1. Studies of prompt photon identification and 0 isolation in first p-p collisions at √s=10 TeV
May 20, 2009
Meeting Frascati
Raphaëlle Ichou

2. Integrated luminosity, nb-1
PHYSICS MOTIVATIONS :
Measure of high-pT prompt photons with EMCal in p-p collisions
Expected pT reach in 1st year EMCal:
Interest of Direct Photons :
1) Comparison with pQCD in p-p at the highest energies ever.
2) Constraint of gluon distribution function at low-x.
3) p-p baseline for photon production in A-A collisions.
Integrated luminosity, nb-1
Run time T
L, cm-2s-1
51027
51028
31030
3 days
1.3 13
780
1 month
130
7800
3 months 39
390
22000
The pT reach is around 40 GeV/c ( with 100 % reconstruction efficiency)

3. Large background of photons from 0-decay.
PHYSICS MOTIVATIONS :
Large background of photons from 0-decay.
Theoretical ratio in p-p collisions at √s= 14 TeV.
At high pT :
times more 0 than direct photons.
Indistinguishable from prompt photons:  
merge in EMCal above pT~ 30 GeV/c

4. WHY ISOLATED PHOTONS Not Isolated Fragmentation Prompt (LO) Isolated
Why isolation ?
- g-frag and photons from 0 come from the fragmentation
of a high pT parton : are mostly accompanied by hadrons.
- Direct photons : most of them (prompt) are isolated.

5. ISOLATED PHOTON MEASUREMENT
Isolation Method:
A photon is isolated if the sum of the pT of
the hadrons in a cone of radius R=( η2 + 2)
around the photon direction is smaller than
a fraction of the candidate pT
with ε = fraction of photon pT
In order to measure the isolated photon spectrum we need to determine:
- Efficiency of our photon isolation method (JETPHOX)
Contamination of background 0 that pass the photon isolation cuts (simulation)
Schematically:
JETPHOX (theoretical)
PYTHIA (full sim)

6. ISOLATED  EXTRACTION (THEORETICAL - I)
JETPHOX = JET-PHOton/hadron X sections:
NLO calculation in pQCD
( both prompt and fragmentation mechanisms)
Study of @ NLO
Find the optimal region in (ε ,R ) to remove a maximum of 0 & g-frag. background
Ideally, R as big as possible, ε as small as possible. But …
Theoretical issue:
when ε ->0 , ln(ε) divergence of pQCD cross section
Experimental issues: underlying event, mininum pT reconstructable in tracker …
Catani et al.JHEP 0205 (2002) 028

7. ISOLATED  EXTRACTION (THEORETICAL - II)
Ratio of isolated over inclusive photon x-sections (fixed R=0.7, =0.13): =
(pT=15-30 GeV/c)
(important contribution of g-frag)

8. ISOLATED  EXTRACTION (THEORETICAL - III)
What is the best choice of εmin ?
Ratio of isolated over inclusive photon X-sections :
@ pT=30 GeV/c for R=0.7
Optimal choice of εmin at pT= 30 GeV/c:
The study of does
not enable to constrain ε.
Underlying event(*) pT(UE)=1-3 GeV/c inside R=0.7 requires:
εmin > pT(UE)/pTg ~
(*) PYTHIA, HERWIG pp 10 TeV

9. ISOLATED 0 CONTAMINATION (SIMULATION - I)
Goal :
- Test isolation method in pT range where we can identify p0 in a model-independent way (= without accessing MC truth, via shower-shape).
- Estimate the isolated photon fraction that are not direct and correspond to isolated 0
Jet-Jet events generation and analysis :
PYTHIA √s = 10 TeV
events in [30-100] GeV/ c with pT0 > 30 GeV/c.
Clusters in EMCAL (-1< η <1 and 70<<200)
AliRoot version16.Rev06
Gustavo Conesa analysis frame work

10. ISOLATED 0 CONTAMINATION (SIMULATION - II)
Efficiency of the 0 identification :
Ratio of identified 0 by shower shape analysis (bayesian method) over simulated 0
The identified 0 obtained from a full detector simulation and reconstruction by shower shape analysis
Last March version
of the analysis frame work
Ongoing developpement with Gustavo…
Shower-shape 0 identification works up to 50 GeV/c with ~30% efficiency

11. How many of these 0 pass the isolation cut (for varying e,R)?
ISOLATED 0 CONTAMINATION (SIMULATION - III)
How many of these 0 pass the isolation cut (for varying e,R)?
The hadronic activity inside the cone is calculated with charged particles (from TPC) and neutral particles in EMCal
ε=0.1, R=0.4
ε=0.2, R=0.4
ε =0.1 and R=0.7:
~2%
ε=0.1, R=0.7
ε=0.2, R=0.7

12. of the analysis frame work
ISOLATED 0 CONTAMINATION (SIMULATION - I)
ISOLATED 0 CONTAMINATION (SIMULATION - IV)
Preliminary MC
ESD
Last March version
of the analysis frame work

13. SUMMARY & CONCLUSIONS ~ 1. JetPHOX: 0.7 PYTHIA: 50 Yellow Report :
~ 1.
JetPHOX:
0.7
PYTHIA: 50
Yellow Report :
0.03
We want to measure prompt photons at high pT with EMCal, where the background from 0 is very large.
At 30 GeV/c, we have ~ 30 times more 0 than direct photons.
Using isolation cut methods, we can get rid of many photons from 0-decay (and g-fragmentation).
The fraction of p0 that pass the isolation cut is ~2% (at pT = 30 GeV/c, R=0.7, ε=0.1).
The efficiency of photon isolation is ~70% (at pT = 30 GeV/c, R=0.7, ε=0.1).
After isolation, we find a ratio of isolated direct g(signal) over 0 (background) of ~ 1.

14. What I’ve shown: PPR Outlook: OUTLOOK
Goal: Measurement of isolated photon spectrum in pp at 10 TeV.
Isolation study at NLO with JETPHOX: how to get isolated over direct photon cross-section and extraction of good isolation parameters.
Preliminary study of the efficiency of the isolation method on background jet events generated with PYTHIA, to determine the background of isolated 0 photons. We can evaluate, in a first approximation (ideal conditions, few events), a remaining background after isolation cuts and improve the S/B ratio using isolation cuts.
PPR Outlook:
Perform the same study of the isolation efficiency in more realistic conditions: more events, full simulation-reconstruction in EMCal (using GRID data files):
0 identification with invariant mass analysis

15. Back up slides

16.  and 0 expected cross-sections
NLO pQCD + CTEQ5M + KPP.
Bands indicate possible
uncertainties in QCD scale
[P. Aurenche, et al.,
Eur. Phys. J. C 13,347 (2000)]
ppX
pp0X
D.Peressounko, QM09