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

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)

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 : - 10-100 times more 0 than direct photons. Indistinguishable from prompt photons: 0  merge in EMCal above pT~ 30 GeV/c

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.

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)

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

ISOLATED  EXTRACTION (THEORETICAL - II) Ratio of isolated over inclusive photon x-sections (fixed R=0.7, =0.13): = 0.4-0.7 (pT=15-30 GeV/c) (important contribution of g-frag)

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 ~ 0.03-0.1 (*) PYTHIA, HERWIG pp 10 TeV

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 pp@ √s = 10 TeV 10 000 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

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

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

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

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.

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

Back up slides

 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