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Hard Probes

Hard Probes with focus on LHC data. γ. jet. Yen-Jie Lee (CERN). Probe the medium. G oal: Un derstand the property of QGP

erica-shannon
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Hard Probes

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  1. Hard Probes with focus on LHC data γ jet Yen-Jie Lee (CERN) Quark Matter 2012 - Hard Probes

  2. Probe the medium Quark Matter 2012 - Hard Probes • Goal: Understand the property of QGP • Problem: the lifetime of QGP is so short (O(fm/c)) such that it is not feasible to probe it with an external source. • Solution: Take the advantage of the large cross-sections of high pT jets, γ/W/Z, quarkonia at the LHC energy, use hard probes produced with the collision. Material External source

  3. Three types of hard probes Quark Matter 2012 - Hard Probes C C Electroweak probes W/Z bosons, high pTγ Quarkonium J/ψ, Υ family Quarks and gluons Jets QGP QGP QGP Jet γ Sensitive to the temperature of QGP Probe the initial state Probe the opacity of QGP

  4. Factorization Quark Matter 2012 - Hard Probes proton proton

  5. Factorization Quark Matter 2012 - Hard Probes Parton Distribution Function (PDF) proton proton

  6. Factorization Quark Matter 2012 - Hard Probes Cross-section of 22 process Parton Distribution Function (PDF) Gluon proton Quark Quark proton

  7. Factorization Quark Matter 2012 - Hard Probes Nuclear Parton Distribution Function (nPDF) Cross-section of 22 process Gluon Quark Quark

  8. Quark Matter 2012 - Hard Probes 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

  9. Quark Matter 2012 - Hard Probes 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 Npart Number of participating nucleons Ncoll Number of binary scatterings Example: Npart = 2 Ncoll = 1 Npart = 5 Ncoll = 6

  10. Quark Matter 2012 - Hard Probes 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 Can also be written as 1/TAA ''NN equivalent integrated luminosity per AA collision'‘Reduces the uncertainty from pp inclusive cross-section

  11. Quark Matter 2012 - Hard Probes 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 Questions: How do we know the Glauber model calculation of Ncoll is correct? Is the nuclear PDF modified with respect to nucleon PDF? Motivates the studies of electroweak probes

  12. Electroweak probes Quark Matter 2012 - Hard Probes • High pT Photons, W and Z bosons: • Colorless Not affected by the QGP • Good theoretical control • Check the validity of Ncoll calculation (ex. from Glauber Model) • Constraint the nuclear parton distribution function (nPDF) nucl-ex/0701025v1 ArXiv:1010.5392 Z bosons Photons PbPb 5.5 TeV ArXiv:1103.1471

  13. Quark Matter 2012 - Hard Probes Photons NLO LO • Ideally: LO photons from hard scattering • Real world: huge background from the decay and fragmentation photons • Need a consistent definition between measurements and theoretical calculations

  14. Quark Matter 2012 - Hard Probes Isolated high pT photons • Solution: measurement of the isolated photons • Decay photons from hadrons in jets such as π0, η γ γ are largely suppressed • UE subtracted isolation variables are developed NLO LO γ γ Isolated γ Non-isolated same object to the detector Isolated Isolated Non-isolated

  15. Quark Matter 2012 - Hard Probes Isolated photon RAA 0-10% PbPb compared to pp Theory • No modification of the photons as expected!

