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1Study of jet quenching using the CMS detectorYen-Jie Lee (CERN)
Yen-Jie Lee (CERN)
for the CMS Collaboration
Rencontres Ion Lourds/Heavy Ion Meeting
18 April, 2013
Study of Jet Quenching using the CMS Detector
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2Study of jet quenching using the CMS detectorYen-Jie Lee (CERN)
Probe the Medium
Final goal: Understand the
thermodynamics and transport
properties of QGP
Problem: the lifetime of QGP is so short
(O(fm/c)) such that it is not yet feasible
to probe it with an external source
Solution: Take the advantage of the
large cross-sections of hard probes
produced with the collision
QGP
source
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3Study of jet quenching using the CMS detectorYen-Jie Lee (CERN)
Probe the Medium
Final goal: Understand the
thermodynamics and transport
properties of QGP
Problem: the lifetime of QGP is so short
(O(fm/c)) such that it is not yet feasible
to probe it with an external source
Solution: Take the advantage of the
large cross-sections of hard probes
produced with the collision
Colorless probes: γ, W/Z bosons
γ/W/Z
QGP
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4Study of jet quenching using the CMS detectorYen-Jie Lee (CERN)
Probe the Medium
Final goal: Understand the
thermodynamics and transport
properties of QGP
Problem: the lifetime of QGP is so short
(O(fm/c)) such that it is not yet feasible
to probe it with an external source
Solution: Take the advantage of the
large cross-sections of hard probes
produced with the collision
Colorless probes: γ, W/Z bosons
Jets: originating from quarks and gluons
γ/W/Z
Jet
Jet
QGP
QGP
In medium parton energy loss
“Jet quenching”(Bjorken, 1982)
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5Study of jet quenching using the CMS detectorYen-Jie Lee (CERN)
CMS Detector
Inner tracker:charged particlesvertex, isolation
solenoid
EM and Hadron calorimetersphotons, isolation
MuonMuon
HCALHCAL
ECALECAL
TrackerTracker |η|< 2.5
|η|< 3.0
|η|< 5.2
|η|< 2.4
Pb
Pb
Calojet
Particle Flow Jet (track pT> 0.9GeV/c)
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6Study of jet quenching using the CMS detectorYen-Jie Lee (CERN)
Heavy Ion Collision Recorded by the CMS Detector
2010 PbPb 7 μb-1
2011 PbPb 150 μb-1
2012 pPb 1 μb-1
2013 pPb 31 nb-1
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7Study of jet quenching using the CMS detectorYen-Jie Lee (CERN)
“Jet Quenching” without jet: Charged Particle RAA
Provide constraints on the parton energy loss models
R AA=σpp
inel
⟨N coll⟩
d2N AA / dpT dη
d2σ pp/ dpT dη
“QC
D M
ediu
m”
“QC
D V
acuu
m”
~
Ncoll
validate by photons
W/Z bosons
If PbPb = superposition of pp ...
EPJC 72 (2012) 1945
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8Study of jet quenching using the CMS detectorYen-Jie Lee (CERN)
Charged Particle Spectra
Absorption? Energy loss?
Single hadron spectra themselves do not provide details of the underlying mechanism
Need direct jet reconstruction and correlation studies
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9Study of jet quenching using the CMS detectorYen-Jie Lee (CERN)
Direct jet reconstruction with CMS
Subleading Jetp
T2
Leading Jetp
T1
Dijet Event Recorded by CMS
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10Study of jet quenching using the CMS detectorYen-Jie Lee (CERN)
To Explain the Suppression of High pT Particles
Do we see strong suppression of high pT jets?Can we collect the radiated energy back?
Soft collinear radiation Hard radiation
PYTHIA inspired models Modified splitting functions
GLV + others(pre-LHC models)
Large angle soft radiation“QGP heating”
AdS/CFTInterference
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11Study of jet quenching using the CMS detectorYen-Jie Lee (CERN)
Inclusive Jet Spectra: Jet RAA
Detector effects unfoldedStrong suppression of inclusive high pT jets
Compare PbPb to pp data
0.5
Anti-kT jets with R = 0.3
If PbPb = superposition of pp
CMS PAS HIN-12-004
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12Study of jet quenching using the CMS detectorYen-Jie Lee (CERN)
Inclusive Jet Spectra: Jet RAA
Strong suppression of inclusive high pT jetsA cone of R=0.2, 0.3, 0.4 doesn’t catch all the radiated energyAre those high pT jets “completely absorbed” by the medium?
