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Review on direct photon measurement and jet correlations with EMCal. Gustavo Conesa Balbastre. Outline. Introduction : Why direct photon measurement and correlation with jets. Photon / p 0 discrimination in EMCal : Shower shape analysis Summary of work done by Cynthia Hadjidakis - PowerPoint PPT Presentation
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4 Convegno sulla Fisica di ALICE
1/20
Review on direct photon measurement
and jet correlations with EMCal
Gustavo Conesa Balbastre
4 Convegno sulla Fisica di ALICE
2/20
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
4 Convegno sulla Fisica di ALICE
3/20
Partons traversing the medium of high colored density are modified. Energy loss through gluon
emission High pT jets are our
tomographical probe of QGP. Constant energy loss E ~ 20
GeV. Jet reconstruction in AA is not
feasible for energies smaller than 50 GeV and in general energy measurement is not very precise.
Direct photon tagged-jets are an useful observable.: EEjet Back-to-back in azimuth
The measurement: Particle species spectra
• S ( pTh)
• RAA = SNN/(Norm x Spp) Fragmentation function
• FF ( z = pTh/Ejet )
• RFF = FFNN/(Norm x FFpp)
Jet
prompt
Nucleus A
Nucleus B
parton parton
QGP
} Medium coefficient transport
Introduction: Why jets and photonsJet-quenching
4 Convegno sulla Fisica di ALICE
4/20
Introduction: Direct photon sources
Thermal radiation
PbPb
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)
γFragmentation
NLOHigh z isolated photonsHigh z isolated photons
γg g
q q
q
g
Jet
Prompt
LOIsolated photonsIsolated photons
γ γ
gg
q
q
q
q
Bremsstrahlung/jet coversion induced by medium
4 Convegno sulla Fisica di ALICE
5/20
Photon sources
Pre-equilibrium: Prompt photons
Equilibrium:Thermal-Bremsstrahlung-jet conversion photons
Freeze-outDecay photons
Photons are produced during all stages of the collision.
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 Convegno sulla Fisica di ALICE
6/20
RHIC : PHENIX RAA
Hadron suppression
of factor 5 at high pT. Leading jet particle
suppression Run 2: No direct
suppression (PRL 94, 232301 (2005)).
Run 4 (QM06): High pT direct suppression Isospin (PDF)
effect Fragmentation
photon suppression? Something else?
4 Convegno sulla Fisica di ALICE
7/20
RHIC : PHENIX RAA
PQM model
RAA is not a very good discriminator to calculate transport coefficient!Need to study what is really quenched, the parton-jet
~13
J. Nagle HP2008
4 Convegno sulla Fisica di ALICE
8/20
Jet fragmentation function with quenching
L. Cunqueiro HP2008
Suppresion of leading particles at low (as observed with RAA)
Increase of soft particles at high
4 Convegno sulla Fisica di ALICE
9/20
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
4 Convegno sulla Fisica di ALICE
10/20
-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
4 Convegno sulla Fisica di ALICE
11/20
/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
Photons have0 < 0.25
4 Convegno sulla Fisica di ALICE
12/20
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
4 Convegno sulla Fisica di ALICE
13/20
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 20< 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 Prompt photons signal larger than jet-jet clusters background for pclusters background for pTT larger than around larger than around
15 GeV/c for pp and quenched PbPb events15 GeV/c for pp and quenched PbPb events
Talk in Prague Physics week
4 Convegno sulla Fisica di ALICE
14/20
pp @ 14 TeVpp @ 14 TeV
Direct photon identification in EMCal:Isolated Spectra
PbPb @ 5.5 TeV, qhat=0PbPb @ 5.5 TeV, qhat=0PbPb @ 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
Talk in Prague Physics week
4 Convegno sulla Fisica di ALICE
15/20
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).
Pythia SimulationsNo detectors, no reconstruction
Talk in Prague, and April PWG4 meeting
4 Convegno sulla Fisica di ALICE
16/20
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. Can we use them safely as normal prompt photons? Isolation studies with hadrons show similar results.
4 Convegno sulla Fisica di ALICE
17/20
Tagging jet with photon
Search identified prompt photon (PHOS or EMCal) with largest pT (E > 20 GeV).