  16. Z boson production in PbPb collisions Quark Matter 2012 - Hard Probes Zμ+μ- Ze+e-

  17. Quark Matter 2012 - Hard Probes Z boson production in PbPb collisions 2010 data 2011 data • Normalized yield is not varying as a function of centrality • No modification is found with respect to the pp reference

  18. W boson Quark Matter 2012 - Hard Probes Wμυ Single high pTμ + Missing pT μ μ W φ υ Wμυ Transverse mass Transverse mass

  19. W boson RAA Quark Matter 2012 - Hard Probes RAA(W) = 1.04 ± 0.07 ± 0.12 • Normalized yield is not varying as a function of centrality

  20. W boson RAA Quark Matter 2012 - Hard Probes RAA(W) = 1.04 ± 0.07 ± 0.12 RAA(W+) = 0.82 ± 0.07 ± 0.09 RAA(W–) = 1.46 ± 0.14 ± 0.16 • Isospin effect is seen if we differentiate W+ and W-

  21. Summary of electroweak probes Quark Matter 2012 - Hard Probes • Electroweak probes are unmodified • Confirmed Ncoll scaling of hard scattering • Constraint nuclear PartonDistribution Function pp PDF Ncoll scaling PbPb nPDF

  22. How about quarks and gluons? Quark Matter 2012 - Hard Probes • Quarks and gluons in pp collisions Gluon Quark

  23. How about quarks and gluons? Quark Matter 2012 - Hard Probes • Want to measure quarks and gluons which carrycolor charge and see how they interact with QGP Gluon Quark

  24. Quarks and gluons Quark Matter 2012 - Hard Probes Color confinement: Quarks and gluons  groups of hadrons

  25. How about quarks and gluons? Quark Matter 2012 - Hard Probes • Want to measure quarks and gluons which carrycolor charge and see how they interact with QGP Gluon Quark

  26. How about out going quarks and gluons? Quark Matter 2012 - Hard Probes • Want to measure quarks and gluons which carrycolor charge and see how they interact with QGP •  Practically: measure hadrons and jets Hadrons “Fragmentation” Jet Gluon Quark “Fragmentation” Jet Hadrons

  27. Quark Matter 2012 - Hard Probes An easier measurement: charged particle RAA If PbPb = superposition of pp ... Ncoll validate by photons W/Z bosons Provide constraints on the parton energy loss models

  28. Charged particle spectra Quark Matter 2012 - Hard Probes Absorption? Energy loss? Single hadron spectra itself do not provide details of the underlying mechanism  Need direct jet reconstruction and correlation studies

  29. Jet events in PbPb collisions at LHC Quark Matter 2012 - Hard Probes ATLAS CMS

  30. Jet reconstruction Quark Matter 2012 - Hard Probes Radius parameter: decide the resolution scale Need rules to group the hadrons A popular algorithm is anti-kT algorithmUsed in ALICE, ATLAS and CMS analyses 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

  31. Jet composition Quark Matter 2012 - Hard Probes 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 Goal: • Make use of the redundancy of measurements from calorimeter and tracker • Improve the sensitivity to low pTparticles in jet Reduce the dependence on MC (ex: PYTHIA)

  32. Underlying event background Quark Matter 2012 - Hard Probes ATLAS Jet Multiple parton interaction Large underlying event from soft scattering Need background subtraction

  33. Summary of jet reconstruction Quark Matter 2012 - Hard Probes correction Raw jet energy Backgroundsubtraction Jet energy correction Jet energy Remove underlying events contribution MC SimulationPYTHIA

  34. Three possible scenarios Quark Matter 2012 - Hard Probes To explain the suppression of high pT particles Soft collinear radiation Hard radiation Large angle soft radiation “QGP heating” PYTHIA inspired models Modified splitting functions GLV + others AdS/CFT

  35. Quark Matter 2012 - Hard Probes ? Jet fragmentation function

  36. Quark Matter 2012 - Hard Probes ? Jet fragmentation function Select Tracks in R=0.3 cone pT> 4 GeV/c Fragmentation pattern of the “hard part” in PbPb collision is consistent with pp Justify the use of PYTHIA for jet energy correction

  37. Inclusive jet RAA, RCP Quark Matter 2012 - Hard Probes RCP: Compare to Compare PbPb to PYTHIA (pp generator) Track Jet Calorimeter Jet Strong suppression of inclusive high pT jets! A cone of R=0.3, 0.4 doesn’t catch all the radiated energy