Compare PbPb to pp data
0.5
Anti-kT jets with R = 0.2, 0.3, 0.4
If PbPb = superposition of pp
CMS PAS HIN-12-004
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13Study of jet quenching using the CMS detectorYen-Jie Lee (CERN)
Dijet and Photon-Jet Energy Imbalance
High pT leading jettriggered sample
High pT photontriggered sample
Photon unmodified jet energy tag
High statistics, with surface bias Lower statistics, without surface bias
Dijet Photon-jet
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14Study of jet quenching using the CMS detectorYen-Jie Lee (CERN)
Dijet Momentum Imbalance
Parton energy loss is observed as a pronounced energy
imbalance in central PbPb collisions
Small AJ
(Balanced dijet)
Large AJ
(Un-balanced dijet)
pp
Jet Cone size R = 0.5
PRC 84 (2011) 024906
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15Study of jet quenching using the CMS detectorYen-Jie Lee (CERN)
Dijet Energy Ratio (imbalance)
PLB 712 (2012) 176
Anti-kT jet R = 0.3
Dijet pT ratio as a function of leading jet pT
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16Study of jet quenching using the CMS detectorYen-Jie Lee (CERN)
Dijet Energy Ratio (imbalance)
• Energy imbalance increases with centrality• Very high pT jets are also quenched PLB 712 (2012) 176
Anti-kT jet R = 0.3
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17Study of jet quenching using the CMS detectorYen-Jie Lee (CERN)
-jet Momentum Imbalance
• Photons serve as an unmodified energy tag for the jet partner• Ratio of the pT of jets to photons (xJ=pT
Jet/pT)
is a direct measure of the jet energy loss• Gradual centrality-dependence of the xJ distribution
PLB 718 (2013) 773
PbPbPbPb
PbPbPbPb
Anti-kT jet R = 0.3
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18Study of jet quenching using the CMS detectorYen-Jie Lee (CERN)
Dijet and photon-jet azimuthal correlation
Given the large momentum imbalanceseen in dijet and photon-jet events
Is the azimuthal correlation modified?
ΔφJet
Jet
QGP
In medium parton energy loss
“Jet quenching”
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19Study of jet quenching using the CMS detectorYen-Jie Lee (CERN)
Dijet Azimuthal Angle Correlations
No apparent modification in the dijet Δφ distribution
for different jet pT (still back-to-back)
Δφ
Jet Cone size R = 0.5PLB 712 (2012) 176
0-20%
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20Study of jet quenching using the CMS detectorYen-Jie Lee (CERN)
Photon-jet Azimuthal Angular Correlation
pp
Azimuthal angle difference between photon and jet
PbPb
pppp
“QGP Rutherfold experiment”
Jet
Jet
Photon
Photon“Backscattering?”
The first photon-jet correlation measurement in heavy ion collisions
PLB 718 (2013) 773
Anti-kT jet R = 0.3
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21Study of jet quenching using the CMS detectorYen-Jie Lee (CERN)
Jet Shape and Fragmentation Function
Differential jet shape: “shape” of the jet as a function of radius (r)
Jet fragmentation function: how transverse momentum is distributed inside the jet cone
Tracks
Large parton energy loss (O(10GeV)) in the medium, out of the jet cone
What about jet structure? Measured using tracks in the jet cone.
r = (Δη2+Δφ2)1/2
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22Study of jet quenching using the CMS detectorYen-Jie Lee (CERN)
PbPbPbPb PbPbPbPb
Differential Jet Shape
Significant modification at large radius (r) with respect to the jet axis, looking at tracks with pT> 1 GeV/c
r = (Δη2+Δφ2)1/2CMS PAS HIN-12-013
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23Study of jet quenching using the CMS detectorYen-Jie Lee (CERN)
Jet Fragmentation Functions
PbPbPbPbPbPbPbPb
Low pT
particlesHigh pT
particlesInside the jet cone: Enhancement of low pT particle
Suppression of intermediate pT particles in cone
CMS PAS HIN-12-013
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24Study of jet quenching using the CMS detectorYen-Jie Lee (CERN)
Track pT Distributions in Jet Cones (R=0.3)
High pT : no change compared to jets in pp collisionsIn (central) PbPb: excess of 1-2 tracks compared to pp at low pT
CMS PAS HIN-12-013
PbPbPbPbPbPbPbPb
(1/G
eV
)
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25Study of jet quenching using the CMS detectorYen-Jie Lee (CERN)
What have we learned so far? (1)
3. Angular correlation of jets not largely modified
2. Large average dijet andphoton-jet pT imbalance
4. pT difference found at low pT particles far away
from the jets
5. Observation of modified jet fragmentation function and jet shape
Inside the jet cone:excess of ~1-2 tracks in PbPb
compared to pp at track pT < 3 GeV
What Have We Learned with CMS PbPb Data?
