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Jet quenching
and
energy loss in CMS
Christof Roland
for the CMS Collaboration
Christof Roland
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Hard Probes 2012, Cagliari
CMS Detector
Muon Detectors
|| < 2.5
Pixel and Silicon-Strip Tracker
Excellent momentum resolution
Large acceptance || < 2.5
3.8 T Solenoid
Hermetic calorimeter coverage
ECAL + HCAL || < 3.0
Forward HCAL 3.0 || < 5.0
DAQ+Trigger: Dedicated triggers for Jets, Photons, Muon, high p T charged particles, etc.
Christof Roland
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Hard Probes 2012, Cagliari
Single particles in the CMS Detector
4T
Christof Roland
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Hard Probes 2012, Cagliari
The “Particle Flow” algorithm to form Jets
Jets from Anti-kT, R = 0.3
Underlying event is subtracted using the
iterative pile-up subtraction method
Christof Roland
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Hard Probes 2012, Cagliari
Isolated Photons
Isolated photon Photon candidate
from jet
Jet, pT: 99.7 GeV/c
Shower Shape
Photon, pT: 142.1 GeV
Two independent handles to measure isolated
photons: isolation and shower shape
Christof Roland
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Hard Probes 2012, Cagliari
Motivation
“Bethe-Bloch curve”
Energy loss of a charged particle in matter (QED)
θ
Ein
muon
Ein-ΔΕ
Δx
Bethe equation
Goal: achieve the same level of understanding for partons
traversing strongly interacting matter (QCD)
Christof Roland
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Hard Probes 2012, Cagliari
Parton Energy Loss in PbPb Collisions
•
Angular deflection ()
– Study angular correlation dijet and gamma jet correlations
• Constrains the scattering mechanisms
•
Study the “lost” energy (Medium Ionization)
– Missing pT studies
•
Energy loss (E)
– Study dijet and gamma jet momentum balance
• Direct observation of parton energy loss
•
The Bethe Curve
– Study the momentum dependence
• Dijet balance vs. pT
•
Different projectiles
– Dijet and gamma-jet momentum balance
• Gluon vs quark dominated processes
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Hard Probes 2012, Cagliari
Initial State
Non Interacting Probes:
Photon, Z0, W
QED:
QCD:
PbPb 0-10% Central
•
No strong modification of the initial state observed
– Use PYTHIA and Ncoll scaling for unquenched reference
– Embed in HYDJET (underlying event)
Christof Roland
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Hard Probes 2012, Cagliari
Dijet Measurements
QED:
Ein- ΔΕ
Ein
QCD:
E-ΔΕ1
E-ΔΕ2
From QM2011: Parton Energy Loss observed
as Dijet Momentum Imbalance
Christof Roland
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Hard Probes 2012, Cagliari
Jet Fragmentation
•
Select dijets with
– pTJet1>100GeV/c
– pTJet2PbPb
>40GeV/c
– 12>2/3
•
Correlate reco. Jet pT with track pT
– Tracks in R=0.3 cone
– pT> 4 GeV/c
– z = p || Track/pJet,  = log (1/z)
 = log (1/z)
dN/dpTJet
Pb
Pb
•
Compare Leading and Subleading Jet
in pp and PbPb
Yetkin Yilmaz, Parallel IVB, Thursday 14:00
Christof Roland
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Hard Probes 2012, Cagliari
Jet Fragmentation in pp and PbPb
PbPb
Pb
Pb
 = log (1/z)
Leading and subleading jet in PbPb fragment like jets of
corresponding energy in pp collisions
Yetkin Yilmaz, Parallel IVB, Thursday 14:00
Christof Roland
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Hard Probes 2012, Cagliari
Fragmentation
jets
PbPb results in Aof
J bins
Christof Roland
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Hard Probes 2012, Cagliari
Fragmentation
jets
PbPb results in Aof
J bins
At the same reconstructed jet energy,
jets in PbPb fragment like jets in pp
– We can reliably reconstruct jets
– We can safely use PYTHIA derived jet
energy scale corrections in PbPb
– Calibrate to final state particle level
Christof Roland
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Hard Probes 2012, Cagliari
Where does the “lost” energy go?
0-30% Central PbPb
E?
