Study of Jet Quenching using the CMS Detector
Yen-Jie Lee (CERN)
for the CMS Collaboration
Rencontres Ion Lourds/Heavy Ion Meeting
18 April, 2013
Yen-Jie Lee (CERN)
Study of jet quenching using the CMS detector
1
Probe the Medium

Final goal: Understand the
thermodynamics and transport
properties of QGP

Problem: the lifetime of QGP is so short
QGP
(O(fm/c)) such that it is not yet feasible
to probe it with an external source

Solution: Take the advantage of the
source
large cross-sections of hard probes
produced with the collision
Yen-Jie Lee (CERN)
Study of jet quenching using the CMS detector
2
Probe the Medium

Final goal: Understand the
γ/W/Z
thermodynamics and transport
properties of QGP

Problem: the lifetime of QGP is so short
(O(fm/c)) such that it is not yet feasible
QGP
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
Yen-Jie Lee (CERN)
Study of jet quenching using the CMS detector
3
Probe the Medium

γ/W/Z
Final goal: Understand the
thermodynamics and transport
properties of QGP

Problem: the lifetime of QGP is so short
QGP
(O(fm/c)) such that it is not yet feasible
to probe it with an external source
Jet

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
Yen-Jie Lee (CERN)
QGP
Jet
Study of jet quenching using the CMS detector
In medium parton energy loss
 “Jet quenching”
(Bjorken, 1982)
4
CMS Detector
EM and Hadron calorimeters
photons, isolation
Pb
Inner tracker:
charged particles
vertex, isolation
solenoid
Pb
Muon
HCAL
ECAL
Tracker
Yen-Jie Lee (CERN)
|η|< 2.4
|η|< 5.2
Calojet
|η|< 3.0
|η|< 2.5
Particle Flow Jet (track pT> 0.9GeV/c)
Study of jet quenching using the CMS detector
5
Heavy Ion Collision Recorded by the CMS Detector
2010 PbPb 7 μb-1
2011 PbPb 150 μb-1
2012 pPb
2013 pPb
Yen-Jie Lee (CERN)
Study of jet quenching using the CMS detector
1 μb-1
31 nb-1
6
~
“QCD Medium”
“QCD Vacuum”
“Jet Quenching” without jet: Charged Particle RAA
2
σ inel
d
N AA / dp T dη
pp
R AA=
⟨ N coll ⟩ d 2 σ pp / dp T dη
If PbPb = superposition of pp ...
Ncoll validate by photons
W/Z bosons
EPJC 72 (2012) 1945
Provide constraints on the parton energy loss models
Yen-Jie Lee (CERN)
Study of jet quenching using the CMS detector
7
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
Yen-Jie Lee (CERN)
Study of jet quenching using the CMS detector
8
Dijet Event Recorded by CMS
Direct jet reconstruction with CMS
Leading Jet
pT1
Subleading Jet
pT2
Yen-Jie Lee (CERN)
Study of jet quenching using the CMS detector
9
To Explain the Suppression of High pT Particles
Large angle soft radiation
“QGP heating”
Soft collinear radiation
Hard radiation
GLV + others
PYTHIA inspired models
Modified splitting functions