1. High pT jet suppressionΔR = 0.2 - 0.4 doesn’t capture all the radiated energy
6. b jets are also quenched(not included in this talk)
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26Study of jet quenching using the CMS detectorYen-Jie Lee (CERN)
The Emerging Picture
pp PbPb
Excess of low pT particles, extends to ΔR>0.8
The bulk is not largely modified
Jet Energy loss ~5-15% in 0-20% central collisions
Excess of ~1-2 charged particles with pT < 4 GeV/cInside the jet cone R<0.3
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27Study of jet quenching using the CMS detectorYen-Jie Lee (CERN)
Coming Back to the Three Scenarios
Soft collinear radiation Hard radiation
PYTHIA inspired models Modified splitting functions
GLV + others(pre-LHC models)
Large angle soft radiation“QGP heating”
AdS/CFTInterference
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28Study of jet quenching using the CMS detectorYen-Jie Lee (CERN)
Summary Before reaching the final goal: understand the properties of QGP, we
are in the position to validate the theoretical understanding of the in-medium parton energy loss
CMS has presented interesting results from dijet, photon-jet and inclusive jet analyses in heavy ion collisions. A detailed picture of jet quenching is emerging.
To go beyond qualitative observation: An iterative feedback cycle between theory (in the form of MC
generator) and experiment is very important
To compare between data and theory: A proper smearing procedure for theorist (a proper unfolding
procedure for experimentalist when applicable) is needed
Summary
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29Study of jet quenching using the CMS detectorYen-Jie Lee (CERN)
Backup slides
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30Study of jet quenching using the CMS detectorYen-Jie Lee (CERN)
Summary Jet reconstruction and background subtraction: Improve jet reconstruction to account for elliptic flow using
forward calorimeter Analysis plan: Finalize QM12’ results
pPb data: Jet quenching in pPb collisions? Shadowing effects in pPb collisions? Corrected inclusive jet spectra in pp, pPb and PbPb
collisions PbPb data:
Further studies on dijet and photon-jet events and compare with high statistics pp sample (~5/pb) collected in 2013
Flavour dependence of jet quenching: study of multijet production and b-jet
Longer term: W/Z+jet analysis
Plan
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31Study of jet quenching using the CMS detectorYen-Jie Lee (CERN)
Systematic uncertainties considered in analysis
X negligible/small effect, * important systematics, ** dominant systematics
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32Study of jet quenching using the CMS detectorYen-Jie Lee (CERN)
How do we extract the medium effect in PbPb collisions?
RAA < 1 (suppression)
“QCD Medium”
“QCD Vacuum”
RAA > 1 (enhancement)
RAA = 1 (no medium effect) ~
pp reference PbPb measurements
One typical way is to compare PbPb data to pp reference measurement
‘Nuclear modification factors’
<Ncoll> Averaged number of binary scattering
R AA=σpp
inel
⟨N coll⟩
d2N AA / dpT dη
d2σ pp/ dpT dη
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33Study of jet quenching using the CMS detectorYen-Jie Lee (CERN)
Background Subtraction
η
φ
-2.0 2.0-ππ
Exc
lude
η reflection MethodJetBkg
η
φ
-2.0 2.0-ππ
Event Mixing Method
Jet Bkg
η
φ
-2.0 2.0-ππ
Jet Event
Jet Event MinBias Event
Main result
(Cross-checks)
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34Study of jet quenching using the CMS detectorYen-Jie Lee (CERN)
Tagging and Counting b-quark Jets Test the theoretical prediction color factor and quark-mass
dependence of in-medium parton energy loss
Secondary vertex tagged using
flight distance significance
Tagging efficiency estimated
in a data-driven way
Purity from template fits
to (tagged) secondary vtx
mass distributions
CMS PAS HIN-12-003
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35Study of jet quenching using the CMS detectorYen-Jie Lee (CERN)
Pb+Pbp+p
Fraction of b-jets among All Jets
b-jet fraction: similar in pp and PbPb → b-jet quenching is comparable to light-jet quenching (RAA0.5), within present systematics
CMS PAS HIN-12-003
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36Study of jet quenching using the CMS detectorYen-Jie Lee (CERN)
Jet Reconstruction and Composition
Anti-kT algorithm is used in most CMS publication
On average, charged hadrons carry 65% of the jet momentum
Measure the known part Correct the rest by MC simulation
