1. Study of Jet Quenching using the CMS Detector
05/11/11
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
Study of jet quenching using the CMS detector

2. Study of jet quenching using the CMS detector
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
Study of jet quenching using the CMS detector

3. Study of jet quenching using the CMS detector
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
Study of jet quenching using the CMS detector

4. Probe the Medium  “Jet quenching” (Bjorken, 1982)
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
QGP
Jet
QGP
In medium parton energy loss
 “Jet quenching”
Jet
(Bjorken, 1982)
Study of jet quenching using the CMS detector

5. EM and Hadron calorimeters
CMS Detector
05/11/11
EM and Hadron calorimeters
photons, isolation Pb
Inner tracker:
charged particles
vertex, isolation
solenoid Pb
Muon
HCAL
ECAL
Tracker
|η|< 2.5
|η|< 3.0
|η|< 5.2
|η|< 2.4
Calojet
Particle Flow Jet (track pT> 0.9GeV/c)
Study of jet quenching using the CMS detector

6. Heavy Ion Collision Recorded by the CMS Detector
05/11/11
2010 PbPb μb-1
PbPb 150 μb-1
2012 pPb μb-1
2013 pPb nb-1
Study of jet quenching using the CMS detector

7. “Jet Quenching” without jet: Charged Particle RAA
05/11/11
If PbPb = superposition of pp ...
Ncoll validate by photons
W/Z bosons
EPJC 72 (2012) 1945
Provide constraints on the parton energy loss models
Study of jet quenching using the CMS detector

8. 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
Study of jet quenching using the CMS detector

9. Direct jet reconstruction with CMS
Dijet Event Recorded by CMS
05/11/11
Direct jet reconstruction with CMS
Leading Jet
pT1
Subleading Jet
pT2
Study of jet quenching using the CMS detector

10. To Explain the Suppression of High pT Particles
Large angle soft radiation
“QGP heating”
Soft collinear radiation
Hard radiation
GLV + others
(pre-LHC models)
PYTHIA inspired models
Modified splitting functions
AdS/CFT
Interference
Do we see strong suppression of high pT jets?
Can we collect the radiated energy back?
Study of jet quenching using the CMS detector

11. Inclusive Jet Spectra: Jet RAA
Compare PbPb to pp data
Anti-kT jets with R = 0.3
0.5
If PbPb = superposition of pp
CMS PAS HIN
Detector effects unfolded
Strong suppression of inclusive high pT jets
Study of jet quenching using the CMS detector

12. 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
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?
Study of jet quenching using the CMS detector

13. Dijet and Photon-Jet Energy Imbalance
High pT photon
triggered sample
Photon  unmodified jet energy tag
High pT leading jet
triggered sample
High statistics, with surface bias
Lower statistics, without surface bias
Study of jet quenching using the CMS detector

14. Dijet Momentum Imbalance
05/11/11
Jet Cone size R = 0.5 pp
Small AJ
(Balanced dijet)
Large AJ
(Un-balanced dijet)
PRC 84 (2011)
Parton energy loss is observed as a pronounced energy imbalance in central PbPb collisions
Study of jet quenching using the CMS detector

15. Dijet Energy Ratio (imbalance)
Anti-kT jet R = 0.3
Dijet pT ratio as a function of leading jet pT
PLB 712 (2012) 176
Study of jet quenching using the CMS detector

16. Dijet Energy Ratio (imbalance)
Anti-kT jet R = 0.3
Energy imbalance increases with centrality
Very high pT jets are also quenched
PLB 712 (2012) 176
Study of jet quenching using the CMS detector

17. -jet Momentum ImbalancePb
Anti-kT jet R = 0.3
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
Study of jet quenching using the CMS detector

18. Dijet and photon-jet azimuthal correlation
Given the large momentum imbalance seen in dijet and photon-jet events
Is the azimuthal correlation modified?
Jet Δφ
QGP
Jet
In medium parton energy loss
 “Jet quenching”
Study of jet quenching using the CMS detector