(pre-LHC models)
AdS/CFT
Interference
Do we see strong suppression of high pT jets?
Can we collect the radiated energy back?
Yen-Jie Lee (CERN)
Study of jet quenching using the CMS detector
10
Inclusive Jet Spectra: Jet RAA
Compare PbPb to pp data
Anti-kT jets with
R = 0.3
0.5
If PbPb = superposition of pp
CMS PAS HIN-12-004
Detector effects unfolded
Strong suppression of inclusive high pT jets
Yen-Jie Lee (CERN)
Study of jet quenching using the CMS detector
11
Inclusive Jet Spectra: Jet RAA
Compare PbPb to pp data
Anti-kT jets with
R = 0.2, 0.3, 0.4
0.5
If PbPb = superposition of pp
CMS PAS HIN-12-004
Strong suppression of inclusive high pT jets
A cone of R=0.2, 0.3, 0.4 doesn’t catch all the radiated energy
Are those high pT jets “completely absorbed” by the medium?
Yen-Jie Lee (CERN)
Study of jet quenching using the CMS detector
12
Dijet and Photon-Jet Energy Imbalance
Photon-jet
Dijet
Photon  unmodified
jet energy tag
High pT leading jet
triggered sample
High statistics, with surface bias
Yen-Jie Lee (CERN)
High pT photon
triggered sample
Lower statistics, without surface bias
Study of jet quenching using the CMS detector
13
Dijet Momentum Imbalance
Jet Cone size R = 0.5
pp
Large AJ
Small AJ
(Balanced dijet) (Un-balanced dijet)
PRC 84 (2011) 024906
Parton energy loss is observed as a pronounced energy
imbalance in central PbPb collisions
Yen-Jie Lee (CERN)
Study of jet quenching using the CMS detector
14
Dijet Energy Ratio (imbalance)
Anti-kT jet R = 0.3
Dijet pT ratio as a function of leading jet pT
Yen-Jie Lee (CERN)
Study of jet quenching using the CMS detector
PLB 712 (2012) 176
15
Dijet Energy Ratio (imbalance)
Anti-kT jet R = 0.3
• Energy imbalance increases with centrality
• Very high pT jets are also quenched
Yen-Jie Lee (CERN)
Study of jet quenching using the CMS detector
PLB 712 (2012) 176
16
-jet Momentum Imbalance
Anti-kT jet R = 0.3
PbPb
Pb
Pb
PLB 718 (2013) 773
• Photons serve as an unmodified energy tag for the jet partner
• Ratio of the pT of jets to photons (xJ=pTJet/pT)
is a direct measure of the jet energy loss
• Gradual centrality-dependence of the xJ distribution
Yen-Jie Lee (CERN)
Study of jet quenching using the CMS detector
17
Dijet and photon-jet azimuthal correlation
Given the large momentum imbalance
seen in dijet and photon-jet events
Is the azimuthal correlation modified?
Δφ
Jet
QGP
Jet
In medium parton energy loss
 “Jet quenching”
Yen-Jie Lee (CERN)
Study of jet quenching using the CMS detector
18
Dijet Azimuthal Angle Correlations
0-20%
Δφ
Jet Cone size R = 0.5
PLB 712 (2012) 176
No apparent modification in the dijet Δφ distribution
for different jet pT (still back-to-back)