Medium ionization
arXiv:1102.1957 [nucl-ex]
balanced jets
Christof Roland
unbalanced jets
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Hard Probes 2012, Cagliari
Missing-pT||
Missing pT||:
0-30% Central PbPb
Calculate projection of pT
on leading jet axis and
average over selected
tracks with
pT > 0.5 GeV/c and
|η| < 2.4
balanced jets
arXiv:1102.1957 [nucl-ex]
unbalanced jets
Leading jet defines direction
Sum all tracks in the event
Christof Roland
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Hard Probes 2012, Cagliari
Missing-pT||
Missing pT||:
0-30% Central PbPb
excess away
from leading jet
excess towards
leading jet
arXiv:1102.1957 [nucl-ex]
balanced jets
unbalanced jets
Integrating over the whole event final state
the momentum balance is restored
Christof Roland
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Hard Probes 2012, Cagliari
Missing-pT||
Missing pT||:
0-30% Central PbPb
excess away
from leading jet
Calculate missing pT in
ranges of track pT:
excess towards
leading jet
arXiv:1102.1957 [nucl-ex]
Phys. Rev. C84 (2011) 024906
balanced jets
unbalanced jets
The momentum difference in the dijet is
balanced by low pT particles
Christof Roland
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Hard Probes 2012, Cagliari
Missing-pT||
in-cone
out-of-cone
0-30% Central PbPb
In-Cone
R<0.8
Out-of-Cone
R<0.8
Phys. Rev. C84 (2011) 024906
balanced jets
unbalanced jets
The momentum difference in the dijet is balanced by low pT
particles at large angles relative to the away side jet axis
Christof Roland
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Hard Probes 2012, Cagliari
DijetAngular
Angular Correlations
Dijet
Correlations
PLB 712 (2012) 176
QED:

)
pT,2> 30GeV/c
QCD:
pT1
pT2
Correlation peak is the same in data and Pythia
across all values of pT
Yetkin Yilmaz, Parallel IVB, Thursday 14:00
Christof Roland
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Hard Probes 2012, Cagliari
pT-dependence of the dijet imbalance
PLB 712 (2012) 176
pT,2> 30GeV/c
12 > 2/3
pT,2/pT,1
Unbalanced
Balanced
Energy loss apparent in
all bins of leading jet pT
Yetkin Yilmaz, Parallel IVB, Thursday 14:00
Christof Roland
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Hard Probes 2012, Cagliari
ppT-dependence
the dijet
dijet imbalance
imbalance
T-dependence of the
PLB 712 (2012) 176
No significant dependence on jet pT
Yetkin Yilmaz, Parallel IVB, Thursday 14:00
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Hard Probes 2012, Cagliari
-Jet Correlations
Jet

)
Ein
Ein-ΔΕ

• Correlate Isolated Photons with Jets
– Photons do not interact with the medium
• Tag initial parton energy and direction
– -Jet channel selects predominantly quark jets
Christof Roland
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Hard Probes 2012, Cagliari
-Jet Angular Correlations
Submitted to PLB, arXiv:1205.0206
pT
>
60 GeV/c || < 1.44
pTJet> 30 GeV/c || < 1.6
Subtract background from
– Multiple hard scatterings
– fragmentation/decay photons
Yue Shi Lai, Parallel VB, Thursday 16:30
Christof Roland
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Hard Probes 2012, Cagliari
-Jet Angular Correlations
Submitted to PLB, arXiv:1205.0206
pT
>
60 GeV/c || < 1.44
pTJet> 30 GeV/c || < 1.6
No significant angular decorrelation
is observed for -jet pairs
Yue Shi Lai, Parallel VB, Thursday 16:30
Christof Roland
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Hard Probes 2012, Cagliari
-Jet Momentum Balance
Submitted to PLB, arXiv:1205.0206
pT
>
60 GeV/c || < 1.44
pTJet> 30 GeV/c || < 1.6
• Momentum ratio shifts/decreases with centrality
– jets shifting below the 30 GeV pT threshold not included
Yue Shi Lai, Parallel VB, Thursday 16:30
Christof Roland
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Hard Probes 2012, Cagliari
-Jet Momentum Balance vs. Centrality
-Fraction without jet: RJ
Mean xJ
CMS
•
•
Significant deviation of〈xJ〉in PbPb compared to PYTHIA + HYDJET
The centrality dependence is mostly visible in RJ
– jet pT shifting below the 30 GeV threshold
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Yue Shi Lai, Parallel VB, Thursday 16:30
Hard Probes 2012, Cagliari
Summary
•
•
Many different aspects of parton energy loss have been
studied
Angular correlation of partons not affected by the medium
