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From yesterday

Goa l: set the Jet Energy Scale Different systematics to take into account (tracking,…) Background fluctuations: the challenge. Jet II: Full Jet Reconstruction. From yesterday. Jet III: Results. p+p and d+Au: reference/control measurements Broadening observed at RHIC.

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From yesterday

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  1. Goal: set the Jet Energy Scale • Different systematics to take into account (tracking,…) • Background fluctuations: the challenge Jet II: Full Jet Reconstruction From yesterday Jet III: Results p+p and d+Au: reference/control measurements Broadening observed at RHIC Elena Bruna (Yale&INFN Torino)

  2. Jet III: Results p+p and d+Au: reference/control measurements Broadening observed at RHIC More on PHENIX vs STAR More quenching observables: di-jets, jet-hadron Today Hard Probes at LHC vs RHIC Results on quenching at LHC Jet IV: The Present: from RHIC to LHC Elena Bruna (Yale&INFN Torino)

  3. Jet-finding in PHENIX CAVEAT: jet-finder based on unmodified jet-shapes ⇒ veto against modified/quenched jets “Anti-quenching” biases! Elena Bruna (Yale&INFN Torino)

  4. Jet-finding in PHENIX Step back: how does jet-finding work in PHENIX? Jet-finding based on a Gaussian kernel (σ=0.3, 0.4) Focuses on the core of the jet Direct rejection of fake jets (i.e. jet by jet): May select specific jet fragmentation 1) Sum pT2 inside a Gaussian kernel to obtain a discriminant: 2) Keep jets with g0.1 > threshold Are those jets that keep a Gaussian shape only minimally interacting? That would explain the suppressed RAA to the level of surface emission Elena Bruna (Yale&INFN Torino)

  5. Jets in A+A: possible biases CAVEAT: jet-finder based on unmodified jet-shapes ⇒ veto against modified/quenched jets “Anti-quenching” biases! pT cut to minimize background ⇒ bias towards less-interacting jets Can we exploit the biases? Elena Bruna (Yale&INFN Torino)

  6. Di-jet measurements Trigger jets are biased towards the surface. Recoil jets are exposed to a maximum path-length in the medium. Large energy loss expected. Anti-kT, R=0.4 Trigger Jet: pT,cut=2 GeV/c, pT(trig)>20 GeV/c Trigger jet σ=6.5 GeV/c EMC trigger Coincidence rate: how often I measure a recoil jet once the trigger jet is found Recoil jet Elena Bruna (Yale&INFN Torino)

  7. di-jet coincidence rate Recoil Jet: R=0.4, pT,cut=0.2 GeV/c Trigger Jet: pT,cut=2 GeV/c pT,cut on trigger jet: allows similar trigger jet population in p+p and Au+Au Recoil jets measured per trigger jet  coincidence rate STAR preliminary Significant suppression in di-jet coincidence measurements  broadening and/or absorption? Elena Bruna (Yale&INFN Torino)

  8. Energy shift? Broadening or absorption? p+p Absorption? Au+Au If broadening: Jet energy spread outside R pTJet(R)<pTJet(true)  shift of spectrum towards lower pT If absorption: Jet is so quenched that the jet-finding algorithms do not find it  Jet is lost Or both? Elena Bruna (Yale&INFN Torino)

  9. Broadening scenario possible interpretation of di-jet suppression How much would the Recoil AuAu spectra need to be shifted in order to recover unbiased pp? (simple assumption = constant shift of the spectra, i.e. constant energy loss) AuAu shifted/pp Compatible with a jet pT shift Δ = 7-8 GeV/c Elena Bruna (Yale&INFN Torino)

  10. Energy shift? Broadening or absorption? p+p Absorption? Au+Au • If we were able to measure unambiguously the jet energy (even in presence of quenching) we could measure the Fragmentation Functions (FF) and: • If absorption: • FF(A+A) = FF(p+p) • jets that come out are pp-like jets • If broadening: • FF(A+A) < FF(p+p) •  jets are modified ! Remark: measure of jet energy (background + possible medium effects) is challenging! Elena Bruna (Yale&INFN Torino)