Yen-Jie Lee (CERN)
Study of jet quenching using the CMS detector
19
Photon-jet Azimuthal Angular Correlation
The first photon-jet correlation measurement in heavy ion collisions
“QGP Rutherfold experiment”
Anti-kT jet R = 0.3
PbPb
Photon
Jet
pp
Photon
pp
“Backscattering?”
Jet
PLB 718 (2013) 773
Yen-Jie Lee (CERN)
Azimuthal angle difference
between photon and jet
Study of jet quenching using the CMS detector
20
Jet Shape and Fragmentation Function
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.
Tracks
r = (Δη2+Δφ2)1/2
Differential jet shape:
“shape” of the jet as a function of
radius (r)
Yen-Jie Lee (CERN)
Jet fragmentation function:
how transverse momentum is
distributed inside the jet cone
Study of jet quenching using the CMS detector
21
Differential Jet Shape
PbPb
CMS PAS HIN-12-013
Pb
Pb
r = (Δη2+Δφ2)1/2
Significant modification at large radius (r) with respect
to the jet axis, looking at tracks with pT> 1 GeV/c
Yen-Jie Lee (CERN)
Study of jet quenching using the CMS detector
22
Jet Fragmentation Functions
Pb
Pb
PbPb
CMS PAS HIN-12-013
Inside the jet cone: Enhancement of low pT particle
Suppression of intermediate pT particles in cone
Yen-Jie Lee (CERN)
Study of jet quenching using the CMS detector
High pT
particles
Low pT
particles
23
Track pT Distributions in Jet Cones (R=0.3)
Pb
Pb
(1/GeV)
PbPb
CMS PAS HIN-12-013
High pT : no change compared to jets in pp collisions
In (central) PbPb: excess of 1-2 tracks compared to pp at low pT
Yen-Jie Lee (CERN)
Study of jet quenching using the CMS detector
24
What HaveWhat
We Learned
withlearned
CMS PbPb
Data?
have we
so far?
1. High pT jet suppression
(1)
ΔR = 0.2 - 0.4 doesn’t capture all
the radiated energy
4. pT difference found at
low pT particles far away
from the jets
2. Large average dijet and
photon-jet pT imbalance
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
3. Angular correlation of jets
not largely modified
6. b jets are also quenched
(not included in this talk)
Yen-Jie Lee (CERN)
Study of jet quenching using the CMS detector
25
The Emerging Picture
pp
PbPb
Jet Energy loss ~5-15% in 0-20% central collisions
The bulk is not
largely modified
Excess of ~1-2 charged
particles with pT < 4 GeV/c
Inside the jet cone R<0.3
Excess of low pT particles, extends to ΔR>0.8
Yen-Jie Lee (CERN)
Study of jet quenching using the CMS detector
26
Coming Back to the Three Scenarios
Large angle soft radiation
“QGP heating”
Soft collinear radiation
Hard radiation
GLV + others
PYTHIA inspired models
Modified splitting functions
(pre-LHC models)
Yen-Jie Lee (CERN)
Study of jet quenching using the CMS detector
AdS/CFT
Interference
27