– Measured in dijets and in photon-jet correlations
– Over a wide range in pT
– Constrains the scattering mechanisms
•
Energy redistribution in the medium
– Momentum difference in the dijet balanced by low pT
particles at large angles relative to the away side jet
•
Large dijet momentum imbalance observed
– Momentum imbalance persists out to the highest pT
•
First measurement of gamma jet correlations in PbPb
– Direct observation of energy loss with respect to the initial
parton energy
– Probes quark energy loss vs. gluon energy loss in dijets
•
The Challenge:
– Develop a description of the energy loss mechanism that
consistently describes all aspects of parton energy loss
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Hard Probes 2012, Cagliari
More news from CMS
•
Plenary Presentations:
–
Jet quenching and energy loss in CMS,
•
–
Hadron correlations in CMS
–
Quarkonia and heavy flavor production in CMS
–
W,Z and photon production in CMS
•
•
•
•
Christof Roland (Massachusetts Inst. of Tech.) Plenary 1b, Monday 28.05
Charles Felix Maguire (Vanderbilt University) , Plenary 2B, Tuesday 10:55
Torsten Dahms (LLR, Ecole Polytech., IN2P3-CNRS) , Plenary 3A, Wednesday 09:30
Begona De La Cruz (CIEMAT), Plenary 4A, Thursday 09:30
Parallel Presentations:
–
Measurement of charged hadron R_AA at high pT in PbPb collisions at sqrt(s)=2.76TeV with CMS
–
Measurement of dN/deta and dET/deta in PbPb collisions at sqrt(s)=2.76TeV with CMS
–
Measurement of charged hadron v_2 at high pT in PbPb collisions at sqrt(s)=2.76TeV with CMS
•
•
•
–
Michael Murray (The Univ. of Kansas) , Parallel IIC, Monday 16:30
Victoria Zhukova (The Univ. of Kansas) , Parallel IC: Correlations, Monday 14:00
Measurement of Fourier components of two-particle correlations in PbPb collisions at sqrt(s)=2.76TeV with CMS
•
–
Krisztian Krajczar (KFKI Research Inst. for Part. & Nucl. Phys.) , Parallel IIC, Monday 17:30
Rylan Towne Conway (UC Davis) , Parallel IC: Correlations, Monday 14:40
Dijet transverse momentum imbalance, fragmentation functions and jet-track correlations in PbPb collisions at sqrt(s)=2.76TeV with CMS
•
Yetkin Yilmaz (Massachusetts Inst. of Tech.) , Parallel IVB, Thursday 14:00
–
Measurement of isolated photon R_AA at high pT in PbPb collisions at 2.76TeV with CMS
–
Measurement of isolated photon-jet correlations in PbPb collisions at sqrt(s)=2.76TeV with CMS
–
Measurement of charmonium production in PbPb collisions at 2.76TeV with CMS,
–
Measurement of bottomonium production in PbPb collisions at 2.76TeV with CMS
•
•
•
•
–
Yue Shi Lai (Massachusetts Inst. of Tech.) , Parallel VB, Thursday 16:30
Dong Ho Moon (Korea University) , Parallel IA: Quarkonia, Monday 14:00
Camelia Mironov (LLR, Ecole Polytech., IN2P3-CNRS) , Parallel VA, Thursday16:30
Measurement of electroweak boson production in PbPb collisions at 2.76TeV with CMS
•
–
Yen-Jie Lee (CERN) , Parallel IIIC: Tuesday 14:35
Lamia Benhabib (LLR, Ecole Polytech., IN2P3-CNRS) , Parallel IIIC: Tuesday14:55
Quarkonium production in pp collisions with CMS
•
Carlos Lourenco (CERN), Parallel IIIA, Tuesday 14:15
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Backup Slides
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Fragmentation Functions
Christof Roland
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pp results and reference for PbPb
Hard
fragments
Soft
fragments
Reference for PbPb data by reweighting pp data
Christof Roland
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Hard Probes 2012, Cagliari
Dijet
Momentum
Data
sample ofBalance
2011
PLB 712 (2012) 176
AJ = (pT,1 - pT,2) / (pT,1 + pT,2)
Yetkin Yilmaz, Parallel IVB, Thursday 14:00
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Hard Probes 2012, Cagliari
DijetCorrelation
correlation and
Dijet
and background
Background
At high pT, only very few jets get completely lost on the away side:
A less biased quantification of energy loss
arXiv:1202.5022
Christof
Roland
Yetkin
Yilmaz
(MIT)
Dijets33
in CMS
Hard Probes 2012, Cagliari
33
Track-Jet Correlations
•
Study charged particle distributions within jet cones
– Use  reflected ( -> -) reference cones for jet-by-jet subtraction
of Pb+Pb underlying event
• This avoids  dependent variations due to elliptic flow
• Exclude |Jet| < 0.8 and |Jet| > 1.6
– Study associated track distributions versus pT and ΔR