  11. Jet-hadron correlations Towards Fragmentation Functions If tangential (halo) emission: • Away side yield in Au+Au similar to p+p, also for low pT,assoc If energy loss: Decrease of high-pT,assoc particles Strong enhancement of low pT,assoc Broadening Elena Bruna (Yale&INFN Torino)

  12. Jet-hadron correlations Trigger jet: Anti-kt R=0.4, pt,cut>2 GeV/c, ptjet>20 GeV/c pt,assoc>2.5 GeV/c 1<pt,assoc<2.5 GeV/c 0.1<pt,assoc<1 GeV/c STAR Preliminary 0-20% Au+Au STAR Preliminary 0-20% Au+Au STAR Preliminary 0-20% Au+Au Open symbols p+p Open symbols p+p Open symbols p+p Significant broadening on the recoil side Observed modification of “Fragmentation Function” Remark: flat bkg subtraction by ZYAM - jet v2 under investigation Elena Bruna (Yale&INFN Torino)

  13. Jet-hadron correlations Broadening π Significant (Gaussian) broadening of the away side. Broadening decreases with jet energy. Out-of-cone (R>0.4) energy ~ 6-9 GeV.  in agreement with broadening scenario in di-jet analysis! Elena Bruna (Yale&INFN Torino)

  14. Jet-hadron correlations Softening π Softening of “jet fragmentation” Significant enhancement at low pT (pT<2 GeV) Suppression at low pT Elena Bruna (Yale&INFN Torino)

  15. Jets IV: The Present:From RHIC to LHC… Elena Bruna (Yale&INFN Torino)

  16. LHC: the hard probes factory Elena Bruna (Yale&INFN Torino)

  17. Remarks on Jet Kinematics (1) Jet production: qq(gq,gg)  qq(gq,gg) Energy-momentum conserved.  jets back-to-back in f Not necessarily back-to-back in h ! Why? jet1 q2 -h +h q1 jet2 Example: q1 + q2 j1 + j2 q1=(x1,0,0,x1) q2=(x2,0,0,-x2) If x1=x2 Dy12=0  jets back-to-back in h !  this is more likely for high-pT jets, where the total energy goes into the transverse plane If x1 ≠x2  Dy12≠0  jets not back-to-back in h ! Elena Bruna (Yale&INFN Torino)

  18. Remarks on Jet Kinematics (1) jet1 Simulation: PYTHIA p+p √sNN= 200 GeV q2 -h +h q1 jet2 h range Simulation: PYTHIA p+p √sNN= 200 GeV The higher the jet pT, the more peaked at mid-rapidity it is Elena Bruna (Yale&INFN Torino)

  19. Remarks on Jet Kinematics: RHIC vs LHC 1) For the same x1 at RHIC and LHC, the higher the √sNN energy, the larger the rapidity gap between the di-jets! 2) For fixed hadron pT, different parton energies are sampled at RHIC vs LHC ! Near side has higher pTparton than away side Fixed pTtrig & pTassoc larger pTparton at LHC di-hadron pTtrig>8 GeV Keep in mind (1) and (2) when comparing di-hadron/di-jets at RHIC vs LHC Elena Bruna (Yale&INFN Torino)

  20. Hard processes: from RHIC to LHC Large increase of jet x-section from RHIC to LHC! Elena Bruna (Yale&INFN Torino)

  21. √s = (h++h-)/2 5500 GeV π0 LHC: the hard probes factory LO p+p y=0 200 GeV 17 GeV LHC RHIC SPS • From RHIC to LHC: • fireball hotter, denser, longer lifetime • huge increase of hard probes! •  need high-pT triggers ! Elena Bruna (Yale&INFN Torino)

  22. LHC: the hard probes factory jet cross section in p+p: RHIC vs LHC Simulation: PYTHIA Anti-kT, R=0.4 huge increase of hard probes!  need high-pT triggers ! Cross-section falls with a smaller (power-law) exponent  less sensitivity to the energy scale: important for background treatment Elena Bruna (Yale&INFN Torino)

  23. Hard processes: from RHIC to LHC xT N. Glover CTEQ, Rhodes, (2006) • Different xT range: • RHIC: 0.15 – 0.45 • LHC: 0.02- 0.2 • RHIC is quark dominated. • LHC is gluon dominated Elena Bruna (Yale&INFN Torino)