Summary
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
Yen-Jie Lee (CERN)
Study of jet quenching using the CMS detector
28
Backup slides
Yen-Jie Lee (CERN)
Study of jet quenching using the CMS detector
29

Plan
Jet reconstruction and background
subtraction:
Summary


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
Yen-Jie Lee (CERN)
Study of jet quenching using the CMS detector
30
Systematic uncertainties considered in analysis
X  negligible/small effect, *  important systematics, ** dominant systematics
Yen-Jie Lee (CERN)
Study of jet quenching using the CMS detector
31
How do we extract the medium effect in PbPb collisions?
One typical way is to compare PbPb data to pp reference measurement
pp reference
PbPb measurements
‘Nuclear modification factors’
R AA =
l
σ ine
pp
d N AA / dp T dη
“QCD Medium”
2
⟨ N coll ⟩ d 2 σ pp / dp T dη
RAA > 1 (enhancement)
RAA = 1 (no medium effect)
~
“QCD Vacuum”
RAA < 1 (suppression)
<Ncoll> Averaged number of binary scattering
Yen-Jie Lee (CERN)
Study of jet quenching using the CMS detector
32
φ
Bkg
Exclude
π
Background Subtraction
η reflection Method
Jet
Main result
-π
Jet Event
-2.0
η
2.0
Bkg
φ
φ
Jet
-2.0
Yen-Jie Lee (CERN)
η
MinBias Event
2.0
-π
Jet Event
-π
(Cross-checks)
π
π
Event Mixing Method
-2.0
Study of jet quenching using the CMS detector
η
2.0
33
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
Yen-Jie Lee (CERN)
Study of jet quenching using the CMS detector
34
Fraction of b-jets among All Jets
b-jet fraction: similar in pp and PbPb
→ b-jet quenching is comparable to light-jet
quenching (RAA0.5), within present systematics
p+p
Pb+Pb
CMS PAS HIN-12-003
Yen-Jie Lee (CERN)
Study of jet quenching using the CMS detector
35
Jet Reconstruction and Composition
Towers
Anti-kT algorithm is used in most
CMS publication
Jet
Δη x Δϕ
0.076 x 0.076 in barrel
Background
subtraction and
jet clustering
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
Yen-Jie Lee (CERN)
Study of jet quenching using the CMS detector
A typical high pT jet
36
Underlying Event Background
Jet
Multiple parton interaction
Large underlying event from soft scattering
Need background subtraction
Yen-Jie Lee (CERN)
Study of jet quenching using the CMS detector
37
Background Subtraction
φ
Yen-Jie Lee (CERN)
Study of jet quenching using the CMS detector
38
Background Subtraction
φ
1. Background energy per tower calculated
in strips of η. Pedestal subtraction
Background level
Yen-Jie Lee (CERN)
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
Study of jet quenching using the CMS detector
39
Background Subtraction
φ
φ
η
η
1. Background energy per tower calculated
in strips of η. Pedestal subtraction
Background level
Yen-Jie Lee (CERN)
Study of jet quenching using the CMS detector
40
Background Subtraction
φ
φ
η
1. Background energy per tower calculated
in strips of η. Pedestal subtraction
η
2. Run anti kT algorithm on background
subtracted towers
Background level
Yen-Jie Lee (CERN)
Study of jet quenching using the CMS detector
41
Background Subtraction
φ
φ
η
1. Background energy per tower calculated
in strips of η. Pedestal subtraction
η
2. Run anti kT algorithm on background
subtracted towers
φ
Background level
Yen-Jie Lee (CERN)
3. Exclude reconstructed jets
Study of jet quenching using the CMS detector
42
Background Subtraction
φ
φ
η
η
1. Background energy per tower calculated
in strips of η. Pedestal subtraction
2. Run anti kT algorithm on background
subtracted towers
φ
φ
η
Background level
Yen-Jie Lee (CERN)
η
3. Exclude reconstructed jets
Recalculate the background energy
Study of jet quenching using the CMS detector
43
Background Subtraction
φ
φ
η
η
1. Background energy per tower calculated
in strips of η. Pedestal subtraction
2. Run anti kT algorithm on background
subtracted towers
φ
φ
η
Background level
Yen-Jie Lee (CERN)
3. Exclude reconstructed jets
Recalculate the background energy
η
4. Run anti kT algorithm on background
subtracted towers to get final jets
Study of jet quenching using the CMS detector
44
Summary of Jet Reconstruction
correction
Raw jet energy
Background
subtraction
Remove underlying
events contribution
Yen-Jie Lee (CERN)
Jet energy
correction
Jet energy
MC Simulation
PYTHIA
Study of jet quenching using the CMS detector
45
Flavor
Candidate(pp
(pp@
@77TeV)
TeV)
Flavor Creation
Creation Candidate
Reconstructed secondary
vertices from b and c quarks
Yen-Jie Lee (CERN)
Study of jet quenching using the CMS detector
46
-jet correlations
RJ = fraction of photons
with jet partner >30 GeV/c
xJ=pTjet/pT
PbPb
Pb
Pb
No -decorrelation
Increasing pT-imbalance
Less jet partners
above threshold
~20% of photons
lose their jet partner
Jets lose ~14% of
their initial energy
PLB 718 (2013) 773
Yen-Jie Lee (CERN)
Study of jet quenching using the CMS detector
47
Path length dependence of jet energy loss?

