1. High pT physics at LHC 1/36 Gustavo Conesa Balbastre High p T direct photon measurement and correlations with hadrons and jets in ALICE

2. High pT physics at LHC 2/36 Direct photons as probes for QGP Direct EM probes convey unperturbed information and their production probe the medium  Tag medium-modified jets: Prompt photons from 2->2 hard process (E  > 10 GeV)  Medium modified production: Fragmentation photons (E  < E jet )  Medium produced photon: Bremsstrahlung and jet conversion (E  < E jet ) Thermal photons (E  < 10 GeV) Challenge:  Disentangle the different sources.  Neutral mesons decay. But decay photons provide a first choice probe of medium effects  Identify real photons (EM calorimetry, trigger) and e+e- from virtual and converted photons (tracking and PID, trigger)

3. High pT physics at LHC 3/36 Direct photon sources prompt LO Isolated photons Jet  Fragmentation NLO High z isolated photons  thermal radiation    Bremsstrahlung/jet coversion induced by medium

4. High pT physics at LHC 4/36 Estimates with a thermal model Turbide, Gale, Jeon, and Moore PRC (2004) Jet bremsstrahlung/fragmentation correlated with hadrons Jet-plasma & thermal, uncorrelated Gale QM2008

5. High pT physics at LHC 5/36 How can we distinguish different direct photon sources? Prompt : R AA = 1, v 2 =0 Fragmentation: R AA 0 Thermal, Bremsstrahlung, Jet Conversion: R AA >1, v 2 <0 Unambiguous signature of medium produced photons

6. High pT physics at LHC 6/36 RHIC News: PHENIX R AA Gale QM2008 Run 2: No  suppression (PRL 94, 232301 (2005)). Run 4 (QM06): High p T  suppression  Isospin (PDF) effect  Fragmentation photon suppresion?  Something else?

7. High pT physics at LHC 7/36 RHIC News : PHENIX v 2 Gale QM2008 v2: small! Consistent with zero (within errors)

8. High pT physics at LHC 8/36 Why photon-tagged jets? Medium effects redistribute (  qL) the parton energy, E jet, inside the hadron jet (multiplicity, j T ). If we measure E  ≈ E jet ^ A B E jet. Redistribution can be best measured with the Fragmentation Function... If we know E jet. HI environment hinders precise reconstruction of E jet. Jet 

9. High pT physics at LHC 9/36 Use Near Side peak to determine direct  associated with h, I.e. fragmentation photons. Statistical subtraction of decay photons. RHIC news : PHENIX  -h correlations Nguyen QM2008

10. High pT physics at LHC 10/36 RHIC news : PHENIX h-  correlations I 10 -6 X10 -6 X10 -3 h-inclusive  h-decay  h-direct   Idea: By triggering on a hadron and looking for near-side direct photon partners one can measure the fragmentation photon yield directly  Measure hadron - inclusive  and hadron - decay  correlations  Decay corr’s are made by tagging  0 and  by invariant mass  Must know tagging efficiency and false tagging rate precisely  dominant source of systematic uncertainty 2.5 < p T,  < 3.5 Nguyen QM2008 Near Side

11. High pT physics at LHC 11/36 Nguyen QM2008 RHIC news : PHENIX h-  correlations II 11  First measurement of it’s kind at RHIC!  Will measure jet shape distributions, e.g. x E, p out  Constrain photon FF?  N frag /N inc ≈ 0.1 at intermediate p T  Measure in Au+Au

12. High pT physics at LHC 12/36 12 The  -rich sample has lower near-side yield than  0. RHIC news : STAR  -h correlations I Hamed QM2008

13. High pT physics at LHC 13/36 "T. Renk and K. Eskola, Phys.Rev.C75:054 910,2007" "X-N Wang & H. Zhang et al (to be published)" I AA of direct  Within the current uncertainty in the scaling the I AA of direct  and  0 are similar. I AA of  0 RHIC news : STAR  -h correlations II Hamed QM2008

14. High pT physics at LHC 14/36 How can we measure direct photons in ALICE? E > 10 GeV   E/E < 1.5%,  x =[0.5,2.5] mm ITS  =110º  |  | < 0.7  =100º  |  | < 0.12  =360º  |  | < 0.9 PHOS TPC EMCal  p/p = 2%,  =1.1º E > 10 GeV   E/E < 3 %  x =[3,50] mm

