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

2. Outline
Introduction: Why direct photon measurement and correlation with jets.
Photon / p0 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 EMCal: Summary of work made by Guénolé Bourdaud
EMCal meeting

3. 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 (Eg > 10 GeV)
Medium modified production:
Fragmentation photons (Eg < Ejet)
Medium produced photon:
Bremsstrahlung and jet conversion (Eg < Ejet)
Thermal photons (Eg < 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)
EMCal meeting

4. Introduction: Why photon-tagged jets?^
Medium effects redistribute (qL) the parton energy, Ejet, inside the hadron jet (multiplicity, jT). A B
Redistribution can be best measured with the Fragmentation Function... If we know Ejet.
HI environment hinders precise reconstruction of Ejet.
If we measure Eg ≈ Ejet
EMCal meeting

5. Introduction: How many direct photons?
… but g/p0 = 0,01-0,1 for pT > 10 GeV/c
We need a good g/p0 PID
10k/year
Large sample of direct LO g-jet for pT < 30 GeV/c in PHOS and pT < 50 GeV/c in EMCal …
EMCal meeting

6. -0 discrimination Three regions of analysis increasing pT
merged clusters
not spherical
 shower shape analysis
GeV/c in EMCal
GeV/c in PHOS
well separated clusters
invariant mass analysis
< 15 GeV/c in EMCal
< 30 GeV/c in PHOS
Opening angle << 1 cell
all 0’s at this energy are in jets
 isolation cut
> 40 GeV/c only method in EMCal
EMCal meeting

7. p0 reconstruction via invariant mass
See Cynthia’s talk in 3rd convegno di Fisica di Alice in November 07
simulation: single particles+HI
only EMCAL
events
g1 g2 invariant mass (GeV)
only EMCAL
single particles
+ HI p0
efficiency
p0: E=9 GeV
s/mp= 11 %
→ effect of the HI environment: efficiency decreases (<10%)
→ measurement with more than 10% efficiency up to 15 GeV/c
EMCal meeting

8. Particle identification with the calorimeters
Different particles produce showers of different shapes.
One of the most sentitive parameter is the main axis of the shower ellipse l0
Identification probability of photons in pp collisions close to 90%, in PbPb around 60-70%
Overlap of p0 decay photons can be rejected in PHOS for 30<E<100 GeV and in EMCal for 5<E<40 GeV. l0 l1
EMCal
PHOS g g g
50 GeV g
70 GeV p0 p0 p0 p0
l20
l20
l20
l20
EMCal meeting
ALICE-INT , ALICE PPR vol II

9. -0 discrimination in EMCal: Shower Shape Analysis
See Cynthia’s H. talk about EMCal PID during PWG4 meeting in 11 December 2007 for more details
-0 discrimination in EMCal: Shower Shape Analysis
Uniform g and p0 distributions, weighted to match INCNLO predictions
identified g: l02 < 0.25
identified p0: l02 > 0.25
p+p
Pb+Pb
g identified as g
g identified as g
PID efficiency
PID efficiency
g identified as p0
g identified as p0
p0 as p0
p0 as p0
p0 as g
p0 as g
EMCal meeting
pT (GeV/c)
pT (GeV/c)

10. g/p0 discrimination in EMCal Shower Shape Analysis
See Cynthia’s H. talk about EMCal PID during PWG4 meeting in 11 December 2007 for more details
g/p0 discrimination in EMCal Shower Shape Analysis
p+p 14 TeV
Ndirect g / Np0
central Pb+Pb 5.5 TeV
factor 5 p0 suppression
pT (GeV/c)
pT (GeV/c)
→ p0 suppression factor = 5-10 for pT =[10-30] GeV/c in p+p
→ p0 suppression factor > 5 for pT =[15-30] GeV/c in Pb+Pb
→ Ndirect g / Ng from p0 ~ 1 in Pb+Pb for pT =[20-30] GeV/c
EMCal meeting

11. g/p0 discrimination in EMCal Shower Shape Analysis, Summary and conclusions
Study presented for the CD2
The PID helps to improve the ratio direct g/p0 in the region from 10 to 40 GeV, but it is still too small.
Problem: Revision of parameters in digitization implies that the PID parameterization must change.
1° Priority: Redo PID studies.
An additional condition to separate direct photons from the rest of clusters (decay photons, hadrons, etc.)is needed: Isolation Cut
EMCal meeting