Participant plane
pp
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):
EP
→ v2 is sensitive to the
path-length dependence
of the energy loss
v2
L3
L2
pT
Yen-Jie Lee (CERN)
Study of jet quenching using the CMS detector
48
Jet and high pT track v2 at the LHC
Jet v2
High pT track v2
PRL 109 (2012) 022301
• Jet and high pT track v2 : non-zero up to very high pT
• Sensitive to the path length dependence of energy loss
Yen-Jie Lee (CERN)
Study of jet quenching using the CMS detector
49
pPb run
Successful 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
Two particle correlation function
PLB 718 (2013) 795
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?
Yen-Jie Lee (CERN)
5x larger than pp!
Elliptic flow?
Color glass condensate?
Modified jet structure?
pp ridge paper: JHEP 1009 (2010) 091
Study of jet quenching using the CMS detector
50
Photon-jet momentum balance
Compare photon-jet momentum
balance
Xjγ = pTJet/pTphoton
PbPb
in vacuum (pp collision) to the QGP
(PbPb collision)
PYTHIA
Xjγ
In addition, 20% of photons
lose their jet partner
(jet pT> 30 GeV/c)
R=0.3
Jets lose about
15% of their initial
energy
PbPb
PbPb
PLB 718 (2013) 773
Yen-Jie Lee (CERN)
Study of jet quenching using the CMS detector
51
Tracking efficiency
Yen-Jie Lee (CERN)
Study of jet quenching using the CMS detector
52
Jet resolution and energy scale
Yen-Jie Lee (CERN)
Study of jet quenching using the CMS detector
53
Subtracted background
Yen-Jie Lee (CERN)
Study of jet quenching using the CMS detector
54
Subtracted background
Yen-Jie Lee (CERN)
Study of jet quenching using the CMS detector
55
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, crosscheck 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
Yen-Jie Lee (CERN)
Study of jet quenching using the CMS detector
56
Selection
of b-jet
Results
CMS
Selection
ofresults
b-jet
Results
from
CMS
b-jet
fromfrom
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
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
Yen-Jie Lee (CERN)
Study of jet quenching using the CMS detector
57
b-jet identification
Identification
b-jet
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%)
Yen-Jie Lee (CERN)
Study of jet quenching using the CMS detector
58
Bottom production
Production
bottom
Flavor Creation (FCR)
Flavor Excitation (FEX)
 LO b-b production (FCR) not
dominant at the LHC
arXiv:0705.1937
pp @ 14 TeV
Gluon Splitting (GSP)
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
Yen-Jie Lee (CERN)
Study of jet quenching using the CMS detector
59
Gluon
Candidate(pp
(pp@@7 7
TeV)
gluon Splitting
splitting candidate
TeV)
Yen-Jie Lee (CERN)
Study of jet quenching using the CMS detector
60
How do we extract the medium effect in PbPb collisions?
One typical way is to compare PbPb data to pp reference measurement
PbPb measurements
pp reference
Npart  Number of participating nucleons
Ncoll  Number of binary scatterings
Example:
Yen-Jie Lee (CERN)
Npart = 2
Ncoll = 1
Npart = 5
Ncoll = 6
Study of jet quenching using the CMS detector
61
How do we extract the medium effect in PbPb collisions?
One typical way is to compare PbPb data to pp reference measurement
pp reference
PbPb measurements
‘Nuclear modification factors’
2
σ inel
d
N AA / dpT dη
pp
RAA =
N coll d 2σ pp / dpT dη
“QCD Medium”
RAA > 1 (enhancement)
RAA = 1 (no medium effect)
~
“QCD Vacuum”
RAA < 1 (suppression)
Ncoll Averaged number of binary scattering
Can also be
written as 1/TAA
Yen-Jie Lee (CERN)
N coll
TAA = inel
σ pp
''NN equivalent integrated luminosity
per AA collision'‘
Reduces the uncertainty from pp inclusive cross-section
Study of jet quenching using the CMS detector
62
RHIC Lesson : Jet Quenching
Jet-quenching
(Bjorken, 1982)
Yen-Jie Lee (CERN)
Study of jet quenching using the CMS detector
63
RHIC Lesson : Jet Quenching
4< pTtrig < 6 GeV/c
pTassoc > 2 GeV/c
Jet-quenching
(Bjorken, 1982)
Yen-Jie Lee (CERN)
Indication of jet quenching:
Suppression of high pT particles
Study of jet quenching using the CMS detector
64
RHIC Lesson : Jet Quenching
Collisional
energy loss
Radiative
energy loss
Parton energy loss models:
Possible modification of jet
fragmentation and broadening