Strategy (event by event):
min max
LeadingJet core
Search leading particle or jet core :
-leading180ºR Reconstruct the jet in TPC and
EMCal (if available): With Standard jet algorithms, only high pT, not enough statistics.
Construct jet with particles around the leading or jet core inside a cone of size R
IP
EMCal
TPC
EMCal/PHOS
MonteCarlo studies: Pythia pp events to generate gamma and jet signal Hijing to generate PbPb background
4 Convegno sulla Fisica di ALICE
18/20
-tagged Fragmentation FunctionF and RFF
photon in PHOS, jets in TPC+EMCal
Systematic errors due to jet(0)-jet background
Sensitive to medium modifications at low z if larger than ~5% in both configurations.
ALICE-INT-2005-014
G. Conesa et al., NIM A 585(2008) 28
No quenching in simulation
PbPb fragmentation functionUE background removed statistically
If signal is quenchedIf signal is quenched If signal is quenchedIf signal is quenched
4 Convegno sulla Fisica di ALICE
19/20
Realistic spectrum simulated: E-jet > 30 GeV Jet core is the jet seed
1/10 of a year statistic. range usable is 0.5 < <
3.2
Tagging jets with photons in EMCal Final Fragmentation function and Nuclear Modification Factor
No quenching
Bkg not substracted
Bkg substracted
PbPb/pp
PbPb/pp With quenching
Deviation from 1 used to calculate systematical errors
G. BourdaudSee July PWG4 meeting
4 Convegno sulla Fisica di ALICE
20/20
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.
4 Convegno sulla Fisica di ALICE
21/20
Back-up
4 Convegno sulla Fisica di ALICE
22/20
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
4 Convegno sulla Fisica di ALICE
23/20pGeV/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
4 Convegno sulla Fisica di ALICE
24/20
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
4 Convegno sulla Fisica di ALICE
25/20
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.
1 +jet in final state (MSEL=10) – jet Prompt is the signal under study.
2) 2 jets in final state (MSEL=1) jet – jet These events constitute the background: decay , fragmentation and
hadrons.1) 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.1) 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.
4 Convegno sulla Fisica di ALICE
26/20
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 cuts
With PIDWith PID With PIDWith PID
4 Convegno sulla Fisica di ALICE
27/20
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
4 Convegno sulla Fisica di ALICE
28/20
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
4 Convegno sulla Fisica di ALICE
29/20
• 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 +φ=
Isolation cut method
R
PHOS/EMCal
TPC
candidate
IP
ALICE-INT-2005-014 - G. Conesa et al, NIM A 580 (2007) 1446
Reminder
4 Convegno sulla Fisica di ALICE
30/20
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
4 Convegno sulla Fisica di ALICE
31/20
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
4 Convegno sulla Fisica di ALICE
32/20
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
4 Convegno sulla Fisica di ALICE
33/20
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
4 Convegno sulla Fisica di ALICE
34/20
PYTHIA pp collisions √s=14 TeV
|| < 0.5
NLO calculation thanks to Lamia B.
Direct photon identification in EMCal:
Prompt & Fragmentation photons: pT
distribution
4 Convegno sulla Fisica di ALICE
35/20
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
4 Convegno sulla Fisica di ALICE
36/20
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
4 Convegno sulla Fisica di ALICE
37/20
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
4 Convegno sulla Fisica di ALICE
38/20
RHIC : PHENIX RAA
Hadron suppression
of factor 5 at high pT. Leading jet particle
suppression Run 2: No direct
suppression (PRL 94, 232301 (2005)).
Run 4 (QM06): High pT direct suppression Isospin (PDF)
effect Fragmentation
photon suppression? Something else?
4 Convegno sulla Fisica di ALICE
39/20
RHIC : PHENIX RAA
PQM model
RAA is not a very good discriminator to calculate transport coefficient!Need to study what is really quenched, the parton-jet
~13
J. Nagle HP2008
4 Convegno sulla Fisica di ALICE
40/20
How can we measure direct photons and jets in ALICE?
E > 10 GeV E/E < 1.5%, x =[0.5,2.5] mm
Tracking System resolution
p/p = 2%, =1.1º
E > 10 GeV E/E < 3 %
x =[3,50] mm
Currently under construction at LNF