1. EMCal meeting 1/34 Gustavo Conesa Balbastre Review on direct photon measurement and jet correlations with EMCal

2. EMCal meeting 2/34 Outline Introduction: Why direct photon measurement and correlation with jets. Photon /  0 discrimination in EMCal: Shower shape analysis  Summary of work done by Cynthia Hadjidakis Direct prompt photon identification in EMCal: Isolation cut  Summary of work done by Gustavo Conesa and Amaya Casanova Tagging jets with photons:  Gamma in PHOS: Summary of work made by Gustavo Conesa  Gamma in EMCal: Summary of work made by Guénolé Bourdaud

3. EMCal meeting 3/34 Introduction: 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)

4. EMCal meeting 4/34 Introduction: 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 

5. EMCal meeting 5/34 Introduction: 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

6. EMCal meeting 6/34  -  0 discrimination Three regions of analysis well separated clusters  invariant mass analysis < 15 GeV/c in EMCal < 30 GeV/c in PHOS merged clusters not spherical  shower shape analysis 5 - 40 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 > 40 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

7. EMCal meeting 7/34 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 At least one count up to ~ 100 GeV, counting rates for correlations good up to 30 GeV. ALICE-INT-2005-014 G. Conesa et al., NIM A 580 (2007) 1446 2 PHOS modules

8. EMCal meeting 8/34 p   GeV/c   -  0 discrimination in EMCal: Shower Shape Analysis  identified as   identified as   Uniform  and  0 distributions, weighted to match INCNLO predictions identified  : 0 2 < 0.25 identified  0 : 0 2 > 0.25 PID efficiency   as     as  See Cynthia’s H. talk about EMCal PID during PWG4 meeting in 11 December 2007 for more details p+p PID efficiency Pb+Pb  identified as   identified as     as     as 

9. EMCal meeting 9/34  /  0 discrimination in EMCal Shower Shape Analysis N direct  / N  from  p   GeV/c  p+p 14 TeV central Pb+Pb 5.5 TeV factor 5   suppression →  0 suppression factor = 5-10 for p T =[10-30] GeV/c in p+p →  0 suppression factor > 5 for p T =[15-30] GeV/c in Pb+Pb → N direct  / N  from  ~ 1 in Pb+Pb for p T =[20-30] GeV/c See Cynthia’s H. talk about EMCal PID during PWG4 meeting in 11 December 2007 for more details

10. EMCal meeting 10/34 Study presented for the CD2 Photons are correctly identified with a 90% efficiency in pp collisions and more than 60% in PbPb collisions in the p T range from10 to 100 GeV/c  0 are misidentified as photons and with an efficiency that varies from 20 to 10 % going from 10 to 20 GeV and increases to 40 % at 40 GeV in pp collisions, similar in PbPb collisions The PID helps to improve the ratio direct  0 in the region from 10 to 40 GeV, but it is still too small. An additional condition to separate direct photons from the rest is needed: Isolation Cut  /  0 discrimination in EMCal Shower Shape Analysis, Summary and conclusions

11. EMCal meeting 11/34 Direct photon 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, to correct for this bias effect.  ~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. Fidutial cut applied to all events. See my talk in Prague ALICE meeting.

12. EMCal meeting 12/34 Direct photon identification in EMCal: Generated and reconstructed+PID 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. PYTHIA

13. EMCal meeting 13/34 Cluster reconstruction efficiency is almost 90%. With shower shape selection the efficiency descends to 45-75% PYTHIA Direct photon identification in EMCal: Generated and reconstructed+PID spectra:  -jet : pp+PbPb @ √5.5 TeV

14. EMCal meeting 14/34 Direct photon identification in EMCal: 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, Overlapped decay  rejection no more feasible

15. EMCal meeting 15/34 Direct photon identification in EMCal: Prompt photon / jet clusters with and without PID pp @ √14 TeV PbPb @ √5.5 TeV, qhat=0 PbPb @ √5.5 TeV, qhat = 50 With PID prompt  to jet clusters (decay , pi0, hadrons) ratio increases significantly but it is not enough.

