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

2. Elena Bruna (Yale&INFN Torino) Today 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 Jet IV: The Present: from RHIC to LHC Hard Probes at LHC vs RHIC Results on quenching at LHC

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

4. Jet-finding in PHENIX Elena Bruna (Yale&INFN Torino) 4 Step back: how does jet-finding work in PHENIX? Direct rejection of fake jets (i.e. jet by jet): May select specific jet fragmentation 1) Sum p T 2 inside a Gaussian kernel to obtain a discriminant: 2) Keep jets with g 0.1 > threshold Jet-finding based on a Gaussian kernel (σ=0.3, 0.4) Focuses on the core of the jet Are those jets that keep a Gaussian shape only minimally interacting? That would explain the suppressed R AA to the level of surface emission

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

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

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

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

9. Broadening scenario Compatible with a jet p T shift Δ = 7-8 GeV/c 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) possible interpretation of di-jet suppression 9 Elena Bruna (Yale&INFN Torino) AuAu shifted/pp

10. Broadening or absorption? Elena Bruna (Yale&INFN Torino) 10 p+p Au+Au Energy shift? Absorption? 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!

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

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

13. Jet-hadron correlations Elena Bruna (Yale&INFN Torino) 13 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! Broadening

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

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

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

17. Remarks on Jet Kinematics (1) Elena Bruna (Yale&INFN Torino) 17 Jet production: qq(gq,gg)  qq(gq,gg) Energy-momentum conserved.  jets back-to-back in  Not necessarily back-to-back in  ! Why? Example: q 1 + q 2  j 1 + j 2 q 1 =(x 1,0,0,x 1 ) q 2 =(x 2,0,0,-x 2 ) -- ++ jet 1 jet 2 q2q2 q1q1 If x 1 =x 2   y 12 =0  jets back-to-back in  !  this is more likely for high-p T jets, where the total energy goes into the transverse plane If x 1 ≠x 2   y 12 ≠0  jets not back-to-back in  !

18. Simulation: PYTHIA p+p √s NN = 200 GeV Simulation: PYTHIA p+p √s NN = 200 GeV  range Remarks on Jet Kinematics (1) Elena Bruna (Yale&INFN Torino) 18 -- ++ jet 1 jet 2 q2q2 q1q1 The higher the jet p T, the more peaked at mid-rapidity it is

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

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

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

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

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

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

25. ALICE EMCal PPR (2009) Jet x-section measurement in ALICE: p+p (PYTHIA) 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 EMCal acceptance: |  |<0.7, Δ  =110 o 25 Elena Bruna (Yale&INFN Torino)

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

27. Underlying background at LHC Elena Bruna (Yale&INFN Torino) 27 ALICE EMCal PPR (2009) PYTHIA jet spectrum √s=5.5 TeV: embedded in HIJING 0-10% Pb+Pb unfolded assuming Gaussian fluctuations with  =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

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

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

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

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

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

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

34. Cacciari, Salam, Soyez, arXiv:1101.2878 Quenching or fluctuations? Elena Bruna (Yale&INFN Torino) 34 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”.

35. Cacciari, Salam, Soyez, arXiv:1101.2878 Quenching or fluctuations? Elena Bruna (Yale&INFN Torino) 35 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

36. Elena Bruna (Yale&INFN Torino) Summary Jet I: Intro & Motivations 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 II: Full Jet Reconstruction 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 III: Results p+p and d+Au: reference/control measurements Broadening observed at RHIC Jet IV: The Present: from RHIC to LHC From RHIC to LHC: huge increase of hard probes! LHC: Less sensitivity to the energy scale: important for background treatment First observation of quenching!

37. Elena Bruna (Yale&INFN Torino) Thanks for this fruitful school ! “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

38. Elena Bruna (Yale&INFN Torino) BACKUP

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

40. Jet-finding and systematics.. Elena Bruna (Yale&INFN Torino) 40 Tracking performance Tracking is limited by misalignment, luminosity, resolution… Rare processes as high-p T 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-p T is expected to deteriorate  need to apply an upper p T cut on tracks PYTHIA simulation: p+p 200 GeV effect of upper p T cut on jet energy scale

41. Jet-finding and systematics.. Elena Bruna (Yale&INFN Torino) 41 Unobserved neutral energy Experiments like STAR and ALICE do not detect neutral, long-lived particles (neutrons, K 0 L ) PYTHIA simulation: p+p at 200 GeV mean missed E ~ 9% median missed E <0.3 % 50% of jets loose no energy model dependent

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

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

44. Fake jets in PHENIX Elena Bruna (Yale&INFN Torino) 44 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

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

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