Optimize the use of calorimeter and tracker Example: “Particle Flow” in CMS A typical high pT jet
Background subtraction and
jet clustering
Towers
Δη x Δϕ0.076 x 0.076 in barrel
Jet
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37Study of jet quenching using the CMS detectorYen-Jie Lee (CERN)
Underlying Event Background
Multiple parton interaction
Large underlying event from soft scattering
Jet
Need background subtraction
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38Study of jet quenching using the CMS detectorYen-Jie Lee (CERN)
Background Subtractionφ
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39Study of jet quenching using the CMS detectorYen-Jie Lee (CERN)
Background Subtractionφ
1. Background energy per tower calculated in strips of η. Pedestal subtraction
Estimate background for each tower ring of constant η estimated background = <pT> + σ(pT)• Captures dN/dη of background• Misses ϕ modulation – to be improved
Background level
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40Study of jet quenching using the CMS detectorYen-Jie Lee (CERN)
Background Subtraction
η
φ
1. Background energy per tower calculated in strips of η. Pedestal subtraction
η
φ
Background level
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41Study of jet quenching using the CMS detectorYen-Jie Lee (CERN)
Background Subtraction
η
φ
1. Background energy per tower calculated in strips of η. Pedestal subtraction
η
φ
2. Run anti kT algorithm on background subtracted towers
Background level
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42Study of jet quenching using the CMS detectorYen-Jie Lee (CERN)
Background Subtraction
η
φ
1. Background energy per tower calculated in strips of η. Pedestal subtraction
η
φ
2. Run anti kT algorithm on background subtracted towers
φ
3. Exclude reconstructed jetsBackground level
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43Study of jet quenching using the CMS detectorYen-Jie Lee (CERN)
Background Subtraction
η
φ
1. Background energy per tower calculated in strips of η. Pedestal subtraction
η
φ
2. Run anti kT algorithm on background subtracted towers
η
φ
3. Exclude reconstructed jetsRecalculate the background energy
η
φ
Background level
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44Study of jet quenching using the CMS detectorYen-Jie Lee (CERN)
Background Subtraction
η
φ
1. Background energy per tower calculated in strips of η. Pedestal subtraction
η
φ
2. Run anti kT algorithm on background subtracted towers
η
φ
3. Exclude reconstructed jetsRecalculate the background energy
η
φ
4. Run anti kT algorithm on background subtracted towers to get final jets
Background level
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45Study of jet quenching using the CMS detectorYen-Jie Lee (CERN)
Summary of Jet Reconstruction
Remove underlying events contribution
MC SimulationPYTHIA
Raw jet energyBackgroundsubtraction
Jet energy correction
Jet energy
correction
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46Study of jet quenching using the CMS detectorYen-Jie Lee (CERN)
Flavor Creation Candidate (pp @ 7 TeV)
Reconstructed secondaryvertices from b and c quarks
Flavor Creation Candidate (pp @ 7 TeV)
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47Study of jet quenching using the CMS detectorYen-Jie Lee (CERN)
-jet correlations
No -decorrelation
Increasing pT-imbalance
Less jet partners above threshold
Jets lose ~14% of their initial energy
~20% of photons lose their jet partner
PbPbPbPbPbPbPbPb
xJ=pTjet/pT
RJ = fraction of photons with jet partner >30 GeV/c
PLB 718 (2013) 773
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48Study of jet quenching using the CMS detectorYen-Jie Lee (CERN)
Overlap zone is almond-shaped→ Parton energy loss is smaller along the short axis→ More high-pT tracks and jets closer to the event plane
→ Azimuthal asymmetry (v2):
→ v2 is sensitive to the path-length dependence of the energy loss
Participant plane pp
EP
Path length dependence of jet energy loss?
pT
v2
L2
L3
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49Study of jet quenching using the CMS detectorYen-Jie Lee (CERN)
Jet and high pT track v2 at the LHC
• Jet and high pT track v2 : non-zero up to very high pT
• Sensitive to the path length dependence of energy loss
Jet v2
High pT track v2
PRL 109 (2012) 022301
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50Study of jet quenching using the CMS detectorYen-Jie Lee (CERN)
pPb runSuccessful pPb data-taking with physics object triggers fully deployed on Sep 2012!
