Dijet Imbalance in 2.76 TeV PbPb Collisions in CMS
David Krofcheck
on behalf of the CMS CollaborationICHEP 2012, Melbourne, July 6th
Jet production in pp collisions
jet-jet correlation in QCD “vaccuum”
D. Krofcheck ICHEP, Melbourne Dijet Imbalance in 2.76 TeV PbPb Collisions 2
Jet production in PbPb collisionsjet-jet correlation in QCD “medium”
γ – jet correlation to probe the medium?
D. Krofcheck ICHEP, Melbourne Dijet Imbalance in 2.76 TeV PbPb Collisions 3
E-ΔΕ1
E-ΔΕ2
Dijet imbalance in PbPb collisionsΔφ
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Phys. Rev. C 84, 024906 (2011)
Pixel and Silicon-Strip TrackerExcellent momentum resolution Large acceptance || < 2.53.8 T Solenoid
Hermetic calorimeter coverageECAL + HCAL || < 3.0
Forward HCAL 3.0 || < 5.0
Muon Detectors|| < 2.5
DAQ+Trigger: Dedicated triggers for Jets, Photons, Muon, high pT charged particles, etc.
Beam Scintillators
The CMS Detector
D. Krofcheck ICHEP, Melbourne Dijet Imbalance in 2.76 TeV PbPb Collisions 5
Selecting jets in PbPb collisions
“peripheral” “central”
D. Krofcheck ICHEP, Melbourne Dijet Imbalance in 2.76 TeV PbPb Collisions 6
Phys. Rev. C 84, 024906 (2011)
Observed angular correlation between the PbPb dijets
D. Krofcheck ICHEP, Melbourne Dijet Imbalance in 2.76 TeV PbPb Collisions 7
Dijets in PbPb are more imbalanced than PYTHIA+HYDJET
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Dijets in PbPb - asymmetry in central collisions
PLB 712 (2012) 176
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Observed angular correlation between γ – jet in PbPb collisions
γ – jet Correlation across PbPb overlap
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• Momentum ratio shifts/decreases with centrality– jets shifting below the 30 GeV pT threshold not included
Submitted to PLB, arXiv:1205.0206
Observed momentum imbalance in γ – jet correlation
D. Krofcheck ICHEP, Melbourne Dijet Imbalance in 2.76 TeV PbPb Collisions 11
CMS
• Significant deviation of x⟨ J in PbPb compared to PYTHIA + HYDJET⟩• The centrality dependence is mostly visible in RJ
– jet pT shifting below the 30 GeV threshold
Mean xJ -Fraction without jet: RJ
Observed deviation between γ – jet from PbPb collisions
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Submitted to PLB, arXiv:1205.0206
• We used dijets to compare PbPb collisions to pp at different impact parameters and transverse momenta
• Dijets can be strongly imbalanced in momentum as a result of passing through the QCD medium
• We observe γ – jet correlationsMeasuring the γ and jet energies may tell us about the energy loss mechanism of in-medium partons.
Summary
D. Krofcheck ICHEP, Melbourne Dijet Imbalance in 2.76 TeV PbPb Collisions 13
Backup slides
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