12. Direct photon identification in EMCal: The Isolation Cut method
ALICE-INT G. Conesa et al, NIM A 580 (2007) 1446
Prompt g are likely to be produced isolated.
Two parameters define g isolation:
Cone size
TPC IP
pT threshold
candidate isolated if:
no particle in cone with pT > pT thres
pT sum in cone, SpT < SpTthres   R
candidate
PHOS/EMCal
EMCal meeting

13. Direct photon identification in EMCal: Event generation
See my talk in Prague ALICE meeting.
Direct photon identification in EMCal: Event generation
pp PYTHIA collisions :
3 simulation cases:
√14 TeV
√5.5 TeV, merged with HIJING, no quenching in PYTHIA.
√5.5 TeV, merged with HIJING, quenching: qhat = 50.
g +jet in final state (MSEL=10)   – jet
Prompt g is the signal under study.
2 jets in final state (MSEL=1)  jet – jet
These events constitute the background: decay g, fragmentation g and hadrons.
~10 k events in different pT hard bins.
Pb-Pb √s = 5.5A TeV: pp simulations 2) and 3) merged with HIJING central events (b < √s = 5.5A TeV.
Simple PID with shower shape used to discriminate photons from other particles: if l02 < 0.25 cluster is a photon (Cynthia H., PWG4 meeting, 12/07)
Full ALICE simulation with AliRoot 4.07 Release.
EMCal meeting

14. See my talk in Prague ALICE meeting.
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).
EMCal meeting
14/40

15. Fidutial cuts Fidutial cut + PID Fidutial cut + PID
See my talk in Prague ALICE meeting.
Fidutial cuts
About 25% of the calorimeter acceptance after the fidutial cut.
Fidutial cut + PID
Fidutial cut + PID
EMCal meeting
15/40

16. See my talk in Prague ALICE meeting.
Direct photon identification in EMCal: Prompt photon reconstruction + PID efficiency
Efficiency = reconstructed / all generated
√5.5 TeV
√14 TeV
50 % of g convert in the material before EMCal (5-10% in ITS-TPC). Cluster reconstruction efficiency in pp collisions is almost 95% but with shower shape selection the efficiency descends to 60-80% due to the non photon shape of some converted clusters. In PbPb collisions efficiency goes from 45 to 75 %
EMCal meeting

17. See my talk in Prague ALICE meeting.
Direct photon identification in EMCal: jet-jet clusters rejection with photon PID
√14 TeV
√5.5 TeV, no quenching
Decay g not overlapped
Overlapped decay g rejection no more feasible
Jet cluster rejection :
pp collisions: from 0.3 (large value due to decay g correctly identified as g) to 0.02
PbPb collisions: from 0.5 to 0.05.
Increase at GeV because PID for larger pT cannot separate effectively overlapped g
EMCal meeting

18. See my talk in Prague ALICE meeting.
Direct photon identification in EMCal: Prompt photon / jet clusters with and without PID
√14 TeV
√5.5 TeV, qhat=0
√5.5 TeV, qhat = 50
With PID prompt g to jet clusters (decay g, pi0, hadrons) ratio increases significantly but it is not enough.
EMCal meeting

19. See my talk in Prague ALICE meeting.
Direct photon identification in EMCal: Isolation Cut : Prompt photon / jet clusters
Ratio isolated clusters in g-jet / isolated clusters in jet-jet
Clusters selected with PID l20< 0.25
√14 TeV
√5.5 TeV, qhat=0
√5.5 TeV, qhat = 50
Prompt photons signal larger than jet-jet clusters background for pT larger than around 15 GeV/c for pp and quenched PbPb events
EMCal meeting

20. See my talk in Prague ALICE meeting.
Direct photon identification in EMCal: Prompt & Fragmentation photons: pT distribution
PYTHIA pp collisions √s=14 TeV
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
Amount of fragmentation photons in previous simulations small.
Pure Pythia Generation, millions of events in several pT hard bins.
Isolation done on the pure PYTHIA particles, R=0.4, pTth = 1 GeV/c.
|| < 1
EMCal meeting

21. See my talk in Prague ALICE meeting.
Direct photon identification in EMCal: Prompt & Fragmentation photons: pT distribution
PYTHIA pp collisions √s=14 TeV
|| < 0.5
NLO calculation thanks to Lamia B.
EMCal meeting

22. See my talk in Prague ALICE meeting.
Direct photon identification in EMCal: Direct & Fragmentation photons Isolation efficiency
FSR photons
Prompt g are isolated, FSR at most 50% are not isolated, high z.
Are isolated fragmentation photons similar to prompt photons?
Isolated prompt and fragmentation g correlate with particles in the away side, not in the near side.
But fragmentation function of both do not follow exactly the same trend because z for isolated fragmentation is not 1.
EMCal meeting