• The first unexpected result already came out:
Observation of long-range near-side angular correlations in proton-lead collisions at the LHC
2013 pPb run: >30/nb recorded!
• Jet quenching in pPb collisions?
• Are jets modified in pPb collisions?
• How shadowing effect and modification on the jet observables?
5x larger than pp!Elliptic flow?Color glass condensate?Modified jet structure?
Two particle correlation function
pp ridge paper: JHEP 1009 (2010) 091
PLB 718 (2013) 795
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51Study of jet quenching using the CMS detectorYen-Jie Lee (CERN)
Compare photon-jet momentum balance
Xjγ = pTJet/pT
photon
in vacuum (pp collision) to the QGP (PbPb collision)
PbPb
PYTHIA
Jets lose about 15% of their initial
energy
In addition, 20% of photons lose their jet partner
(jet pT> 30 GeV/c)
PbPbPbPb
Photon-jet momentum balance
R=0.3
Xjγ
PLB 718 (2013) 773
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52Study of jet quenching using the CMS detectorYen-Jie Lee (CERN)
Tracking efficiency
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53Study of jet quenching using the CMS detectorYen-Jie Lee (CERN)
Jet resolution and energy scale
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54Study of jet quenching using the CMS detectorYen-Jie Lee (CERN)
Subtracted background
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55Study of jet quenching using the CMS detectorYen-Jie Lee (CERN)
Subtracted background
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56Study of jet quenching using the CMS detectorYen-Jie Lee (CERN)
Effects to be considered in analysis
• Impact of background subtraction• Jet response dependence on the jet event configuraiton (ex: 3-jet v.s. 2-jet
event) studied with PYTHIA & PYTHIA+HYDJET MC
• Jet flavour dependence (Quark v.s. gluon, modified jet shape and FF pattern) studied PYTHIA & PYTHIA+HYDJET MC, cross-check with PYQUEN generator
• Shape of medium response (?)• Sensitivity to tracking efficiency (and fluctuation) studied with
PYTHIA+HYDJET
• Impact of jet energy resolution• Resolution of calorimeter resolution studied PYTHIA & PYTHIA+HYDJET MC, cross-
check with PYQUEN generator
• Possible bias toward positive UE fluctuation Random cone & PYTHIA+HYDJET
• Impact to jet energy and pointing resolution PYTHIA+HYDJET
• Fake jets from UE fluctuation PYTHIA+HYDJET, data driven from dijet Δφ correlation
• Inefficiency due to downward UE fluctuation PYTHIA+HYDJET • Impact of flow and event plane dependence PYTHIA+HYDJET
• Centrality determination• Inference of the presence of a jet to centrality determination PYTHIA+HYDJET,
cross-checks with other detectors
• Detector related effects• Calorimeter noise data driven rejection studied with dijet Δφ correlation
• Fake tracks PYTHIA+HYDJET, studied with dijet Δφ correlation
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57Study of jet quenching using the CMS detectorYen-Jie Lee (CERN)
Selection of b-jet Results from CMS
Identification of b-quark jets with the CMS experiment
CMS Physics Analysis Summary BTV-11-004
Inclusive b-jet production in pp collisions at √s = 7 TeV
JHEP 1204 (2012) 084, arXiv:1202.4617
Measurement of BB Angular Correlations based on Secondary
Vertex Reconstruction at √s = 7 TeV
JHEP 1103 (2011) 136, arXiv:1102.3194
Measurement of the b-jet to inclusive jet ratio in PbPb and pp collisions at √sNN = 2.76 TeV with the CMS detector
CMS Physics Analysis Summary HIN-12-003
Selection of b-jet Results from CMSb-jet results from CMS
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58Study of jet quenching using the CMS detectorYen-Jie Lee (CERN)
b-jet Identification
Long lifetime of b (~1.5 ps) leads to measurable (mm or
cm) displaced secondary vertices (SV)
Subsequent charm decay may lead to a tertiary vertex
Several classes of b-jet taggers using:o Reconstructed SV’s, employing
discriminating variables such as SV mass, flight distance, etc.