15. High pT physics at LHC 15/36 How many direct photons? 10k/year Large sample of direct LO  -jet for p T < 30 GeV/c in PHOS and p T < 50 GeV/c in EMCal …  … but  0 = 0,01-0,1 for p T > 10 GeV/c We need a good  0 PID

16. High pT physics at LHC 16/36  -  0 discrimination Three regions of analysis well separated clusters  invariant mass analysis < 10 GeV/c in EMCal < 30 GeV/c in PHOS merged clusters not spherical  shower shape analysis 10 - 30 GeV/c in EMCal 30 - 100 GeV/c in PHOS increasing p T Opening angle << 1 cell all  0 ’s at this energy are in jets  isolation cut > 30 GeV/c only method in EMCal   R PHOS/EMCal TPC candidate IP Isolated if: no particle in cone with p T > p T thres or p T sum in cone,  p T <  p T thres

17. High pT physics at LHC 17/36 PHOS identified spectrum pp and PbPb annual statistics Yaxian M. poster QM2008 IC: R =0.3,  (p T )=2 GeV/c IC: R =0.2, p T >2 GeV/c Statistics limits to ~ 100 GeV ALICE-INT-2005-014 G. Conesa et al., NIM A 580 (2007) 1446

18. High pT physics at LHC 18/36 Direct  identification in EMCal: Event generation pp PYTHIA collisions :  3 simulation cases: 1) pp @ √14 TeV 2) pp @ √5.5 TeV, merged with HIJING, no quenching in PYTHIA. 3) pp @ √5.5 TeV, merged with HIJING, quenching: qhat = 50.   +jet in final state ( MSEL=10)   – jet Prompt  is the signal under study.  2 jets in final state ( MSEL=1)  jet – jet These events constitute the background: decay , fragmentation  and hadrons. Jet-Jet Event generation with jets containing at least one  0 with p T >5 GeV/c in the acceptance of EMCAL.: production of fragmentation photons suppressed.  Also did a pp simulation without trigger in the same bins, similar number of events.  10 k events in different p T hard bins. Pb-Pb collisions @ √s = 5.5A TeV: pp simulations 2) and 3) merged with HIJING central events (b < 3fm) @ √s = 5.5A TeV. Simple PID with shower shape used to discriminate photons from other particles: if 0 2 < 0.25 cluster is a photon (Cynthia H., PWG4 meeting, 12/07) Full ALICE simulation with AliRoot 4.07 Release.

19. High pT physics at LHC 19/36 Generated spectra  jet : pp @ √14 TeV 50 % of  convert in the material before EMCal (5-10% in ITS-TPC). Cluster reconstruction efficiency is almost 95% but with shower shape selection the efficiency descends to 60-80% due to the non photon shape of some converted clusters. PID rejects 15% of clusters generated by real photons and from 50 to 25% of converted photons. With PID PYTHIA

20. High pT physics at LHC 20/36 Generated spectra  jet : pp+PbPb @ √5.5 TeV With PID Cluster reconstruction efficiency is almost 90%. With shower shape selection the efficiency descends to 45-75% PID rejects from 45% to 10% of clusters generated by photons and from 60 to 30% of converted photons.

21. High pT physics at LHC 21/36 jet-jet clusters rejection with photon PID pp @ √14 TeV PbPb @ √5.5 TeV The jet cluster rejection goes from 0.3 (large value due to decay  correctly identified as , no invariant mass analysis is done) to around 0.02 in pp collisions. There is an increase at 30- 40/c GeV because PID for larger p T cannot separate effectively overlapped  from  0 decay. In Pb-Pb rejection worsens, from 0.5 to 0.05. Decay  not overlapped,  0 invariant mass not done Overlapped decay  rejection no more feasible

22. High pT physics at LHC 22/36 Prompt  / jet clusters pp @ √14 TeV PbPb @ √5.5 TeV With PID prompt  to jet clusters ratio increases significatively but it is not enough. PbPb @ √5.5 TeV, qhat = 50

23. High pT physics at LHC 23/36 Isolation Cut : Signal/Background Ratio isolated clusters in  -jet / isolated clusters in jet-jet Clusters selected with PID 2 0 < 0.25 pp @ √14 TeV PbPb @ √5.5 TeV PbPb @ √5.5 TeV, qhat = 50 Prompt photons signal larger than jet-jet clusters background for p T larger than around 15 GeV/c for pp and quenched PbPb events

