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EMCal meeting 1/34
Gustavo Conesa Balbastre
Review on direct photon measurement and jet
correlations with EMCal
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
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 < Ejet) Medium produced photon:
Bremsstrahlung and jet conversion (E < Ejet) 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)
EMCal meeting 4/34
Introduction:Why photon-tagged jets?
Medium effects redistribute (qL) the parton energy, Ejet, inside the hadron
jet (multiplicity, jT).
If we measure E ≈ Ejet
^̂AA BB
Redistribution can be best measured with the Fragmentation Function... If we know E Ejetjet..
HI environment hinders precise reconstruction of Ejet.
Jet
EMCal meeting 5/34
Introduction: How many direct photons?
10k/year
Large sample of direct LO -jet for pT < 30 GeV/c in PHOS and pT < 50 GeV/c in EMCal …
… but0 = 0,01-0,1 for pT > 10 GeV/cWe need a good 0 PID
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 clustersnot spherical shower shape analysis
5 - 40 GeV/c in EMCal 30 - 100 GeV/c in PHOS
increasing pT
Opening angle << 1 cellall 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 pT > pT thres
• or pT sum in cone, pT < pT
thres
EMCal meeting 7/34
PHOS identified spectrum pp and PbPb annual statistics
Yaxian M. poster QM2008
IC: R =0.3, (pT)=2 GeV/c IC: R =0.2, pT>2 GeV/c
At least one count up to ~100 GeV, counting rates for correlations good up to 30 GeV.
ALICE-INT-2005-014G. Conesa et al., NIM A 580 (2007) 1446
2 PHOS modules
EMCal meeting 8/34pGeV/cpGeV/c
-0 discrimination in EMCal: Shower Shape Analysis
identified as identified as
Uniform and 0 distributions, weighted to match INCNLO predictions
identified : 02 < 0.25
identified 0: 02 > 0.25
PID
eff
icie
ncy
as
as
See Cynthia’s H. talk about EMCal PID during PWG4 meeting in 11 December 2007 for more details
p+p
PID
eff
icie
ncy
Pb+Pb
identified as
identified as
as
as
EMCal meeting 9/34
/0 discrimination in EMCal Shower Shape Analysis
Nd
irec
t
/ N
from
pGeV/c pGeV/c
p+p 14 TeV
central Pb+Pb 5.5 TeVfactor 5 suppression
→ 0 suppression factor = 5-10 for pT =[10-30] GeV/c in p+p
→ 0 suppression factor > 5 for pT =[15-30] GeV/c in Pb+Pb
→ Ndirect / Nfrom ~ 1 in Pb+Pb for pT =[20-30] GeV/c
See Cynthia’s H. talk about EMCal PID during PWG4 meeting in 11 December 2007 for more details
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 pT 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
EMCal meeting 11/34
Direct photon identification in EMCal:Event generation
pp PYTHIA collisions : 3 simulation cases:
1) pp @ √14 TeV2) 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 pT>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 pT hard bins.
1) Pb-Pb collisions @ √s = 5.5A TeV: pp simulations 2) and 3) merged with HIJING central events (b < 3fm) @ √s = 5.5A TeV.
2) 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)
3) Full ALICE simulation with AliRoot 4.07 Release.4) Fidutial cut applied to all events.
See my talk in Prague ALICE meeting.
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.
PYTHIAPYTHIA
EMCal meeting 13/34
Cluster reconstruction efficiency is almost 90%.With shower shape selection the efficiency descends to 45-75%
PYTHIAPYTHIA
Direct photon identification in EMCal: Generated and reconstructed+PID spectra: -jet : pp+PbPb @ √5.5
TeV
EMCal meeting 14/34
Direct photon identification in EMCal: jet-jet clusters rejection with photon PID
pp @ pp @ √14 TeV√14 TeV PbPb @ PbPb @ √5.5 TeV√5.5 TeV
The jet cluster rejection goes from 0.3 (large value due to decay The jet cluster rejection goes from 0.3 (large value due to decay correctly identified correctly identified as as , no invariant mass analysis is done) to around 0.02 in pp collisions. There is an , 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 pincrease at 30-40/c GeV because PID for larger pTT cannot separate effectively cannot separate effectively
overlapped overlapped from from 00 decay. In Pb-Pb rejection worsens, from 0.5 to 0.05. decay. In Pb-Pb rejection worsens, from 0.5 to 0.05.