– Uncertainties in background subtraction limit this
method to pT > 1 GeV/c and ΔR < 0.8
Dijet Partner
η-reflected
Background Cone
Signal Jet Cone
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Hard Probes 2012, Cagliari
Track-Jet Correlations
Phys. Rev. C84 (2011) 024906
MC
Pb
Pb
Data
unbalanced jets
balanced jets
Increasing AJ
Christof Roland
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Hard Probes 2012, Cagliari
Track-Jet Correlations
Phys. Rev. C84 (2011) 024906
MC
• In dijet events with a large
imbalance, AJ>0.33, we find
more energy in tracks below pT
Pb
Pb
of 4GeV/c
at large R
MC
Data– But, not nearly enough to restore
Data
the dijet balance
• No indication of cone like
structures around the jet up to a
cone size of 0.8
• Not in the region currently
balanced jets
accessible, to be extended later
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unbalanced jets
Hard Probes 2012, Cagliari
Jet Reconstruction
Calorimeter based Jet Finder (IC5)
– Iterative Cone Algorithm
– R = 0.5
– arXiv:1102.1957 [nucl-ex]
Particle Flow Jet Finder (PF)
– Anti-kT Clustering Algorithm*
– R = 0.3
– See Presentation by M. Nguyen
Parallel Session, Friday
Underlying event subtraction
– Iterative PileUp subtraction**
* M. Cacciari, G. P. Salam, and G. Soyez, JHEP 04 (2008) 063
** O. Kodolova, I. Vardanian, A. Nikitenko et al., Eur. Phys. J. C50
(2007) 117
37
Christof Roland
Hard Probes 2012, Cagliari
Jet Reconstruction
Calorimeter based Jet Finder (IC5)
– Iterative Cone Algorithm
– R = 0.5
Particle Flow Jet Finder (PF)
– Anti-kT Clustering Algorithm
– R = 0.3
Underlying event subtraction
– Iterative PileUp subtraction
Jet Reconstruction fully efficient above:
– 50 GeV/c, Calorimeter Jets
– 40 GeV/c, Particle Flow Jets
Christof Roland
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Hard Probes 2012, Cagliari
Jet Reconstruction
Pb Pb
Calorimeter based Jet Finder (IC5)
PbPb
– Iterative Cone Algorithm
– R = 0.5
Particle Flow Jet Finder (PF)
– Anti-kT Clustering Algorithm
– R = 0.3
Underlying event subtraction
– Iterative PileUp subtraction
•
•
Good Jet pT resolution
Jet pT corrected to generator final state particle level
– Correction derived from PYTHIA
Christof Roland
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Hard Probes 2012, Cagliari
Pb
Pb
-Jet Signal Definition
Leading order
Higher orders
γ
γ
γ
Final state
not
Isolated
ISOLATED Non-isolated
differentiable
•
•
•
•
•
Isolated
ISOLATED NONISOLATED
SumIso = uncorrected Track + ECAL + HCAL ET in R < 0.4
GenIso = generator level particle energy in R < 0.4
Isolated prompt (non-decay) photons with SumIso < 1 GeV
Comparison to MC definition GenIso < 5 GeV
SumIso ≠ GenIso due to PbPb underlying event fluctuation
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Hard Probes 2012, Cagliari
-Jet Statistical Background Subtraction
Data, cf. Slide 13
Photon-Jet
Background Photon–
Jet
Background Photon–UE
Combinatorics
Estimated from event mixing
method using minimum-bias
data
Photon–UE
Combinatorics
Δφ
Christof Roland
Signal
region
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Hard Probes 2012, Cagliari
Fragmentation/decay photon subtraction
Statistical Subtraction
Photon-Jet
Background Photon–
Jet
Estimated from shower shape
sideband & purity
Δφ
Christof Roland
Signal
region
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Hard Probes 2012, Cagliari
Analysis Flow Chart
Super Clusters
spike rejection
Passing loose photon ID
H/E<0.1
Photon isolation
Energy correction
Shower shape
Signal like
σηη<0.01
Shower shape
Background like
0.011<σηη<0.017
Background
clusters
(Fakes from jet)
Z to ee
Cross check the
energy correction
Electron rejection
Remaining contribution
1-3%
Selected Jet
candidates
Leading Cluster
Shower shape
Signal like
σηη<0.01
Exclude photon candidate
|ΔR| > 0.3 w/r to leading photon
Photon
Candidates
Background template
Selected Jet
Photon Candidates- AllJet
|Δφ| > 7π/8
Purity
Determination
Dijet Background subtraction
Christof Roland
Anti-kT 0.3
pile-up subtracted
Jet candidates |η|<1.6
Event Mixing
Photon Candidates- All Jet
Signal region
Photon-Jet correlations in
MIXED events
Photon- All Jet
Signal region
Photon-Jet
Final result
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Hard Probes 2012, Cagliari