23. Direct photon identification in EMCal : Summary & conclusions
A simple PID and Isolation Cut have been applied and the measurements seem feasible for prompt g energies larger than 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.
EMCal meeting

24. Tagging jets with photons Old approach
ALICE-INT
G. Conesa et al., NIM A 585(2008) 28
Strategy (event by event):
Search identified prompt photon (PHOS or EMCal) with largest pT (E g > 20 GeV).
Search leading particle :
g-leading180º
Eleading > 0.1 Eg
Reconstruct the jet in TPC alone or TPC+EMCal:
With Standard jet algorithms, only high pT, not enough statistics.
Used method: Particles around the leading with pT > 0.5 GeV/c ,inside a cone of R = 0.3
Main conclusion of this study, prompt g in PHOS:
Measurement feasible for 20 < p Tg < 30 GeV/c
Sensitive to RAA of FF modifications larger than 5% for 0.1<z<0.5 where z = p T jet hadron /Eg
Study must be redone with new simulations and newer approach.
min
max
EMCal R
leading IP
TPC g
EMCal/PHOS
EMCal meeting

25. Tagging jets with photons in EMCal Study done by Guénolé Bourdaud
See Guénolé’s talk in last PWG4 meeting.
Tagging jets with photons in EMCal Study done by Guénolé Bourdaud
Generation of gamma-jet events, with Eg>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 Df = Dh = 0.01
Jet axis must be in Df=p  0.5
Construct fragmentation function with x
EMCal meeting

26. Subtract HI background
See Guénolé’s talk in last PWG4 meeting.
Tagging jets with photons in EMCal Fragmentation function calculation
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 x range.
EMCal meeting

27. See Guénolé’s talk in last PWG4 meeting.
Tagging jets with photons in EMCal Fragmentation function, Ejet = 100 GeV
Ratio pp / PbPb
EMCal meeting

28. See Guénolé’s talk in last PWG4 meeting.
Tagging jets with photons in EMCal Final Fragmentation function and Nuclear Modification Factor
Realistic spectrum simulated:
Eg-jet > 30 GeV
pp events with PYTHIA.
Pb-Pb events with PYTHIA+ »PYQUENafterburner«  merged in HIJING events.
1/10 of a year statistic.
x range usable is 0.5 < x < 3.2
No quenching
PbPb/pp
Bkg not substracted
With quenching
Bkg substracted
Deviation from 1 used to calculate systematical errors
PbPb/pp
EMCal meeting

29. Conclusion of gamma tagging jets with gamma in EMCal
The modification of the fragmentation function is usable with 30 GeV for 0.5 < x < 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.
EMCal meeting

30. PPR: To Do, Who Task Sub Task Who PID
Re-evaluate PID parameters with last AliRoot version
SUBATECH, 1 person
Re-study the g/p0 discrimination
EMCAL (gamma) - Central barrel (conversion) combination for Inv, Mass of p0 (not for PPR?)
1 person
Direct photons
Isolation cut
Re-evaluate IC with larger amount of events, specially jet-jet, PYTHIA
Gustavo
Compare predictions for different generators and theory
Decay photon subtraction with Invariant Mass
Gamma-jet
Improve HI background rejection
Guenole?
Estimate effect of jet-jet contamination after Isolation Cut
Code implementation in Analysis Frame
EMCal meeting

31. PPR: Data sets available and needed
Available (if the grid works, the SE is not down, God is kind, …)
Check
pp events
LHC08u: gamma (PHOS) – jet, 15 < Eg < -1 GeV, 3M events
LHC08p: gamma (PHOS) - jet quenched, qhat=50, 15 < Eg < -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
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?
EMCal meeting

32. Back-up
EMCal meeting

33. PHOS identified spectrum pp and PbPb annual statistics
ALICE-INT
G. Conesa et al., NIM A 580 (2007) 1446
Yaxian M. poster QM2008
IC: R =0.3, S(pT)=2 GeV/c
IC: R =0.2, pT>2 GeV/c
Statistics limits to ~100 GeV
EMCal meeting

34. jet-jet clusters: effect of the p0 trigger 1 p0 with pT > 5 GeV/c in EMCal per event
f(pT<30) = e pT
Number of clusters found in p0 triggered pp jet-jet events divided by clusters found in non triggered events.
From now on pp p0 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
EMCal meeting

