hard probesenergy loss in the medium

• Motivation
• RHIC Physics Results
• High-pT suppression ? medium induced parton
  energy loss
• Initial gluon medium density dNg/dy
• Medium diffusion properties (transport
  coefficient q)
• Disappearance of back-to-back jets
• RHIC ? LHC Increased hard scattering cross
  section and luminosity
• CMS detector and triggering capabilities provide
  extended pT reach for charged hadrons and for
  fully reconstructed jets

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Hard probesCMS Capabilities

• Large acceptance calorimetry (ECalHCal)
• Fully reconstruct jets in heavy ion collisions
• Photon reconstruction in ECal
• 4T magnetic field
• Momentum resolution lt2
• Low fake rates
• High-Level Triggering
• Online inspection of all events provides 20 to
  300 times statistical reach

PbPb dNch/dhy03500
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Hard probesReconstructing Jets

• Inclusive jet spectra
• utilizes Hcal and Ecal
• Iterative cone (R0.5) Background subtraction
• High efficiency and purity for ETgt50 GeV jets
• Good energy resolution for ETgt100 GeV
• Jet spectra reconstructed up to ET 0.5 TeV
• Estimated for one year of running PbPb 0.5 nb-1
• (or 3.9x109 events,106 sec)

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Hard probesg-Jet

• Direct probe for in-medium energy loss,
  DEEg-Ehjet
• Reconstruction
• Photon ID combine Ecal/Hcal/tracker to form g
  isolation cuts
• Use of Multivariate analysis
• For e 60, fake g 3.5, S/B4.5
• Away-side jet selection
• ET gt 30 GeV, hlt 2,
• Df(g,jet) gt 1720
• Calculate dN/d?
• Charged tracks in R0.5 cone around jet axis

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Hard probesg-Jet

• Direct probe for in-medium energy loss,
  DEEg-Ehjet
• Final Measurement
• Reconstruction using non-quenched and quenched MC
• Fragmentation functions differ
• Medium modification of fragmentation functions
  can be discriminated with high significance

Significant difference between Non-quenched and
Quenched Analysis method has discriminatory power
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Heavy FlavorJ/y and y
PbPb2500
hlt2.4
Di-muon mass reconstruction
S/B1.2

• Direct probe of QGP formation
• Step suppression of charmonium/bottomonium
  resonances
• Sensitive to QGP temperature
• Reconstruction performance
• Excellent dimuon mass resolution
• 1 of the quarkonium mass for full h
• Best Signal/Background at LHC
• Clean separation of the states
• Broad h-coverage and high-pT reach
• Using HLT selection

sJ/y35MeV/c2
pT (GeV/c)
Broad h coverage
h
NJ/y1.8105
1-year statistical reach
J/y acceptance
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Heavy Flavor family
Di-muon mass reconstruction

• Direct probe of QGP formation
• Step suppression of charmonium/bottomonium
  resonances
• Sensitive to QGP temperature
• Reconstruction performance
• Excellent dimuon mass resolution
• 1 of the quarkonium mass for full h
• Best Signal/Background at LHC
• Clean separation of the states
• Broad h-coverage and high-pT reach
• Using HLT selection

S/B1
PbPb2500
pT (GeV/c)
Broad h coverage
h
N2.5 104
1-year statistical reach
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Ultra peripheral collisions photo-production

• At LHC the accelerated Pb nucleus can produce
  strong electromagnetic field
• due to the coherent action of the Z 82 proton
  charges
• Equivalent photon flux Egmax 80 GeV
• gPb cm Emax 1. TeV/n (3ep HERA)
• gg cm Emax 160 GeV (LEP)
• Measure the gluon distribution function in the
  nucleus (gPb)
• low background
• simpler initial state
• gPb? photo-production in CMS
• Unexplored (x,Q2) regime
• Pin down amount of low-x suppression in the Pb
  nuclear PDF (compared to the proton PDF)

dAu
eA
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Summary

• CMS has a broad and exciting heavy ion program,
  including
• Bulk observables (soft physics)

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Summary

• CMS has a broad and exciting heavy ion program,
  including
• Jet physics
• Quarkonia and heavy-quarks
• Ultra peripheral collisions

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Backup slides
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Soft Physics
Charged particle tracking

• Pixel tripletsvertexstrips
• reconstructing down to pT0.075 GeV/c with high
  efficiency (80-90) and acceptance
• The pT resolution is about 1-2 in the barrel
  region
• Fake track rate
• around per mille level in pp, below 10 in
  central PbPb for pT gt 0.4 GeV/c
• Steps at 1 and 2 GeV/c are due to stricter
  requirements (points on track)
• Close to flat and smooth in the mid-rapidity
  region

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Jet quenching

• At RHIC, suppression of leading particles
• Interpretated by parton energy loss models in
  the medium
• Loose energy by gluon strahlung
• transport coefficient hqi, characterizing the
  scattering power of the medium
• GLV Gyulassy M, Levai P and Vitev I
  nucl-th/0006010,hep-ph/0209161
• BDMPSBaier R, Dokshitzer Y L, Mueller A H,
  Peigne S and Schiff hep-ph/9608322,
  hep-ph/0002198, hep-ph/0005129, hep-ph/0302184

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gJetIn medium modified fragmentation function
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Generated events
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Reconstruction/Photon ID
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Jet finding bias
Jet finding (away side)
Setting working point
Main contribution to systematic
uncertainty Biased to parton with high ET (high
pt particles)