o Impact parameter (IP) of tracks associated to the jet, w/o requiring a reco’d SV
o Muons in jets, exploiting the large branching ratio (20%)
b-jet identification
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59Study of jet quenching using the CMS detectorYen-Jie Lee (CERN)
Bottom Production
At NLO
Excitation of sea quarks b(b) + light
dijet, w/ b(b) at beam rapidity
Gluon splitting into b and b which can
be reconstructed as a single jet
Flavor Creation (FCR) Flavor Excitation (FEX) Gluon Splitting (GSP)
LO b-b production (FCR) not dominant at the LHC
arXiv:0705.1937
pp @ 14 TeV
bottom production
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60Study of jet quenching using the CMS detectorYen-Jie Lee (CERN)
Gluon Splitting Candidate (pp @ 7 TeV)gluon splitting candidate (pp @ 7 TeV)
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61Study of jet quenching using the CMS detectorYen-Jie Lee (CERN)
How do we extract the medium effect in PbPb collisions?
pp reference PbPb measurements
One typical way is to compare PbPb data to pp reference measurement
Ncoll Number of binary scatterings
Npart Number of participating nucleons
Npart = 2 Ncoll = 1
Npart = 5 Ncoll = 6
Example:
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62Study of jet quenching using the CMS detectorYen-Jie Lee (CERN)
How do we extract the medium effect in PbPb collisions?
RAA < 1 (suppression)dηdpσd
dηdpNd
N
σ=R
Tpp
TAA
coll
inelpp
AA /
/2
2 “QCD Medium”
“QCD Vacuum”
RAA > 1 (enhancement)
RAA = 1 (no medium effect) ~
pp reference PbPb measurements
inelpp
collAA σ
N=T ''NN equivalent integrated luminosity
per AA collision'‘Reduces the uncertainty from pp inclusive cross-section
One typical way is to compare PbPb data to pp reference measurement
‘Nuclear modification factors’
Can also be written as 1/TAA
Ncoll Averaged number of binary scattering
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63Study of jet quenching using the CMS detectorYen-Jie Lee (CERN)
RHIC Lesson : Jet Quenching
Jet-quenching (Bjorken, 1982)
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64Study of jet quenching using the CMS detectorYen-Jie Lee (CERN)
RHIC Lesson : Jet Quenching
4< pT
trig < 6 GeV/c
pT
assoc > 2 GeV/c
Indication of jet quenching: Suppression of high p
T particles
Jet-quenching (Bjorken, 1982)
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65Study of jet quenching using the CMS detectorYen-Jie Lee (CERN)
Parton energy loss models:Possible modification of jet fragmentation and broadening of the dijet ΔΦ distribution
Difficulty at RHIC:High-p
T probes are rare and direct jet reconstruction is very hard
Collisionalenergy loss
Radiativeenergy loss
RHIC Lesson : Jet Quenching
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66Study of jet quenching using the CMS detectorYen-Jie Lee (CERN)
Need rules to group the hadrons
A popular algorithm is anti-kT algorithm Used in ALICE, ATLAS and CMS analyses
Cacciari, Salam, Soyez, JHEP 0804 (2008) 063
Small radius parameter jet spliting
Large radius parameter
ΔR = 0.2, 0.3, 0.4, 0.5 are used in LHC analyses
Radius parameter: decide the resolution scale
Jet reconstruction
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67Study of jet quenching using the CMS detectorYen-Jie Lee (CERN)
Effects to be considered in analysis
• Impact of background subtraction• Jet response dependence on the jet event configuraiton (ex: 3-jet v.s. 2-jet
event) studied with PYTHIA & PYTHIA+HYDJET MC
• Jet flavour dependence (Quark v.s. gluon, modified jet shape and FF) studied PYTHIA & PYTHIA+HYDJET MC, cross-check with PYQUEN generator
• Shape of medium response • Sensitivity to tracking efficiency (and fluctuation) studied with
PYTHIA+HYDJET
• Impact of jet energy resolution• Resolution of calorimeter resolution studied PYTHIA & PYTHIA+HYDJET MC, cross-
check with PYQUEN generator
• Possible bias toward positive UE fluctuation Random cone & PYTHIA+HYDJET
• Impact to jet energy and pointing resolution PYTHIA+HYDJET
• Fake jets from UE fluctuation PYTHIA+HYDJET, data driven from dijet Δφ correlation
• Inefficiency due to downward UE fluctuation PYTHIA+HYDJET • Impact of flow and event plane dependence PYTHIA+HYDJET
