Transcript PHOS off-line activities

Review on direct photon
measurement and jet
correlations with EMCal
Gustavo Conesa Balbastre
4 Convegno sulla Fisica di ALICE
1/20
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 PHOS: Summary of work made by Gustavo Conesa
 Gamma in EMCal: Summary of work made by Guénolé Bourdaud
4 Convegno sulla Fisica di ALICE
2/20
Introduction: Why jets and photons
Jet-quenching


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 DE ~ 20 GeV.
Jet reconstruction in AA is not
feasible for energies smaller than 50
GeV and in general energy
measurement is not very precise.




Jet
Nucleus A
parton
parton
Nucleus B
QGP
Direct photon tagged-jets are an
useful observable.:

EEjet

Back-to-back in azimuth
 prompt
The measurement:

1.
1.

Particle species spectra
S ( pTh)
RAA = SNN/(Norm x Spp)
Fragmentation function
1.
FF ( z = pTh/Ejet )
1.
RFF = FFNN/(Norm x FFpp)
}
Medium coefficient
transport
4 Convegno sulla Fisica di ALICE
3/20

Introduction: Direct
photon sources
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)

Fragmentation photons (E < Ejet)
Medium modified production:
Medium produced photon:

q
q
Prompt
q
γ
g
LO
g

γ
Fragmentation
g
q

q
g

Jet
Bremsstrahlung/jet
coversion induced
by medium
Pb
γ
q
γ

Isolated photons

Bremsstrahlung and jet conversion (E < Ejet)
Thermal photons (E < 10 GeV)
g
NLO
q
High z isolated photons
QGP
Pb

Thermal radiation

4 Convegno sulla Fisica di ALICE
4/20
Photon sources


Photons are produced during all stages of
the collision.
Challenge:


Disentangle the different sources.
Neutral mesons decay.


Pre-equilibrium:
Prompt photons
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)
Equilibrium:
Thermal-Bremsstrahlungjet conversion photons
Freeze-out
Decay photons
4 Convegno sulla Fisica di ALICE
5/20
RHIC : PHENIX RAA

Hadron suppression of
factor 5 at high pT.



Run 2: No direct 
suppression (PRL 94,
232301 (2005)).
Run 4 (QM06): High pT
direct  suppression



4 Convegno sulla Fisica di ALICE
Leading jet particle
suppression
Isospin (PDF) effect
Fragmentation
photon suppression?
Something else?
6/20
RHIC : PHENIX RAA
PQM model
J. Nagle HP2008
~13
RAA is not a very good discriminator to calculate transport coefficient!
Need to study what is really quenched, the parton-jet
4 Convegno sulla Fisica di ALICE
7/20
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
8/20
Introduction: How many direct photons?
10k/year
… but /p0 = 0,01-0,1 for pT > 10 GeV/c
We need a good /p0 PID
Large sample of direct LO -jet
for pT < 30 GeV/c in PHOS and pT
4 Convegno sulla Fisica di ALICE
< 50 GeV/c in EMCal …
9/20
-p0 discrimination
Three regions of analysis
increasing pT
well separated
clusters
 invariant mass
analysis
merged clusters
not spherical
 shower shape
analysis
Opening angle << 1 cell
all p0’s at this energy
are in jets
 isolation cut
< 15 GeV/c in EMCal
< 30 GeV/c in PHOS
5 - 40 GeV/c in EMCal
30 - 100 GeV/c in PHOS
> 40 GeV/c only method in
EMCal
IP

TPC

R
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Isolated if:
•no particle in
cone with
pT > pT thres
• or pT sum in
cone,
SpT < SpTthres
candidate
PHOS/EMCal
10/20
/p0 discrimination in EMCal
Ndirect / Nfrom p0
Shower Shape Analysis
See Cynthia’s H. talk
about EMCal PID
during PWG4 meeting
in 11 December 2007
for more details
p+p 14 TeV
central Pb+Pb 5.5 TeV
factor 5 p0 suppression
pT(GeV/c)
pT(GeV/c)
Photons have0 < 0.25
→ 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 / Nfrom p0 ~ 1 in Pb+Pb for pT =[20-30] GeV/c
4 Convegno sulla Fisica di ALICE
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/p0 discrimination in EMCal
Shower Shape Analysis, Summary and conclusions





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
p0 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 /p0 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
4 Convegno sulla Fisica di ALICE
12/20
Direct photon identification in EMCal:
Isolation Cut : Prompt photon / jet clusters
Talk in Prague
Physics week
Ratio isolated clusters in -jet / isolated clusters in jet-jet
Clusters selected with PID 20< 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 pT larger than around 15 GeV/c for pp and
4 Convegno sulla Fisica di ALICE
quenched PbPb events
13/20
Direct photon identification in EMCal:
Isolated Spectra
Talk in Prague
Physics week
IC Parameters: R=0.4, pTth = 0.5 (pp), 2 (PbPb) GeV/c
pp @ 14 TeV
PbPb @ 5.5 TeV, qhat=0
PbPb @ 5.5 TeV, qhat=50
With PID
With PID
With PID
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
4 Convegno sulla Fisica di ALICE
14/20
Direct photon identification in EMCal:
Direct & Fragmentation photons Isolation
Talk in Prague,
and April
efficiency. PWG4 meeting
Pythia Simulations
No detectors,
no reconstruction