of the dijet ΔΦ distribution
Difficulty at RHIC:
High-pT probes are rare and direct jet reconstruction is very hard
Yen-Jie Lee (CERN)
Study of jet quenching using the CMS detector
65
Jet reconstruction
Need rules to group the hadrons
A popular algorithm is anti-kT algorithm
Used in ALICE, ATLAS and CMS analyses
Radius parameter:
decide the resolution scale
Large radius parameter
Small radius parameter
 jet spliting
Cacciari, Salam, Soyez, JHEP 0804 (2008) 063
ΔR = 0.2, 0.3, 0.4, 0.5 are used in LHC analyses
Yen-Jie Lee (CERN)
Study of jet quenching using the CMS detector
66
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, crosscheck 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
Yen-Jie Lee (CERN)
Study of jet quenching using the CMS detector
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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
Anti-kT jet R = 0.3
PLB 712 (2012) 176
• Fake away side jet rate is < 4%
Yen-Jie Lee (CERN)
Study of jet quenching using the CMS detector
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Colorless Probes in Heavy Ion Collisions
Isolated photon
Zμ+μγ/W/Z
μ
QGP
Wμυ
Yen-Jie Lee (CERN)
Study of jet quenching using the CMS detector
69
(Non-) Suppression of Colorless Probes
σ inel
d 2 N AA / dp T dη
pp
R AA=
⟨ N coll ⟩ d 2 σ pp / dp T dη
Isolated photon
PLB 710 (2012) 256
Z0
PRL 106 (2011) 212301
CMS-PAS HIN-12-008
m+m-
If PbPb = superposition of pp ...
W mn using
single muon recoil
against missing pT
Ncoll scaling confirmed
in PbPb collisions at 2.76 TeV
Yen-Jie Lee (CERN)
Study of jet quenching using the CMS detector
arXiv:1205.6334
PLBarXiv:1205.6334
715 (2012) 66
70
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Study of jet quenching using the CMS detector
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Yen-Jie Lee (CERN)
Study of jet quenching using the CMS detector
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Where Does the Energy Go?
Photon RAA and RCP
• 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?
Yen-Jie Lee (CERN)
Study of jet quenching using the CMS detector
73
Missing-pT||
Missing pT||:
0-30% Central PbPb
Sum over all tracks in the event
Calculate projection of pT on
leading jet axis and average
over selected tracks with
pT > 0.5 GeV/c and
|η| < 2.4
pTTrack
p
Track ||
arXiv:1102.1957
T [nucl-ex]
ΔΦ
Underlying events
cancels
unbalanced
jets
balanced jets
Yen-Jie Lee (CERN)
Study of jet quenching using the CMS detector
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Missing-pT||
Missing pT||:
Track
pT > 0.5 GeV/c
0-30% Central PbPb
excess away from
leading jet
Balanced!!
excess towards
leading jet
balanced jets
pTTrack
ΔΦ
pTTrack ||
unbalanced jets
Integrating over the whole event final state
the dijet momentum balance is restored
Yen-Jie Lee (CERN)
Study of jet quenching using the CMS detector
75
Missing-pT||
Missing pT||:
Out of the jet cones
Excess towards sub-leading jet
0-30% Central PbPb
R=0.5
balanced jets
Inside the jet cones
Excess towards leading jet
unbalanced jets
All tracks
Tracks in
the jet cone
ΔR<0.8
Tracks out of
the jet cone
ΔR>0.8
The momentum difference in the dijet is
balanced by low pT particles outside the jet cone
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Study of jet quenching using the CMS detector
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
To Go Beyond Qualitative Observation
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
Used to derive correction or to compare with data
Generator
Experiment
Feedback and improve the 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
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Study of jet quenching using the CMS detector
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Impact of Jet Energy Smearing
Dijet pT ratio
Generator level jets from PYTHIA
Generator level leading and
subleading jets matches reco level
Anti-kT jet R = 0.3
0-20%
Jet energy smearing
Smearing function from
PLB 718 (2013) 773
Yen-Jie Lee (CERN)
Subleading jet is replaced by third jet
Swapped leading and subleading jet
Study of jet quenching using the CMS detector
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