  24. LHC detectors for Jet analysis muon • Complimentary measurements: • large acceptance for charged hadrons, leptons and neutral energy (ATLAS, CMS) • Hadron PID in ALICE (|h|<1) • ALICE: full tracking to very low pT • ATLAS,CMS also low pT with vertex detector (pp) HCAL ECAL tracking hadron PID counters lumi. Elena Bruna (Yale&INFN Torino)

  25. Jet x-section measurement in ALICE: p+p (PYTHIA) ALICE EMCal PPR (2009) EMCal acceptance: |η|<0.7, Δφ=110o EMCal needed for triggering and for neutral jet energy component TPC used for charged tracks Large kinematical reach in 1 year ALICE p+p running Elena Bruna (Yale&INFN Torino)

  26. Jet x-section measurement in ALICE: Pb+Pb (qPYTHIA) ALICE EMCal PPR (2009) EMCal acceptance: |η|<0.7, Δφ=110o Large kinematical reach in 1 year ALICE running Precise measurement: Effect of background fluctuations in jet spectrum suppressed due to harder underlying partonic spectrum! Elena Bruna (Yale&INFN Torino)

  27. Underlying background at LHC ALICE EMCal PPR (2009) • PYTHIA jet spectrum √s=5.5 TeV: • embedded in HIJING 0-10% Pb+Pb • unfolded assuming Gaussian fluctuations with s=12 GeV/c • unfolded spectrum within 20% of the input spectrum! •  background fluctuations under control because of the harder jet spectrum at LHC wrt RHIC ! LHC: background less dangerous because of the harder parton spectrum Elena Bruna (Yale&INFN Torino)

  28. Underlying background at LHC Jet p+p x-sec (PYTHIA) Ratio: Jet p+p x-sec / Jet p+p ✕ Bkg fluctuations Simulation: PYTHIA Anti-kT, R=0.4 LHC: background less dangerous because of the harder parton spectrum Elena Bruna (Yale&INFN Torino)

  29. Let’s look at the data.. Elena Bruna (Yale&INFN Torino)

  30. Jet quenching at the LHC ALICE, Phys. Lett. B 696 (2011) 30. Central Pb+Pb suppressed ! Peripheral suppressed less Elena Bruna (Yale&INFN Torino)

  31. Jet quenching at the LHC ALICE, Phys. Lett. B 696 (2011) 30. • LHC RAA: • sharp rise above 7 GeV • minimum at ~ 0.5 RHIC RAA • Next: • PID • increase statistics • take pp reference at 2.76 TeV LHC RAA<RHIC RAA RHIC: high-pT hadrons hadronize from quarks LHC: high-pT hadrons hadronize from gluons (larger color charge) Elena Bruna (Yale&INFN Torino)

  32. Jet quenching at the LHC Di-Jet asymmetry: arXiv:1011.6182 Anti-kT R=0.4 Elena Bruna (Yale&INFN Torino)

  33. Jet quenching at the LHC Di-Jet asymmetry: CMS arXiv:1102.1957 Anti-kT Iterative cone R=0.5 Elena Bruna (Yale&INFN Torino)

  34. Quenching or fluctuations? Cacciari, Salam, Soyez, arXiv:1101.2878 • Use HYDJET instead of HIJING • Fluctuations might potentially have an impact on the dijet asymmetry • From the paper: “It is not our intention to claim that the striking di-jet asymmetry results are an artifact of fluctuations. Nevertheless fluctuations can significantly affect the main observable Aj”. Elena Bruna (Yale&INFN Torino)

  35. Quenching or fluctuations? Cacciari, Salam, Soyez, arXiv:1101.2878 • Use HYDJET instead of HIJING • Fluctuations might potentially have an impact on the dijet asymmetry • From the paper: “It is not our intention to claim that the striking di-jet asymmetry results are an artifact of fluctuations. Nevertheless fluctuations can significantly affect the main observable Aj”. • Next steps: • - Other observables: jet energy profile (jet core), R-dependence of Aj, jet-hadron correlations • More exhaustive investigation of different scenarios of fluctuations and quenching Elena Bruna (Yale&INFN Torino)