19. Dijet Azimuthal Angle Correlations
0-20% Δφ
No apparent modification in the dijet Δφ distribution
for different jet pT (still back-to-back)
Jet Cone size R = 0.5
PLB 712 (2012) 176
Study of jet quenching using the CMS detector

20. 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 pp pp
Photon
“Backscattering?”
Jet
Azimuthal angle difference between photon and jet
PLB 718 (2013) 773
Study of jet quenching using the CMS detector

21. 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)
Jet fragmentation function: how transverse momentum is distributed inside the jet cone
Study of jet quenching using the CMS detector

22. Differential Jet ShapePb
CMS PAS HIN
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
Study of jet quenching using the CMS detector

23. Jet Fragmentation FunctionsPb Pb
CMS PAS HIN
High pT
particles
Low pT
particles
Inside the jet cone: Enhancement of low pT particle
Suppression of intermediate pT particles in cone
Study of jet quenching using the CMS detector

24. Track pT Distributions in Jet Cones (R=0.3)Pb Pb
(1/GeV)
CMS PAS HIN
High pT : no change compared to jets in pp collisions
In (central) PbPb: excess of 1-2 tracks compared to pp at low pT
Study of jet quenching using the CMS detector

25. What have we learned so far? (1)
What Have We Learned with CMS PbPb Data?
05/11/11
What have we learned so far? (1)
1. High pT jet suppression
ΔR = 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)
Study of jet quenching using the CMS detector

26. 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
Study of jet quenching using the CMS detector

27. Coming Back to the Three Scenarios
Large angle soft radiation
“QGP heating”
Soft collinear radiation
Hard radiation
GLV + others
(pre-LHC models)
PYTHIA inspired models
Modified splitting functions
AdS/CFT
Interference
Study of jet quenching using the CMS detector

28. Study of jet quenching using the CMS detector
Summary
05/11/11
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
Study of jet quenching using the CMS detector

29. Study of jet quenching using the CMS detector
Backup slides
05/11/11
Study of jet quenching using the CMS detector

30. Study of jet quenching using the CMS detector
Plan
05/11/11
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
Study of jet quenching using the CMS detector

31. Systematic uncertainties considered in analysis
X  negligible/small effect, *  important systematics, ** dominant systematics
Study of jet quenching using the CMS detector

32. How do we extract the medium effect in PbPb collisions?
05/11/11
One typical way is to compare PbPb data to pp reference measurement
PbPb measurements
pp reference
‘Nuclear modification factors’
RAA > 1 (enhancement)
“QCD Medium”
RAA = 1 (no medium effect) ~
“QCD Vacuum”
RAA < 1 (suppression)
<Ncoll> Averaged number of binary scattering
Study of jet quenching using the CMS detector

33. Background Subtractionπ
η reflection Method
Bkg
Jet
Main result
Exclude φ
Jet Event -π
-2.0 η
2.0
Event Mixing Method
(Cross-checks) π π
Jet
Bkg φ φ
Jet Event
MinBias Event -π -π
-2.0 η
2.0
-2.0 η
2.0
Study of jet quenching using the CMS detector

34. 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
Study of jet quenching using the CMS detector

35. 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
Study of jet quenching using the CMS detector

36. Jet Reconstruction and Composition
Towers
Jet
Δη x Δϕ
0.076 x in barrel
Background subtraction and
jet clustering
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
Study of jet quenching using the CMS detector

37. Underlying Event Background
Jet
Multiple parton interaction
Large underlying event from soft scattering
Need background subtraction
Study of jet quenching using the CMS detector

38. Background Subtractionφ
Study of jet quenching using the CMS detector

39. 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
Study of jet quenching using the CMS detector

40. Background Subtractionφ φ η η
1. Background energy per tower calculated in strips of η. Pedestal subtraction
Background level
Study of jet quenching using the CMS detector

41. Background Subtractionφ φ η η
1. Background energy per tower calculated in strips of η. Pedestal subtraction
2. Run anti kT algorithm on background subtracted towers
Background level
Study of jet quenching using the CMS detector