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
Convegno
sulla all
Fisica
ALICE energy (more in back-up).
fragmentation photons4 do
not carry
thediparton
15/20
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
16/20
Tagging jet with photon

Strategy (event by event):


Search identified prompt photon (PHOS or
EMCal) with largest pT (E  > 20 GeV).
Search leading particle or jet core :



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
MonteCarlo studies:


Pythia pp events to generate gamma and jet signal
Hijing to generate PbPb background
4 Convegno sulla Fisica di ALICE
max
EMCal
Reconstruct the jet in TPC and EMCal (if
available):


-leading180º
min
R
Leading
Jet core
TPC
IP

EMCal/PHOS
17/20
-tagged Fragmentation FunctionF and RFF
photon in PHOS, jets in TPC+EMCal
ALICE-INT-2005-014
G. Conesa et al., NIM A 585(2008) 28
PbPb fragmentation function
UE background removed statistically
Systematic errors due
to jet(p0)-jet background
No quenching
in simulation
If signal is quenched
If signal is quenched
Sensitive to medium modifications at low z if larger than
~5% in both configurations.
4 Convegno sulla Fisica di ALICE
18/20
Tagging jets with photons in EMCal
G. Bourdaud
See July PWG4
meeting
Final Fragmentation function and Nuclear Modification Factor
Realistic spectrum simulated:



PbPb/pp

E-jet > 30 GeV
Jet core is the jet seed
1/10 of a year statistic.
 range usable is 0.5 <  < 3.2
No quenching
Bkg not substracted
Bkg substracted
PbPb/pp

With quenching
Deviation from 1 used to calculate systematical errors
4 Convegno sulla Fisica di ALICE
19/20
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
20/20
Back-up
4 Convegno sulla Fisica di ALICE
21/20
PHOS identified spectrum
pp and PbPb annual statistics
Yaxian M. poster QM2008
ALICE-INT-2005-014
G. Conesa et al., NIM A 580 (2007) 1446
2 PHOS modules
IC: R =0.3, S(pT)=2 GeV/c
IC: R =0.2, pT>2 GeV/c
At least one count up to ~100 GeV, counting rates for
4 Convegno sulla Fisica di ALICE
correlations good up to 30 GeV.
22/20
See Cynthia’s H. talk
about EMCal PID
during PWG4 meeting
in 11 December 2007
for more details
-p0 discrimination in EMCal:
Shower Shape Analysis
 identified as 
 identified as p0
PID efficiency
PID efficiency
Uniform  and p0 distributions, weighted to match
INCNLO predictions
identified : 02 < 0.25
0:  2 > 0.25
identified
p
0
Pb+Pb
p+p
p0 as p0
p0 as 
pT(GeV/c)
4 Convegno sulla Fisica di ALICE
 identified as 
 identified as p0
p0 as p0
p0 as 
pT(GeV/c)
23/20
PHOS identified spectrum
pp and PbPb annual statistics
ALICE-INT-2005-014
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
4 Convegno sulla Fisica di ALICE
24/20
See my talk in
Prague ALICE
meeting.
Direct photon identification in EMCal:
Event generation

pp PYTHIA collisions :

3 simulation cases:
1)
2)
3)

+jet in final state







(MSEL=10)
Prompt  is the signal under study.
2 jets in final state


pp @ √14 TeV
pp @ √5.5 TeV, merged with HIJING, no quenching in PYTHIA.
pp @ √5.5 TeV, merged with HIJING, quenching: qhat = 50.
  – jet
(MSEL=1)
 jet – jet
These events constitute the background: decay , fragmentation  and hadrons.
Jet-Jet Event generation with jets containing at least one p0 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.
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 02 < 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.
4 Convegno sulla Fisica di ALICE
25/20
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 cuts
With PID
With fidutial cuts
With PID
4 Convegno sulla Fisica di ALICE
26/20
Direct photon identification in EMCal:
Generated and reconstructed+PID spectra: -jet : pp @ √14 TeV
PYTHIA
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
4 Convegno sulla Fisica di ALICE
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of some converted clusters.
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 pT > 20
GeV/c (very very rough stimations)