24. High pT physics at LHC 24/36 pp @ 14 TeV Isolated Spectra in EMCal No PID PbPb @ 5.5 TeV, qhat=0 PbPb @ 5.5 TeV, qhat=50 No PID With PID No PID With PID IC Parameters: R=0.4, p T th = 0.5 (pp), 2 (PbPb) GeV/c

25. High pT physics at LHC 25/36 Direct photon in EMCal : Summary & conclusions jet-jet and  -jet events, pp and Pb-Pb collisions, quenched and not quenched, have been generated and fully reconstructed. A simple PID and Isolation Cut have been applied and the measurements seem feasible for prompt  energies larger than 10-20 GeV/c in pp and Pb-Pb quenched collisions. Need a large production of jet-jet events to improve background estimation after isolation and use a more sophisticated PID.

26. High pT physics at LHC 26/36 Other approach: photon conversions Study performed by Ana Marin (GSI) Loss of efficiency at high p T under investigation Needs to be improved ! Identify photons converting in the beampipe, ITS and TPC  Clean photon identification  Provide directional information  Non vertex background (important source of systematic errors in measurement of direct photons) can be rejected. Independent measurement of the same quantities, with different systematics compared to PHOS/EMCAL. Increase level of confidence in the results Counting annual statistics for p T  > 20 GeV/c (very very rough stimations) pp @ √s=14 TeV PbPb @ √s=5.5A TeV EMCal20000 PHOS3000 Tracking4000

27. High pT physics at LHC 27/36 Azimuthal correlation: Direct  converted– charged particles Study performed by Ana Marin (GSI)  detected in Central Barrel Isolation Cut: R=0.2, p T >0.7GeV

28. High pT physics at LHC 28/36 Prompt & Fragmentation  in PYTHIA Have fragmentation and prompt photon similar properties? Looks the correlation with jets the same for both kind of photons? I have studied the prompt and fragmentation photon production generating several millions of jet-jet and  -jet events in same p T bins as in previous study.  pp collisions √s=14 TeV.  Pure PYTHIA generation. I have looked to the isolation cut and correlation and fragmentation function with the back to back jet at this generation level. Only difference with previous simulations now is the Parton Distribution Function (PDF). Before CTEQ4L now CTEQ5L.

29. High pT physics at LHC 29/36 Prompt & Fragmentation  p T distribution. PYTHIA pp collisions √s=14 TeV |  | < 1 |  | < 0.5 NLO calculation thanks to Lamia B. Pythia produces non hadronic decay photons:  Prompt photons: Compton and annihilation processes  Jet-Jet events: Initial state photons (ISR), radiated by hard parton before scattering. Few Final state photons (FSR), scattered partons radiate / fragment into bremsstrahlung / fragmentation photons Spectrum of all photons in |  | < 1 in all plots. Isolation done on the pure PYTHIA particles, R=0.4, p T th = 1 GeV/c. Particles with status code 1 except neutrinos enter in the cone.

30. High pT physics at LHC 30/36 Isolation efficiency Direct / Fragmentation |  |<1 Direct & Fragmentation  Isolation efficiency. Prompt and ISR are isolated, FSR at most 50% are not isolated. Fragmentation photon (FSR) yield is larger than prompt photon for p T < 50 GeV/c but with isolation at p T < 20 GeV/c ISR yield is small, I will not consider it in next slides. No away side correlation. PYTHIA pp collisions √s=14 TeV

31. High pT physics at LHC 31/36 Fraction of parton energy carried by the fragmentation photon p T  > 5 GeV/c PYTHIA pp collisions √s=14 TeV Obviously, fragmentation photon isolation is more efficient when it has an small part of the parton (near side jet) energy. Is hadron correlation with fragmentation photons and prompt photons the same? Could we reduce further the amount of low z fragmentation photons correlating with the away jet and putting a threshold on the energy? I will check.