Decay Decay not overlapped, not overlapped,
Overlapped decay Overlapped decay rejection no more feasible rejection no more feasible
EMCal meeting 15/34
Direct photon identification in EMCal: Prompt photon / jet clusters with and without PID
pp @ pp @ √14 TeV√14 TeV PbPb @ PbPb @ √5.5 TeV, qhat=0√5.5 TeV, qhat=0 PbPb @ PbPb @ √5.5 TeV, qhat = 50√5.5 TeV, qhat = 50
With PID prompt With PID prompt to jet clusters (decay to jet clusters (decay , pi0, hadrons) , pi0, hadrons) ratio increases significantly but it is not enough.ratio increases significantly but it is not enough.
EMCal meeting 16/34
Direct photon identification in EMCal: Isolation Cut : Prompt photon / jet clusters
Ratio isolated clusters in -jet / isolated clusters in jet-jetClusters selected with PID 2
0< 0.25
pp @ pp @ √14 TeV√14 TeV PbPb @ PbPb @ √5.5 TeV√5.5 TeV PbPb @ PbPb @ √5.5 TeV, qhat = 50√5.5 TeV, qhat = 50
Prompt photons signal larger than jet-jet clusters Prompt photons signal larger than jet-jet clusters background for pbackground for pTT larger than around 15 GeV/c for pp larger than around 15 GeV/c for pp
and quenched PbPb eventsand quenched PbPb events
EMCal meeting 17/34
pp @ 14 TeVpp @ 14 TeV
Direct photon identification in EMCal:Isolated Spectra
PbPb @ 5.5 TeV, qhat=0PbPb @ 5.5 TeV, qhat=0 PbPb @ 5.5 TeV, qhat=50PbPb @ 5.5 TeV, qhat=50
With PIDWith PID
With PIDWith PID
With PIDWith PID
IC Parameters: R=0.4, pTth = 0.5 (pp), 2 (PbPb) GeV/c
Isolation seems to work in pp and PbPb quenched events for pT > 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 pT
EMCal meeting 18/34
Direct photon identification in EMCal: Prompt & Fragmentation photons: pT 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 pT hard bins.
Spectrum of all photons in || < 1 in all plots.
Isolation done on the pure PYTHIA particles, R=0.4, pT
th = 1 GeV/c. Particles with status code 1
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: pT distribution
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).
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.
EMCal meeting 22/34
Tagging jets with photonsOld approach
Search identified prompt photon (PHOS or EMCal) with largest pT (E > 20 GeV).
Strategy (event by event):
min max
leading
Search leading particle : -leading180º Eleading > 0.1 E
R
Reconstruct the jet in TPC and EMCal (if available):
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. PYTHIA events generated in a similar way as in
previous analysis.
IP
EMCal
TPC
ALICE-INT-2005-014
G. Conesa et al., NIM A 585(2008) 28
EMCal/PHOS
EMCal meeting 23/34
Pb-Pb collisions, pT, part > 0.5 GeV/c
Tagging jets with photons in PHOSReconstructed jet selection 40 GeV jets
TPC alone
TPC+EMCAL
p-p collisions, pT, part > 0.5 GeV/c
ALICE-INT-2005-014
G. Conesa et al., NIM A 585(2008) 28
EMCal meeting 24/34
TPC alone
TPC+EMCAL
p-p collisions, pT, part > 0.5 GeV/c
TPC alone
TPC+EMCAL
Pb-Pb collisions, pT, part > 2 GeV/c
ALICE-INT-2005-014
G. Conesa et al., NIM A 585(2008) 28
Tagging jets with photons in PHOSReconstructed jet selection 40 GeV jets
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 = pT, jet particle /E
Prompt identified in PHOS
If signal is quenchedIf 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 PHOSFragmentation function
EMCal meeting 26/34
Charged + EMWith quenched 0
PNNCAB
PAB
AB dT
dNR
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 PHOSFragmentation function RFF
EMCal meeting 27/34
Enough counting rate to do the measurement only in the pT 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: 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.
Tagging jets with photons in PHOSConclusions and Summary
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 EMCalStudy done by Guénolé Bourdaud
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
EMCal meeting 30/34
Ratio pp / PbPb
Tagging jets with photons in EMCal Fragmentation function, Ejet = 100 GeV
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 RFF
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.
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.
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 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?