42. Background Subtractionφ 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
Background level
3. Exclude reconstructed jets
Study of jet quenching using the CMS detector

43. Background Subtractionφ 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 η
Background level
3. Exclude reconstructed jets
Recalculate the background energy
Study of jet quenching using the CMS detector

44. Background Subtractionφ 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 η
Background level
3. Exclude reconstructed jets
Recalculate the background energy
4. Run anti kT algorithm on background subtracted towers to get final jets
Study of jet quenching using the CMS detector

45. Summary of Jet Reconstruction
correction
Raw jet energy
Background subtraction
Jet energy correction
Jet energy
Remove underlying events contribution
MC Simulation PYTHIA
Study of jet quenching using the CMS detector

46. Flavor Creation Candidate (pp @ 7 TeV)
Reconstructed secondary vertices from b and c quarks
Study of jet quenching using the CMS detector

47. -jet correlations xJ=pTjet/pT Less jet partners above threshold
RJ = fraction of photons with jet partner >30 GeV/c
xJ=pTjet/pT Pb Pb
Less jet partners above threshold
No -decorrelation
Increasing pT-imbalance
~20% of photons lose their jet partner
Jets lose ~14% of their initial energy
PLB 718 (2013) 773
Study of jet quenching using the CMS detector

48. 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):
→ v2 is sensitive to the path-length dependence of the energy loss EP v2 L3 L2 pT
Study of jet quenching using the CMS detector

49. Jet and high pT track v2 at the LHC
Jet v2
High pT track v2
PRL 109 (2012)
Jet and high pT track v2 : non-zero up to very high pT
Sensitive to the path length dependence of energy loss
Study of jet quenching using the CMS detector

50. Study of jet quenching using the CMS detector
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
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?
Two particle correlation function
PLB 718 (2013) 795
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

51. Photon-jet momentum balance
Compare photon-jet momentum balance
Xjγ = pTJet/pTphoton
in vacuum (pp collision) to the QGP (PbPb collision)
PbPb
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
Study of jet quenching using the CMS detector

52. Study of jet quenching using the CMS detector
Tracking efficiency
Study of jet quenching using the CMS detector

53. Jet resolution and energy scale
Study of jet quenching using the CMS detector

54. Subtracted background
Study of jet quenching using the CMS detector

55. Subtracted background
Study of jet quenching using the CMS detector

56. 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
Study of jet quenching using the CMS detector

57. Selection of b-jet Results from CMS
Identification of b-quark jets with the CMS experiment CMS Physics Analysis Summary BTV Inclusive b-jet production in pp collisions at √s = 7 TeV JHEP 1204 (2012) 084, arXiv: Measurement of BB Angular Correlations based on Secondary Vertex Reconstruction at √s = 7 TeV JHEP 1103 (2011) 136, arXiv: 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
Study of jet quenching using the CMS detector

58. Study of jet quenching using the CMS detector
b-jet Identification
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:
Reconstructed SV’s, employing discriminating variables such as SV mass, flight distance, etc.
Impact parameter (IP) of tracks associated to the jet, w/o requiring a reco’d SV
Muons in jets, exploiting the large branching ratio (20%)
Study of jet quenching using the CMS detector

59. Study of jet quenching using the CMS detector
Bottom Production
bottom production
Flavor Creation (FCR)
Flavor Excitation (FEX)
Gluon Splitting (GSP)
LO b-b production (FCR) not dominant at the LHC
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
arXiv:
14 TeV
Study of jet quenching using the CMS detector

60. Gluon Splitting Candidate (pp @ 7 TeV)
Study of jet quenching using the CMS detector

61. Npart  Number of participating nucleons
How do we extract the medium effect in PbPb collisions?
05/11/11
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:
Npart = Ncoll = 1
Npart = Ncoll = 6
Study of jet quenching using the CMS detector