•other detectors
• Detector related effects• Calorimeter noise data driven rejection studied with dijet Δφ correlation
• Fake tracks PYTHIA+HYDJET, studied with dijet Δφ correlation
• Centrality determination• Inference of the jet to centrality determination PYTHIA+HYDJET, cross-checks with
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68Study of jet quenching using the CMS detectorYen-Jie Lee (CERN)
Fraction of jets with an away side jet• Given a leading jet with pT > 150 GeV/c, >90% of them has a away side partner
• Fake away side jet rate is < 4%
Anti-kT jet R = 0.3
PLB 712 (2012) 176
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69Study of jet quenching using the CMS detectorYen-Jie Lee (CERN)
Colorless Probes in Heavy Ion Collisions
ZZμμ++μμ--
μ
Wμυ
Isolated photon
γ/W/Z
QGP
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70Study of jet quenching using the CMS detectorYen-Jie Lee (CERN)
Ncoll scaling confirmedin PbPb collisions at 2.76 TeV
W using single muon recoil against missing pT
Z0
arXiv:1205.6334
PRL 106 (2011) 212301CMS-PAS HIN-12-008
Isolated photon
arXiv:1205.6334PLB 715 (2012) 66
PLB 710 (2012) 256
(Non-) Suppression of Colorless Probes
R AA=σpp
inel
⟨N coll⟩
d2N AA / dpT dη
d2σ pp/ dpT dη
If PbPb = superposition of pp ...
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71Study of jet quenching using the CMS detectorYen-Jie Lee (CERN)
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72Study of jet quenching using the CMS detectorYen-Jie Lee (CERN)
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73Study of jet quenching using the CMS detectorYen-Jie Lee (CERN)
Photon RAA
and RCP
Where Does the Energy Go?
• Suppression of high pT jets• Large dijet (photon-jet) energy (momentum) imbalance
ΔET ~ O(10) GeV,
~10% shift in <dijet pT ratio>
Where does the energy go?
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74Study of jet quenching using the CMS detectorYen-Jie Lee (CERN)
arXiv:1102.1957 [nucl-ex]
0-30% Central PbPb
balanced jets unbalanced jets
Missing pT||:
Missing-pT||
Calculate projection of pT on leading jet axis and average over selected tracks with
pT > 0.5 GeV/c and |η| < 2.4
Underlying events cancels
Sum over all tracks in the event
pT
Track
pT
Track ||
ΔΦ
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75Study of jet quenching using the CMS detectorYen-Jie Lee (CERN)
0-30% Central PbPb
balanced jets unbalanced jets
Missing pT||:
Integrating over the whole event final statethe dijet momentum balance is restored
excess away from leading jet
excess towards leading jet p
TTrack
pT
Track ||
ΔΦ
Balanced!!
Missing-pT||
TrackpT > 0.5 GeV/c
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76Study of jet quenching using the CMS detectorYen-Jie Lee (CERN)
0-30% Central PbPb
balanced jets
Missing pT||:
The momentum difference in the dijet is
balanced by low pT particles outside the jet cone
All tracks
Tracks inthe jet coneΔR<0.8
Tracks out ofthe jet coneΔR>0.8
unbalanced jets
Inside the jet conesExcess towards leading jet
Out of the jet conesExcess towards sub-leading jet
Missing-pT||
R=0.5
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77Study of jet quenching using the CMS detectorYen-Jie Lee (CERN)
Summary Need realistic MC generator (for both jet and UE)
Iterative feedback cycle is very important (like PYTHIA vs pp
data in high energy community)
CMS is willing to use and check the simulated results if you
offer a jet event generator
Need to include reconstruction effect before comparing to data
Generator level parton Energy loss + hadronization
apply jet energy smearing apply jet selection
compare the result
To Go Beyond Qualitative Observation
Generator Experiment
Used to derive correction or to compare with data
Feedback and improve the generator
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78Study of jet quenching using the CMS detectorYen-Jie Lee (CERN)
Impact of Jet Energy Smearing
0-20%
Jet energy smearing
Swapped leading and subleading jet
Subleading jet is replaced by third jet
Generator level leading and subleading jets matches reco level
Dijet pT ratio Generator level jets from PYTHIAAnti-kT jet R = 0.3
Smearing function fromPLB 718 (2013) 773