35. Fidutial cuts With fidutial cuts With PID
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
EMCal meeting

36. Other approach: photon conversions Study performed by Ana Marin (GSI)
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 pTg > 20 GeV/c (very very rough stimations)
pp @
√s=14 TeV
√s=5.5A TeV
EMCal
20000
PHOS
3000
Tracking
4000
Loss of efficiency at high pT under investigation
Needs to be improved !
EMCal meeting

37. Azimuthal correlation: Direct g converted– charged particles Study performed by Ana Marin (GSI)
g detected in
Central Barrel
Isolation Cut:
R=0.2, pT>0.7GeV
EMCal meeting

38. Direct photon identification in EMCal: Generated and reconstructed+PID spectra: g-jet : pp @ √14 TeV
PYTHIA
50 % of g 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.
EMCal meeting

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

40. Generated spectra g-jet : pp @ √14 TeV
PYTHIA
50 % of g 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.
EMCal meeting

41. Generated spectra g-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.
EMCal meeting

42. Isolation Cut: Efficiency
Ratio isolated clusters / total clusters
No PID
√14 TeV
√5.5 TeV
√5.5 TeV, qhat = 50
g-jet
jet-jet
Isolation rejection for jet clusters can be better than 99 % in pp and quenched PbPb events and pT > 20 GeV/c
EMCal meeting

43. Isolation Cut: Efficiency pp @ √14 TeV
Ratio isolated clusters / total clusters
No PID
R=0.3
pTth = 0.5 GeV/c
g-jet
g-jet
jet-jet
jet-jet
EMCal meeting

44. Isolation Cut: Efficiency pp+PbPb @ √5.5 TeV, no quenching
Ratio isolated clusters / total clusters
No PID
R=0.3
pTth = 2 GeV/c
g-jet
g-jet
jet-jet
jet-jet
EMCal meeting

45. Isolation Cut: Efficiency pp+PbPb @ √5.5 TeV, qhat = 50
Ratio isolated clusters / total clusters
No PID
R=0.3
pTth = 2 GeV/c
g-jet
g-jet
jet-jet
jet-jet
EMCal meeting

46. Direct photon identification in EMCal:
Isolated Spectra
IC Parameters: R=0.4, pTth = 0.5 (pp), 2 (PbPb) GeV/c
14 TeV
5.5 TeV, qhat=0
5.5 TeV, qhat=50
Isolation + PID seem to work in pp and PbPb quenched events for pT > GeV/c.
I would like to have larger amount of jet-jet simulations to quantify better the effect of other particles background at high pT
EMCal meeting

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

48. Fraction of parton energy carried by the fragmentation photon
PYTHIA pp collisions √s=14 TeV
pTg > 5 GeV/c
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.
EMCal meeting

49. g-jet energy/phi smearing
PYTHIA pp collisions √s=14 TeV
pTg/pTjet > 0.9
pTg/pTjet > 0.5
Jet energy reconstructed with the PYTHIA jet finder.
We have to be careful when we say that g is back and has the energy of the jet. There is an smearing to take into account.
EMCal meeting

50. Correlation Function: Df
PYTHIA pp collisions √s=14 TeV
All charged hadrons with pT > 2 GeV/c
Plots for quark jets (similar for gluon jets)
As expected, without isolation fragmentation g have a correlation in the near and away sides, direct g only in the away side. Isolated fragmentation g only correlate in the away side
To understand: why away side descends a bit in isolated FSR photons.
EMCal meeting

51. Fragmentation Function: zT g
PYTHIA pp collisions √s=14 TeV
All charged hadrons with pT > 0.1 GeV/c inside cone of size R=1 around jet axis
pT g FSR / pT jet > 0.5
Gluon Jet
Quark Jet
Difference in FF for fragmentation and prompt due to z<1 for fragmentation photons?
EMCal meeting

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

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

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

55. Tagging jets with photons in PHOS Fragmentation function RFF
ALICE-INT
G. Conesa et al., NIM A 585(2008) 28
Systematic errors due
to jet(p0)-jet background
With quenched p0
Charged + EM
Sensitive to medium modifications at low z if larger than
~5% in both configurations.
Same result using TPC alone to reconstruct the jet
EMCal meeting

56. Tagging jets with photons in PHOS Conclusions and Summary
Enough counting rate to do the measurement only in the pT range 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:
pT smearing of generated particles
EMCal resolution similar to PHOS …
Method selects jets as a function of pT 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 pT cut.
Should be repeated with fully reconstructed events and a fine tuned method.
EMCal meeting

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