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

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

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

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

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

Heavy Ion Collision Recorded by the CMS Detector 05/11/11 2010 PbPb 7 μb-1 PbPb 150 μb-1 2012 pPb 1 μb-1 2013 pPb 31 nb-1 Study of jet quenching using the CMS detector

“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

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

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

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

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 Study of jet quenching using the CMS detector

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? Study of jet quenching using the CMS detector

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

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) 024906 Parton energy loss is observed as a pronounced energy imbalance in central PbPb collisions Study of jet quenching using the CMS detector

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

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

-jet Momentum Imbalance Pb 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

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

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

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

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

Differential Jet Shape Pb CMS PAS HIN-12-013 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

Jet Fragmentation Functions Pb Pb CMS PAS HIN-12-013 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

Track pT Distributions in Jet Cones (R=0.3) Pb Pb (1/GeV) 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 Study of jet quenching using the CMS detector

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 = 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) Study of jet quenching using the CMS detector

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

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

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

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

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

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

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

Background Subtraction π 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

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 Study of jet quenching using the CMS detector

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 Study of jet quenching using the CMS detector

Jet Reconstruction and Composition Towers Jet Δη x Δϕ 0.076 x 0.076 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

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

Background Subtraction φ Study of jet quenching using the CMS detector

Background Subtraction φ 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

Background Subtraction φ 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

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 Background level Study of jet quenching using the CMS detector

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

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

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 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

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

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

-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

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

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 Study of jet quenching using the CMS detector

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

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

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

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

Subtracted background Study of jet quenching using the CMS detector

Subtracted background Study of jet quenching using the CMS detector

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

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 Study of jet quenching using the CMS detector

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

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:0705.1937 pp @ 14 TeV Study of jet quenching using the CMS detector

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

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 = 2 Ncoll = 1 Npart = 5 Ncoll = 6 Study of jet quenching using the CMS detector

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

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

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

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

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

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

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

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

(Non-) Suppression of Colorless Probes Isolated photon PRL 106 (2011) 212301 CMS-PAS HIN-12-008 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:1205.6334 PLB 715 (2012) 66 arXiv:1205.6334 Study of jet quenching using the CMS detector

Study of jet quenching using the CMS detector

Study of jet quenching using the CMS detector

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

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:1102.1957 [nucl-ex] ΔΦ balanced jets unbalanced jets Study of jet quenching using the CMS detector 74

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

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

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

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