EMCal meeting 35/34
Back-up
EMCal meeting 36/34
PHOS identified spectrum pp and PbPb annual statistics
Yaxian M. poster QM2008
IC: R =0.3, (pT)=2 GeV/c IC: R =0.2, pT>2 GeV/c
Statistics limits to ~100 GeV
ALICE-INT-2005-014G. Conesa et al., NIM A 580 (2007) 1446
EMCal meeting 37/34
jet-jet clusters: effect of the 0 trigger 1 0 with pT > 5 GeV/c in EMCal per event
Number of clusters found Number of clusters found in in 00 triggered pp jet-jet triggered pp jet-jet events divided by events divided by clusters found in non clusters found in non triggered events.triggered events.
From now on pp From now on pp triggered pp jet-jet triggered pp jet-jet events will be multiplied events will be multiplied by the function fitted in by the function fitted in the figure to apply the figure to apply correction for the correction for the triggering bias on the triggering bias on the hadron contribution. hadron contribution.
The fragmentation The fragmentation photon yield is much photon yield is much more suppressed (not more suppressed (not considered in the cluster considered in the cluster points and next plots)points and next plots)
Reconstructed clusters
Pythia generated particles (more events than in rec. clusters)Where the
correction should be more or less
f(pT<30) =0.86+1.55 e -0.072 pT
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 cutsWith fidutial cuts With fidutial cutsWith fidutial cutsWith PIDWith PID With PIDWith PID
EMCal meeting 39/34
Other approach: photon conversionsStudy performed by Ana Marin (GSI)
Loss of efficiency at high pT under investigation
Needs to be improved !
Identify photons converting in the beampipe, ITS and TPC
Clean photon identificationProvide 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 pT > 20 GeV/c (very very rough stimations)
pp @
√s=14 TeV
PbPb @ √s=5.5A TeV
EMCal 20000 20000
PHOS 3000 3000
Tracking 4000 4000
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, pT>0.7GeV
EMCal meeting 41/34
• pT threshold candidate isolated if:
• no particle in cone with pT > pT thres
• pT sum in cone, pT < pTthres
• 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 22R
Isolation cut method
R
PHOS/EMCal
TPC
candidate
IP
ALICE-INT-2005-014 - G. Conesa et al, NIM A 580 (2007) 1446
Reminder
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.
PYTHIAPYTHIA
EMCal meeting 43/34
Generated spectra jet : pp+PbPb @ √5.5 TeV
With PIDWith PID
With PIDWith 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 44/34
Isolation Cut: EfficiencyRatio isolated clusters / total clusters
No PID
-jet
jet-jet
pp @ pp @ √14 TeV√14 TeV PbPb @ PbPb @ √5.5 TeV√5.5 TeV PbPb @ PbPb @ √5.5 TeV, qhat = 50√5.5 TeV, qhat = 50
Isolation rejection for jet clusters can be better than 99 Isolation rejection for jet clusters can be better than 99 % in pp and quenched PbPb events and p% in pp and quenched PbPb events and pTT > 20 GeV/c > 20 GeV/c
EMCal meeting 45/34
Isolation Cut: Efficiency pp @ √14 TeV
R=0.3Ratio isolated clusters / total clusters
No PID pTth = 0.5 GeV/c
-jet
jet-jet jet-jet
-jet
EMCal meeting 46/34
Isolation Cut: Efficiency pp+PbPb @ √5.5 TeV, no quenching
R=0.3Ratio isolated clusters / total clusters
No PID pTth = 2 GeV/c
-jet
jet-jetjet-jet
-jet
EMCal meeting 47/34
Isolation Cut: Efficiency pp+PbPb @ √5.5 TeV, qhat = 50
R=0.3Ratio isolated clusters / total clusters
No PID pTth = 2 GeV/c
-jet
jet-jet jet-jet
-jet
EMCal meeting 48/34
Isolation efficiencyDirect / 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 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.
PYTHIA pp collisions √s=14 TeV
EMCal meeting 49/34
Fraction of parton energy carried by the fragmentation photon
pT > 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.
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
pT/pTjet > 0.5pT/pTjet > 0.9
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 sideTo understand: why away side descends a bit in isolated FSR photons.
All charged hadrons with pT > 2 GeV/c
PYTHIA pp collisions √s=14 TeV
EMCal meeting 52/34
Fragmentation Function: zT
All charged hadrons with pT > 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 Jet Quark JetpT FSR / pT jet > 0.5
EMCal meeting 53/34
100 GeV -jets, no bkg.
Fraction of reconstructed jet energy vs RReconstructed jet energy in HI /HI Background fluctuation
For Ejet = 30 GeV and R = 0.25 ! Ejet / fluctuations bkg = 2.
Tagging jets with photons in EMCalJet energy reconstruction