32. High pT physics at LHC 32/36 Photon and back jet type Probability that a photon is back to a gluon or a quark Prompt photons are mainly back to quark jets (Compton). Fragmentation photons are mainly back to gluon jets PYTHIA pp collisions √s=14 TeV

33. High pT physics at LHC 33/36  -jet energy/phi smearing Jet energy reconstructed with the PYTHIA jet finder. We have to be careful when we say that  is back and has the energy of the jet. There is an smearing to take into account. PYTHIA pp collisions √s=14 TeV p T  /p Tjet > 0.5 p T  /p Tjet > 0.9

34. High pT physics at LHC 34/36 Correlation Function:  Plots for quark jets (similar for gluon jets) As expected, without isolation fragmentation  have a correlation in the near and away sides, direct  only in the away side. Isolated fragmentation  only correlate in the away side To understand: why away side descends a bit in isolated FSR photons. All charged hadrons with p T > 2 GeV/c PYTHIA pp collisions √s=14 TeV

35. High pT physics at LHC 35/36 Fragmentation Function: z T  All charged hadrons with p T > 0.1 GeV/c inside cone of size R=1 around jet axis Difference in FF for fragmentation and prompt due to z<1 for fragmentation photons? PYTHIA pp collisions √s=14 TeV Gluon JetQuark Jet p T  FSR / p T jet > 0.5

36. High pT physics at LHC 36/36 PYTHIA predicts that prompt and fragmentation photons are produced in pp collisions at 14 TeV, being their ratio 0.6 at 10 GeV and increasing linearly to 1.4 at 90 GeV. Isolation cut rejects few of the fragmentation photons, at best around 50%.  Remaining fragmentation photons carry a significant amount of the original parton/jet energy. Are isolated fragmentation photons similar to prompt photons?  Prompt photons and isolated fragmentation photons have a correlation with particles in the away side, not in the near side.  But correlation/fragmentation function of both isolated fragmentation and prompt photons seems not to follow the same trend due to the fact that isolated fragmentation photons do not carry all the parton energy, and something else? Prompt and Fragmentation  in PYTHIA: Summary & conclusions

37. High pT physics at LHC 37/36 Back-up

38. High pT physics at LHC 38/36 p   GeV/c   -  0 discrimination: Shower Shape Analysis PHOS (PPR Vol2) Bayesian  identified as   identified as   EMCal identified  : 0 2 < 0.25 identified  0 : 0 2 > 0.25 PID efficiency p+p   as     as  see Cynthia’s H. talk about EMCal PID during PWG4 meeting in December for more details Low particle environment

39. High pT physics at LHC 39/36 PHOS identified spectrum Pb+Pb annual statistics Particles identified as  IC: R =0.2, p T >2 GeV/c Factor 5 suppression Signal Background Statistics limits to ~ 100 GeV Corrected spectrum, systematic errors Reminder ALICE-INT-2005-014 - G. Conesa et al, NIM A 580 (2007) 1446

40. High pT physics at LHC 40/36 Direct  identification in EMCal: Event generation and full reconstruction II pp PYTHIA collisions:  k T default value (1 GeV/c).  3 simulation cases: 1) pp @ √14 TeV 2) pp @ √5.5 TeV, merged with HIJING, no quenching in PYTHIA. 3) pp @ √5.5 TeV, merged with HIJING, quenching: qhat = 50. Pb-Pb collisions @ √s = 5.5A TeV: pp simulations 2) and 3) merged with HIJING central events (b < 3fm) @ √s = 5.5A TeV. Simple PID with shower shape used to discriminate photons from other particles: if 0 2 < 0.25 cluster is a photon (see talk from Cynthia in December PWG4) Full ALICE simulation with AliRoot 4.07 Release. New

41. High pT physics at LHC 41/36 Direct  identification in EMCal: Event generation and full reconstruction I pp PYTHIA collisions:   +jet in final state ( MSEL=10)   – jet Prompt  is the signal under study. Generated p T hard bins (GeV): [5-10], [10-20], [20-30], [30-40], [40- 50], [50-60], [60-70], [70-80], [80-90], [90-100] and [>100] with ~10k events per bin.  2 jets in final state ( MSEL=1)  jet – jet These events constitute the background: decay , fragmentation  and hadrons. Generated p T hard bins (GeV): [12-16], [16-20], [20-24], [24-29], [29- 35], [35-41], [41-50], [50-60], [60-72], [72-86], [86-104], [104-124], [124-149], [149-179], [179-215], [215-258] and [>258] with ~10k events per bin. Jet-Jet Event generation with jets containing at least one  0 with p T >5 GeV/c in the acceptance of EMCAL.  Also did a pp simulation without trigger in the same bins, similar number of events. New

42. High pT physics at LHC 42/36 Prompt  /  0 At generation level PYTHIA Theory Ratios for pp collisions at √s=14 and 5.5 TeV agree with theoretical predictions. With qhat=50 ratio a factor ~2 larger than prediction.