  38. Quark Matter 2012 - Hard Probes Large AJ (Un-balanced dijet) Small AJ (Balanced dijet) Correlation study: Di-jet imbalance

  39. Quark Matter 2012 - Hard Probes Correlation study: Di-jet imbalance 40-100% 20-40% 10-20% 0-10% Balanced dijet Asymmetric dijet pp reference Large AJ (Un-balanced dijet) Small AJ (Balanced dijet) π π π π Δφ Parton energy loss is observed as apronounced energy imbalance in central PbPb collisions No apparent modification in the dijet Δφ distribution(Dijet pairs are still pretty back-to-back in azimuthal angle)

  40. Quark Matter 2012 - Hard Probes Correlation study: Di-jet imbalance 40-100% 20-40% 10-20% 0-10% Δφ Large AJ (Un-balanced dijet) Small AJ (Balanced dijet) π π π π Δφ Δφ Δφ Δφ Parton energy loss is observed as apronounced energy imbalance in central PbPb collisions No apparent modification in the dijet Δφ distribution(Dijet pairs are stillback-to-back in azimuthal angle)

  41. Leading jet and subleading jet pT ratio pT1 pT2 Theshift in <pT2/pT1> increases monotonically with collision centrality, and is largely independent of the leading jet pT. Quark Matter 2012 - Hard Probes

  42. Photon RAA and RCP Where does the energy go? • Suppression of high pT jets • Large dijet energy(momentum) imbalance Jets lose energy when passing through the medium ΔET ~ O(10) GeV, ~10% shift in <dijet pT ratio> Where does the energy go? Quark Matter 2012 - Hard Probes

  43. Missing-pT|| Quark Matter 2012 - Hard Probes Missing pT||: Calculate projection of pT on leading jet axis and average over selected tracks with pT > 0.5 GeV/c and |η| < 2.4 Underlying events cancels 0-30% Central PbPb Where does the energy go? pTTrack pTTrack || arXiv:1102.1957 [nucl-ex] ΔΦ unbalanced jets balanced jets Sum over all tracks in the event

  44. Missing-pT|| Quark Matter 2012 - Hard Probes Missing pT||: 0-30% Central PbPb 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

  45. Missing-pT|| Quark Matter 2012 - Hard Probes Missing pT||: 0-30% Central PbPb Calculate missing pT in ranges of track pT: excess away from leading jet excess towards leading jet balanced jets unbalanced jets The momentum difference in the dijet is balanced by low pT particles

  46. Missing-pT|| Quark Matter 2012 - Hard Probes Missing pT||: Out of the jet cones Excess towards sub-leading jet 0-30% Central PbPb Inside the jet conesExcess towards leading jet balanced jets unbalanced jets Tracks inthe jet cone ΔR<0.8 Tracks out ofthe jet cone ΔR>0.8 All tracks The momentum difference in the dijet is balanced by low pT particles outside the jet cone

  47. High pT jet in PbPb collisions Quark Matter 2012 - Hard Probes

  48. Low pT jets in PbPb collisions Quark Matter 2012 - Hard Probes Two particle correlation from ALICE: Jet like near side correlation with background subtraction Strong centrality dependence, widening of the angular correlation 2 < pT,trig < 3 1 < pT,assoc < 2 0-10% 60-70% pp Dh Dh Dh Dj pT Dj Dj • Motivates jet shape analysis and fragmentation function with low pT particles • Look at low pT reconstructed jet

  49. Problem of jet as a trigger: surface bias Quark Matter 2012 - Hard Probes • Selection on a high pT leading jet (charged particle) may bias the position of the hard scattering in the QGP High pT leading jet Triggered sample All hard collisions Can happen in any place in the QGP

  50. How about correlate photons and jets? Quark Matter 2012 - Hard Probes pTphoton ~ pTJet Hadrons photon+jet Surface bias is removed! Jet Gluon Quark “quark-gluon compton scattering” Photon

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