1 / 78

Yen-Jie Lee (CERN) for the CMS Collaboration Rencontres Ion Lourds/Heavy Ion Meeting

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. Probe the Medium. Final g oal: Un derstand the thermodynamics and transport properties of QGP

glenda
Download Presentation

Yen-Jie Lee (CERN) for the CMS Collaboration Rencontres Ion Lourds/Heavy Ion Meeting

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. 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

  2. Probe the Medium Study of jet quenching using the CMS detector • Final goal: Understand the thermodynamics and transportproperties 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

  3. Probe the Medium Study of jet quenching using the CMS detector • Final goal: Understand the thermodynamics and transportproperties 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

  4. Probe the Medium Study of jet quenching using the CMS detector • Final goal: Understand the thermodynamics and transportproperties 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)

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

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

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

  8. Charged Particle Spectra Study of jet quenching using the CMS detector Absorption? Energy loss? Single hadron spectra themselves do not provide details of the underlying mechanism  Need direct jet reconstruction and correlation studies

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

  10. To Explain the Suppression of High pT Particles Study of jet quenching using the CMS detector Large angle soft radiation “QGP heating” Hard radiation Soft collinear 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?

  11. Inclusive Jet Spectra: Jet RAA Study of jet quenching using the CMS detector 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

  12. Inclusive Jet Spectra: Jet RAA Study of jet quenching using the CMS detector 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 “completelyabsorbed” by the medium?

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

  14. Study of jet quenching using the CMS detector Dijet Momentum Imbalance Jet Cone size R = 0.5 pp Large AJ (Un-balanced dijet) Small AJ (Balanced dijet) PRC 84 (2011) 024906 Parton energy loss is observed as apronounced energy imbalance in central PbPb collisions

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

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

  17. -jet Momentum Imbalance Study of jet quenching using the CMS detector Pb Pb Pb 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 directmeasure of the jet energy loss • Gradual centrality-dependence of the xJ distribution

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

  19. Dijet Azimuthal Angle Correlations Study of jet quenching using the CMS detector 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

  20. Photon-jet Azimuthal Angular Correlation Study of jet quenching using the CMS detector 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

  21. Jet Shape and Fragmentation Function Study of jet quenching using the CMS detector 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

  22. Differential Jet Shape Study of jet quenching using the CMS detector Pb Pb Pb 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

  23. Jet Fragmentation Functions Study of jet quenching using the CMS detector Pb Pb 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

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

  25. What have we learned so far? (1) What Have We Learned with CMS PbPb Data? 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 andphoton-jet pTimbalance 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 Study of jet quenching using the CMS detector 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

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

  28. Summary Summary Study of jet quenching using the CMS detector • 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

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

  30. Plan Summary Study of jet quenching using the CMS detector • 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

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

  32. Study of jet quenching using the CMS detector 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’ RAA > 1 (enhancement) “QCD Medium” RAA = 1 (no medium effect) ~ “QCD Vacuum” RAA < 1 (suppression) <Ncoll> Averaged number of binary scattering

  33. Background Subtraction Study of jet quenching using the CMS detector 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

  34. Tagging and Counting b-quark Jets Study of jet quenching using the CMS detector Test the theoretical prediction color factor and quark-mass dependence of in-medium parton energy loss Secondary vertex tagged usingflight 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

  35. Fraction of b-jets among All Jets Study of jet quenching using the CMS detector • 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

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

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

  38. Background Subtraction Study of jet quenching using the CMS detector φ

  39. Background Subtraction Study of jet quenching using the CMS detector 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

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

  41. Background Subtraction Study of jet quenching using the CMS detector 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

  42. Background Subtraction Study of jet quenching using the CMS detector 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

  43. Background Subtraction Study of jet quenching using the CMS detector 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

  44. Background Subtraction Study of jet quenching using the CMS detector 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

  45. Summary of Jet Reconstruction Study of jet quenching using the CMS detector correction Raw jet energy Backgroundsubtraction Jet energy correction Jet energy Remove underlying events contribution MC SimulationPYTHIA

  46. Study of jet quenching using the CMS detector Flavor Creation Candidate (pp @ 7 TeV) Flavor Creation Candidate (pp @ 7 TeV) Reconstructed secondaryvertices from b and cquarks

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

  48. Path length dependence of jet energy loss? Study of jet quenching using the CMS detector  pp Participant plane 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

  49. Jet and high pT trackv2 at the LHC Study of jet quenching using the CMS detector 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

  50. pPb run Study of jet quenching using the CMS detector 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 

More Related