  36. Why jets in heavy ion collisions? Jet Tomography! • Access kinematics of the binary hard-scattering • Characterize the parton energy loss in the hot QCD medium • Study medium response to parton energy loss Jet I: Intro & Motivations Summary • Jet-finding connects Theory and Experiment • Goal: set the Jet Energy Scale • Different systematics to take into account (double counting,…) • Background fluctuations: the challenge Jet II: Full Jet Reconstruction p+p and d+Au: reference/control measurements Broadening observed at RHIC Jet III: Results • From RHIC to LHC: huge increase of hard probes! • LHC: Less sensitivity to the energy scale: important for background treatment • First observation of quenching! Jet IV: The Present: from RHIC to LHC Elena Bruna (Yale&INFN Torino)

  37. “Science is a way to teach how something gets to be known, what is not known, to what extent things are known (for nothing is known absolutely), how to think about things so that judgments can be made, how to distinguish truth from fraud, and from show”. R. Feynman Thanks for this fruitful school ! Elena Bruna (Yale&INFN Torino)

  38. BACKUP Elena Bruna (Yale&INFN Torino)

  39. Jet Energy resolution with di-jets Particle-Detector jet Res: pTJet(Part.Lev) – pTJet(Det.Lev) ~10-25 % di-jet Res: pTJet 1– pTJet 2 (PY Det. Lev.) ~ (dijet data) : good! But: (dijet PY Det. Lev.) > (Part-Det) • di-jet imbalance includes both energy resolution and kT (initial state) effect! • [kT=pTjet sinDfdijet] • kT: good agreement between data and simulation Use PYTHIA to determine the jet energy resolution Elena Bruna (Yale&INFN Torino)

  40. Jet-finding and systematics.. Tracking performance • Tracking is limited by misalignment, luminosity, resolution… • Rare processes as high-pT jets are likely to come from high luminosity runs • Example of high-luminosity distortion? Space-charge effect  accumulation of space charge in the TPC that causes an anomalous transport of drifting electrons in the TPC, affecting the tracking performance by shifting the momentum up or down (depending on the charge) • Tracking resolution at high-pT is • expected to deteriorate  need to apply • an upper pT cut on tracks PYTHIA simulation: p+p 200 GeV effect of upper pT cut on jet energy scale Elena Bruna (Yale&INFN Torino)

  41. Jet-finding and systematics.. Unobserved neutral energy Experiments like STAR and ALICE do not detect neutral, long-lived particles (neutrons, K0L) • PYTHIA simulation: • p+p at 200 GeV • mean missed E ~ 9% • median missed E <0.3 % • 50% of jets loose no energy • model dependent Elena Bruna (Yale&INFN Torino)

  42. Fragmentation Functions Jet energy determined in R=0.4 large uncertainties due to background (further systematic evaluation needed) • pT Jet(trig)>20 GeV • pTcut=2 GeV AuAu (Jet+Bkg) AuAu (Bkg) high z low z Charged particle FF: R(FF)=0.7 STAR preliminary xrec=ln( pT,Jet rec / pT,hadr) AuAu: FF(Jet)=FF(Jet+Bkg)-FF(bkg) Bkg estimated from charged particle spectra out of jet cones Bkg dominates at low pT Elena Bruna (Yale&INFN Torino)

  43. Fragmentation Functions “trigger” jet “recoil” jet EMC trigger No apparent modification of FF of recoil jets with pTrec>25 GeV would imply non-interacting jets, but: Jet broadeningEnergy shift harder FF Need to better determine the jet energy Elena Bruna (Yale&INFN Torino)

  44. Fake jets in PHENIX Pedestal comes from combinatorics of residual fake jets When 17.8 (GeV/c)2 used as standard fake rejection cut level:  < 10% contamination at 7.5 GeV/c Elena Bruna (Yale&INFN Torino)

  45. Jet Yields in ALICE Elena Bruna (Yale&INFN Torino)

  46. DCal for Di-Jet analysis @ ALICE Elena Bruna (Yale&INFN Torino)

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