62. How do we extract the medium effect in PbPb collisions?
05/11/11
One typical way is to compare PbPb data to pp reference measurement
PbPb measurements
pp reference
‘Nuclear modification factors’
RAA > 1 (enhancement)
“QCD Medium”
RAA = 1 (no medium effect) ~
“QCD Vacuum”
RAA < 1 (suppression)
Ncoll Averaged number of binary scattering
Can also be written as 1/TAA
''NN equivalent integrated luminosity per AA collision'‘ Reduces the uncertainty from pp inclusive cross-section
Study of jet quenching using the CMS detector

63. RHIC Lesson : Jet Quenching
05/11/11
Two particle correlation
Away side
Jet-quenching (Bjorken, 1982)
Study of jet quenching using the CMS detector

64. RHIC Lesson : Jet Quenching
05/11/11
4< pTtrig < 6 GeV/c
pTassoc > 2 GeV/c
Two particle correlation
Away side
Jet-quenching (Bjorken, 1982)
Indication of jet quenching: Suppression of high pT particles
Study of jet quenching using the CMS detector

65. RHIC Lesson : Jet Quenching
05/11/11
Collisional
energy loss
Radiative
energy loss
Parton energy loss models:
Possible modification of jet fragmentation and broadening of the dijet ΔΦ distribution
High pT
rare
Difficulty at RHIC:
High-pT probes are rare and direct jet reconstruction is very hard
Study of jet quenching using the CMS detector

66. Jet reconstruction Radius parameter: 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
Small radius parameter  jet spliting
Large radius parameter
Cacciari, Salam, Soyez, JHEP 0804 (2008) 063
ΔR = 0.2, 0.3, 0.4, 0.5 are used in LHC analyses
Study of jet quenching using the CMS detector

67. 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
Study of jet quenching using the CMS detector

68. 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%
Study of jet quenching using the CMS detector

69. Colorless Probes in Heavy Ion Collisions
Isolated photon
Zμ+μ-
γ/W/Z μ
QGP
Wμυ
Study of jet quenching using the CMS detector

70. (Non-) Suppression of Colorless Probes
Isolated photon
PRL 106 (2011)
CMS-PAS HIN
PLB 710 (2012) 256
Z0 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
arXiv:
PLB 715 (2012) 66
arXiv:
Study of jet quenching using the CMS detector

71. Study of jet quenching using the CMS detector

72. Study of jet quenching using the CMS detector

73. Photon RAA and RCP Where Does the Energy Go? Where does the energy go?
05/11/11
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?
Study of jet quenching using the CMS detector

74. Study of jet quenching using the CMS detector
Missing-pT||
05/11/11
Missing pT||:
Sum over all tracks in the event
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
Underlying events cancels
pTTrack
pTTrack ||
arXiv: [nucl-ex] ΔΦ
balanced jets
unbalanced jets
Study of jet quenching using the CMS detector 74

75. Missing-pT|| Missing pT||:
05/11/11
Missing pT||:
0-30% Central PbPb
Track pT > 0.5 GeV/c
excess away from leading jet
Balanced!!
excess towards leading jet
pTTrack
pTTrack || ΔΦ
balanced jets
unbalanced jets
Integrating over the whole event final state
the dijet momentum balance is restored
Study of jet quenching using the CMS detector 75

76. Missing-pT|| Missing pT||: The momentum difference in the dijet is
05/11/11
Missing pT||:
Out of the jet cones
Excess towards sub-leading jet
0-30% Central PbPb
R=0.5
Inside the jet cones Excess towards leading jet
balanced jets
unbalanced jets
Tracks in the jet cone
ΔR<0.8
Tracks out of the jet cone
ΔR>0.8
All tracks
The momentum difference in the dijet is
balanced by low pT particles outside the jet cone
Study of jet quenching using the CMS detector 76

77. To Go Beyond Qualitative Observation Summary
05/11/11
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
Used to derive correction or to compare with data
Generator
Experiment
Feedback and improve the generator
Study of jet quenching using the CMS detector

78. 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
Subleading jet is replaced by third jet
Swapped leading and subleading jet
Study of jet quenching using the CMS detector