16. EMCal meeting 16/34 Direct photon identification in EMCal: Isolation Cut : Prompt photon / jet clusters 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

17. EMCal meeting 17/34 pp @ 14 TeV Direct photon identification in EMCal: Isolated Spectra PbPb @ 5.5 TeV, qhat=0 PbPb @ 5.5 TeV, qhat=50 With PID IC Parameters: R=0.4, p T th = 0.5 (pp), 2 (PbPb) GeV/c Isolation seems to work in pp and PbPb quenched events for p T > 15-20 GeV/c. I would like to have larger amount of jet-jet simulations to quantify better the effect of other particles background at high p T

18. EMCal meeting 18/34 Direct photon identification in EMCal: Prompt & Fragmentation photons: p T distribution PYTHIA pp collisions √s=14 TeV |  | < 1 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 Pure Pythia Generation, millions of events in several p T hard bins. 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

19. EMCal meeting 19/34 PYTHIA pp collisions √s=14 TeV |  | < 0.5 NLO calculation thanks to Lamia B. Direct photon identification in EMCal: Prompt & Fragmentation photons: p T distribution

20. EMCal meeting 20/34 Direct photon identification in EMCal: Direct & Fragmentation photons Isolation efficiency. Prompt and ISR are isolated, FSR at most 50% are not isolated, high z. 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 exactly the same trend due to the fact that isolated fragmentation photons do not carry all the parton energy (more in back-up).

21. EMCal meeting 21/34 Direct photon identification 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. PYTHIA predicts that fragmentation photons in jet-jet events are produced in a same amount as prompt photons. Most of them are isolated, carry a large value of the parton energy.

22. EMCal meeting 22/34 Tagging jets with photons Old approach  Search identified prompt photon (PHOS or EMCal) with largest p T (E  > 20 GeV). Strategy (event by event) :  min  max leading Search leading particle :   -  leading  180º E leading > 0.1 E  R Reconstruct the jet in TPC and EMCal (if available): With Standard jet algorithms, only high p T, not enough statistics. Used method: Particles around the leading with p T > 0.5 GeV/c,inside a cone of R = 0.3. PYTHIA events generated in a similar way as in previous analysis. IP EMCa l TPC ALICE-INT-2005-014 G. Conesa et al., NIM A 585(2008) 28 EMCal/PHOS

23. EMCal meeting 23/34 Pb-Pb collisions, p T, part > 0.5 GeV/c Tagging jets with photons in PHOS Reconstructed jet selection 40 GeV jets TPC alone TPC+EMCAL p-p collisions, p T, part > 0.5 GeV/c ALICE-INT-2005-014 G. Conesa et al., NIM A 585(2008) 28

24. EMCal meeting 24/34 TPC alone TPC+EMCAL p-p collisions, p T, part > 0.5 GeV/c TPC alone TPC+EMCAL Pb-Pb collisions, p T, part > 2 GeV/c ALICE-INT-2005-014 G. Conesa et al., NIM A 585(2008) 28 Tagging jets with photons in PHOS Reconstructed jet selection 40 GeV jets

25. EMCal meeting 25/34 E  > 20 GeV/c; TPC+EMCal detect jet particles, PHOS  Pb-Pb collisions Background Signal HIC background Any neutral signal in PHOS z = p T, jet particle /E  Prompt  identified in PHOS If signal is quenched ALICE-INT-2005-014 G. Conesa et al., NIM A 585(2008) 28 HI Background subtracted statistically, accumulate a fake fragmentation function with particle in cone at opposite  and same  as jet Tagging jets with photons in PHOS Fragmentation function

26. EMCal meeting 26/34 Charged + EM With quenched  0 Systematic errors due to jet(  0 )-jet background Sensitive to medium modifications at low z if larger than ~5% in both configurations. Same result using TPC alone to reconstruct the jet ALICE-INT-2005-014 G. Conesa et al., NIM A 585(2008) 28 Tagging jets with photons in PHOS Fragmentation function R FF

27. EMCal meeting 27/34 Enough counting rate to do the measurement only in the p T range 20-30 GeV. Good sensitivity to modifications in the jet, being the jet inside or outside EMCal. 0.1 < z < 0.5, region where measurement is sensitive.  For z < 0.1, too large HI background fluctuations  For z > 0.5, too large statistical fluctuations. Study was done with fast reconstruction of the jet:  p T smearing of generated particles  EMCal resolution similar to PHOS  … Method selects jets as a function of p T of the leading, applying a cut depending on the photon energy.  Possible bias, use better a grid in the detectors as done in normal jet finders and avoid the p T cut. Should be repeated with fully reconstructed events and a fine tuned method. Tagging jets with photons in PHOS Conclusions and Summary