43. High pT physics at LHC 43/36 jet-jet clusters: effect of the  0 trigger 1  0 with p T > 5 GeV/c in EMCal per event Number of clusters found in  0 triggered pp jet-jet events divided by clusters found in non triggered events. From now on pp   triggered pp jet-jet events will be multiplied by the function fitted in the figure to apply correction for the triggering bias on the hadron contribution. The fragmentation photon yield is much more suppressed (not considered in the cluster points and next plots) Reconstructed clusters Pythia generated particles (more events than in rec. clusters) Where the correction should be more or less f(p T <30) =0.86+1.55 e -0.072 pT

44. High pT physics at LHC 44/36 p T threshold candidate isolated if: no particle in cone with p T > p T thres p T sum in cone,  pT <  pT thres p T min of all particles (charged and neutral) in cone is at least 0.5 GeV/c Consider in cone particles : Charged in TPC acceptance: |  |<0.7, 0º<  <360º Neutral in EMCal acceptance: |  |<0.7, 80º<  <190º PHOS acceptance: |  |<0.12, 220º<  <320º Prompt  are likely to be produced isolated. Two parameters define  isolation: Cone size Isolation cut method   R PHOS/EMCal TPC candidate IP ALICE-INT-2005-014 - G. Conesa et al, NIM A 580 (2007) 1446 Reminder

45. High pT physics at LHC 45/36 Isolation Cut: Efficiency Ratio isolated clusters / total clusters No PID  -jet jet-jet pp @ √14 TeV PbPb @ √5.5 TeV PbPb @ √5.5 TeV, qhat = 50 Isolation rejection for jet clusters can be better than 99 % in pp and quenched PbPb events and p T > 20 GeV/c

46. High pT physics at LHC 46/36 Isolation Cut: Efficiency pp @ √14 TeV R=0.3 Ratio isolated clusters / total clusters No PID p T th = 0.5 GeV/c  -jet jet-jet  -jet

47. High pT physics at LHC 47/36 Isolation Cut: Efficiency pp+PbPb @ √5.5 TeV, no quenching R=0.3 Ratio isolated clusters / total clusters No PID p T th = 2 GeV/c  -jet jet-jet  -jet

48. High pT physics at LHC 48/36 Isolation Cut: Efficiency pp+PbPb @ √5.5 TeV, qhat = 50 R=0.3 Ratio isolated clusters / total clusters No PID p T th = 2 GeV/c  -jet jet-jet  -jet

49. High pT physics at LHC 49/36 Prompt photon at parton level Partonic photons and back-to-back quarks / gluons do not have the same momentum distribution, there is an energy smearing, and they are not exactly back-to-back. There is also a difference in p T (not in azimuth) between the partonic photon (status 21) and the final state photon (status 1). Where does it come from?, k T ? This difference is usually small but not always.

50. High pT physics at LHC 50/36 Direct & Fragmentation  p T distribution and ratio p T spectra (full  and , and |  |<0.7 ) Direct/Fragmentation ratio (full  and , and |  |<0.7)

51. High pT physics at LHC 51/36 Direct & Fragmentation  distribution and ratio Particle production seems independent of pseudo-rapidity in the range -5 to 5. I have studied the isolation cut in the EMCal  acceptance, results should be equivalent for the PHOS  acceptance p T spectra (full  p T > 5 GeV/c) Direct/Fragmentation ratio (full , pT > 5 GeV/c)

52. High pT physics at LHC 52/36 Correlation fuction for ISR

53. High pT physics at LHC 53/36 Correlation Function: z T All charged hadrons with p T > 2 GeV/c and at |   | < 1, hadrons in all eta. 0.5  <   –  hadron < 1.5  The correlation function of isolated fragmentation photons seems not to be the same as of prompt photons, effect of the z<1 for fragmentation photons? Gluon JetQuark Jet PYTHIA pp collisions √s=14 TeV

54. High pT physics at LHC 54/36 CF versus FF Gluon Jet Quark Jet Prompt Fragmentation PYTHIA pp collisions √s=14 TeV Don’t understand prompt-gluon ratios. Other plots show similar trend, needs check.