Loss of efficiency at high pT under
investigation
Needs to be improved !
pp @
√s=14 TeV
PbPb @
√s=5.5A TeV
EMCal
20000
20000
PHOS
3000
3000
Tracking
4000
4000
4 Convegno sulla Fisica di ALICE
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Isolation cut method
Reminder
ALICE-INT-2005-014 - G. Conesa et al, NIM A 580 (2007) 1446
•
•
Prompt  are likely to be produced isolated.
Two parameters define  isolation:
• Cone size R  D2  D2
• pT threshold candidate isolated if:
• no particle in cone with pT > pT thres
• pT sum in cone, SpT < SpTthres
• p T min of all particles (charged and neutral)
in cone is at least 0.5 GeV/c
• Consider in cone particles :
IP

TPC

R
candidate
PHOS/EMCal
• Charged in TPC acceptance: ||<0.7, 0º<<360º
• Neutral in
• EMCal acceptance: ||<0.7, 80º<<190º
• PHOS acceptance: ||<0.12, 220º<<320º
4 Convegno sulla Fisica di ALICE
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Tagging jets with photons in PHOS
Reconstructed jet selection 40 GeV jets
p-p collisions, pT, part > 0.5 GeV/c
ALICE-INT-2005-014
G. Conesa et al., NIM A 585(2008) 28
Pb-Pb collisions, pT, part > 0.5 GeV/c
TPC alone
TPC+EMCAL
4 Convegno sulla Fisica di ALICE
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Tagging jets with photons in PHOS
Reconstructed jet selection 40 GeV jets
p-p collisions, pT, part > 0.5 GeV/c
TPC alone
ALICE-INT-2005-014
G. Conesa et al., NIM A 585(2008) 28
Pb-Pb collisions, pT, part > 2 GeV/c
TPC alone
TPC+EMCAL
TPC+EMCAL
4 Convegno sulla Fisica di ALICE
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Tagging jets with photons in PHOS
Fragmentation function
ALICE-INT-2005-014
G. Conesa et al., NIM A 585(2008) 28
z = pT, jet particle /E 
Any neutral signal in PHOS
Background
Prompt  identified in PHOS
If signal is quenched
HIC background
Pb-Pb collisions Signal
E  > 20 GeV/c; TPC+EMCal detect jet particles, PHOS 
HI Background subtracted statistically, accumulate a fake fragmentation
4 Convegno sulla Fisica di ALICE
function with particle in cone at opposite  and same  as jet
32/20
Direct photon identification in EMCal:
Prompt & Fragmentation photons: pT distribution
PYTHIA pp collisions √s=14 TeV

Pythia produces non hadronic
decay photons:

|| < 1




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, pTth = 1
GeV/c. Particles with status
code 1
4 Convegno sulla Fisica di ALICE
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Direct photon identification in EMCal:
Prompt & Fragmentation photons: pT distribution
PYTHIA pp collisions √s=14 TeV
|| < 0.5
NLO calculation
thanks to Lamia B.
4 Convegno sulla Fisica di ALICE
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Correlation Function: D
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  have a correlation in the
near and away sides, direct  only in the away side. Isolated fragmentation
 only correlate in the away side
4 Convegno sulla Fisica di ALICE
To understand: why away side descends a bit in isolated FSR photons.
35/20
Fragmentation Function: zT 
PYTHIA pp collisions √s=14 TeV
All charged hadrons with pT > 0.1 GeV/c inside cone of size R=1 around jet axis
Gluon Jet
pT  FSR / pT jet > 0.5
Quark Jet
Difference in FF for fragmentation and prompt due to z<1 for
4 Convegno sulla Fisica di ALICE
fragmentation photons?
36/20
Tagging jets with photons in EMCal
Jet energy reconstruction
Fraction of reconstructed jet energy vs R
Reconstructed jet energy in HI /
HI Background fluctuation
100 GeV -jets, no bkg.
For Ejet = 30 GeV and R = 0.25 ! Ejet / fluctuations bkg = 2.
4 Convegno sulla Fisica di ALICE
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RHIC : PHENIX RAA

Hadron suppression of
factor 5 at high pT.



Run 2: No direct 
suppression (PRL 94,
232301 (2005)).
Run 4 (QM06): High pT
direct  suppression



4 Convegno sulla Fisica di ALICE
Leading jet particle
suppression
Isospin (PDF) effect
Fragmentation
photon suppression?
Something else?
38/20
RHIC : PHENIX RAA
PQM model
J. Nagle HP2008
~13
RAA is not a very good discriminator to calculate transport coefficient!
Need to study what is really quenched, the parton-jet
4 Convegno sulla Fisica di ALICE
39/20
How can we measure direct photons and jets in
ALICE?
Currently under
construction at LNF
E > 10 GeV  DE/E < 3 %
sx =[3,50] mm
Tracking System resolution
Dp/p = 2%,  =1.1º
E > 10 GeV  DE/E < 1.5%,
sx =[0.5,2.5] mm
4 Convegno sulla Fisica di ALICE
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