28. EMCal meeting 28/34 Generation of gamma-jet events, with E  >30 GeV,  pp, pp+HI and quenched pp + HI. PID and isolation cut applied to the photons. New approach:  Search jet core in all event UA1 like, core defined as   Jet axis must be in  Construct fragmentation function with  Tagging jets with photons in EMCal Study done by Guénolé Bourdaud

29. EMCal meeting 29/34 Jet selected, collect all charged particles around jet axis in a cone of R=0.7 Subtract HI background  FFin = signal + bkg.  FFout = bkg only.  FFcorr : = FFsignal. ! Available  range. Tagging jets with photons in EMCal Fragmentation function calculation

30. EMCal meeting 30/34 Ratio pp / PbPb Tagging jets with photons in EMCal Fragmentation function, E jet = 100 GeV

31. EMCal meeting 31/34 Realistic spectrum simulated: 30 < E  -jet GeV pp events with PYTHIA. Pb-Pb events with PYTHIA+"PYQUEN afterburner« merged in HIJING events. 1/10 of a year statistic.  range usable is 0.5 <  < 3.2 Tagging jets with photons in EMCal Final Fragmentation function and R FF

32. EMCal meeting 32/34 Conclusion of gamma tagging jets with gamma in EMCal The modification of the fragmentation function is usable with 30 GeV for 0.5 <  < 3.2. HI Background is the main source of error.  More studies of bkg subtraction are needed. bkg area, min pT cut,  testing other kind of algorithm... Jet-Jet background still not studied.

33. EMCal meeting 33/34 TO DO Photon Isolation:  Larger sample of background (  0, hadrons) needed.  Check predictions with other generators Gamma-Jet tagging  Study from photons at 20 GeV, PHOS and EMCAL?  Study how to improve background subtraction.  Check the effect of the jet-jet background.

34. EMCal meeting 34/34 GRID data sets Available (if the grid works, the SE is not down, God is kind, …)  Check http://pcalimonitor.cern.ch/job_details.jsp http://pcalimonitor.cern.ch/job_details.jsp  pp events LHC08u: gamma (PHOS) – jet, 15 < E  < -1 GeV, 3M events LHC08p: gamma (PHOS) - jet quenched, qhat=50, 15 < E  < -1, 4M events LHC08v: jet-jet, 15<Ejet<50 GeV, 4M events LHC08r: jet-jet, 50<Ejet<-1 GeV, 3M events LHC08q: jet-jet, 100<Ejet<-1 GeV, 800k events Events stored in directory: /alice/sim/PDC_08/LHC08* Need to check if this amount of jet-jet events are enough for gamma isolation.  PbPb events: HIJING central, 100k events Desirable  pp events Gamma (EMCAL) – jet  pp+PbPb events: Currently testing the grid scripts Generate new pp and merge immediately with the available PbPb Gamma (EMCAL/PHOS) – jet (quenched, not quenched) jet-jet (quenched/not quenched) A couple of quenching scenarios? q= 20 and q=50, models PQM, PYQUEN?

35. EMCal meeting 35/34 Back-up

36. EMCal meeting 36/34 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

37. EMCal meeting 37/34 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

38. EMCal meeting 38/34 Fidutial cuts Structures in front of EMCAL increase the amount of conversions, more than the 50% expected. Apply fidutial cuts to get rid of extra particles in analysis of candidates to prompt photons (but extra clusters can be inside the isolation cone). About 25% of the calorimeter acceptance after the cuts is taken out. With fidutial cuts With PID

39. EMCal meeting 39/34 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

40. EMCal meeting 40/34 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

41. EMCal meeting 41/34 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

42. EMCal meeting 42/34 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. PYTHIA

43. EMCal meeting 43/34 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.

44. EMCal meeting 44/34 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

45. EMCal meeting 45/34 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

46. EMCal meeting 46/34 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

47. EMCal meeting 47/34 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

48. EMCal meeting 48/34 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

49. EMCal meeting 49/34 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.

50. EMCal meeting 50/34  -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

51. EMCal meeting 51/34 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

52. EMCal meeting 52/34 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

53. EMCal meeting 53/34 100 GeV  -jets, no bkg. Fraction of reconstructed jet energy vs R Reconstructed jet energy in HI / HI Background fluctuation For E jet = 30 GeV and R = 0.25 ! E jet / fluctuations bkg = 2. Tagging jets with photons in EMCal Jet energy reconstruction