1. 1 Guénolé BOURDAUD Guenole.bourdaud@subatech.in2p3.fr Gamma-jet physics with the Electromagnetic Calorimeter (EMCal) in ALICE experiment at LHC

2. 2 Hard Probes in QGP Study, historical view ~98%~ 50%~ 2%  hard /  tot Alice was designed before RHIC results. Lessons from RHIC : Need dedicated detectors for high p T and Hard probes > EMCal QGP Initial state (partonic) observations. Explosion of hard probes New matter state. Final state (hadronic) observations. Emergence of hard probes. Measurement 200820001994 Global Observables ~1994~1990~1980 Start of construction LHCRHICSPS

3. 3 Hard probes : interest of LHC Heavy ion dedicated experiment : ALICE More jets, higher p T and access to gamma-jet correlations with enough statistics. 2008 : first pp collisions at 14 TeV… 2009 : collisions Pb-Pb low luminositiy 2010 : collisions Pb-Pb high luminositiy

4. 4 Jet quenching at RHIC Hard scatterings in nucleon collisions produce jets of particles. In the presence of a color-deconfined medium, the partons strongly interact loosing a significant part of their energy. Energy loss : “Jet Quenching” Redistribution of jet energy : - less particles at high pT : interaction with medium : seen @ RHIC + more particles at low pT : radiations of gluons : visible @ ALICE ?

5. 5 ALICE Central tracking + EMCal : Well designed detectors to study jet quenching effect Alice central traking :  = +/- 0.9  = 360° Charged particles High particle identification capabilities Low p T possibilities For neutral particles : PHOS : high granularity spectrometer EMCAL : large aceptance calorimeter

6. 6 ALICE Central tracking + EMCal : Well designed detectors to study jet quenching effect EMCal & TPC : Possibility to reconstruct jet event by event. Full jet reconstruction (neutral and charged) Gamma detection (for gamma-jet). EMCal : U.S.-Italy-France project  = +/- 0.7  = 110° 11 modules Ready for run 2009 (1 module)

7. 7 Direct prompt  g+q   +q (Compton) q+q   +g (Annihilation) – Parton in-medium modification perturb final hadronic state (jet-quenching) – Prompt photons are not perturbed by the medium – Prompt photons gives jet energy. Initial Photon (  prompt). Initial parton Leading Particle Jet Want to study in medium interaction by jet modification (p-p vs Pb-Pb) Need to obtain the jet energy : gamma-jet is a good solution. Gamma-jet _

8. 8 gamma-jet in ALICE Gamma-jet in EMCal is one order of magnitude higher than in PHOS. ~ 10k events for energy higher than 30 GeV / year Need the high geometrical acceptance of EMCal Not seen at RHIC (not enough statistic) PHOS has a lower geometrical acceptance, can not see (enough) Gamma-jets Hard to observe : background from jet-jet and heavy-ion collision.

9. 9 x Dashed red : quenched black : unquenched Schematic example Fragmentation function & Hump-backed Plateau Depletion of High energy particles to increase the number of low energy particles

10. 10 180° EMCal p T leading > 10% p T  prompt p T Jet ~ p T  prompt Beam axis TPC Moyens Algorithm for gamma-jet reconstruction

11. 11 Gamma selection Particle identification : Pid (based on Shower Shape Analysis : SSA) Method developped for EMCal, can distiguish ,  0 and other hadrons. Efficiency ~55 % and Purity ~70 % for  Gamma energy higher than 30 GeV to avoid backgroud photons (thermal, decay, pi 0 …) Isolation cut :  energy higher than 10% of the energy in a cone centered around it. If exact,  is isolated and so considered as a prompt . SSA

12. 12 Jet reconstruction Cuts & parameters Azimutal correlation, angular selection :  (  - jet) < 0,1 rad Leading particle E lead /E  < 0,1 Reduced cone algorithme R=0.3 =  (  ²+  ²) (backgroud study in progress) Try to use same parameters in p-p and Pb-Pb

13. 13 x  =ln(1/x) 1/N dN/d  Fragmentation function Ejet=30GeV. Humpbacked plateau 1/N dN/dx Reconstruction of Fragmentation Fonction & humpbacked plateau. quenched but no background unquenched Pythia simulation, full geant reconstruction Quenching with pyquen. I.P. Lokhtin, A.M. Snigirev, Eur. Phys. J. C 46 (2006)211-217 q ~ 30 GeV 2 /fm No background 71% of gammas found 72 % of jet found if gamma found. ^

14. 14 Estimating the heavy-ion background The modification of the hump-backed plateau is dominated by background in Pb-Pb collisions. Measured S/B : S/B ~ 10 -2 for  > 2.5 (p T < 2.46 GeV) quenched signal only : no background Signal + background Measured S/B

15. 15 With the heavy-ion background & errors The modification of the hump-backed plateau is dominated by background in Pb-Pb collisions. Need background corrections. Actually : Can see depletion of high energy particles (seen at RHIC) Need bkg. subtraction unquenched quenched with background quenched but no background ratio unquenched/quenched With background Need bkg substraction ratio unquenched/quenched Without background Only Signal  =ln(1/x) 1/N dN/d  1/N dN/dx x Hb. pLat. PB/p

16. 16 Conclusion With the EMCAL, ALICE will greatly improve its direct photon and jet physics capabilities. These new, more accessible, probes, will help ALICE to study the Quark Gluon Plasma in greater detail. gamma-jets give access to redistribution of the jet energy. Show the depletion for high energy particles. The low pT particles increase has to be studied by background subtraction.

17. 17

18. 18 More …

19. 19 Jet-quenching at RHIC  s = 200A GeV 1/N trigger dN/d(  ) Near side Away side

20. 20 Jet-quenching at RHIC  s = 200A GeV 1/N trigger dN/d(  ) Near side Away side

21. 21 Jet-quenching at RHIC  s = 200A GeV 1/N trigger dN/d(  ) The back to back jet correlation is lost due to hard interaction in plasma. The rare process (Hard Scattering or “Jets”) is the probe of whether the soft production products form a medium.

22. 22 Conclusion – gamma-JETS It is possible to show the decrease of high energy particles, not yet the increase of low energy particle (Background subtraction in progress) Need: Systematic study of cuts in algorithm Jet-jet pollution of signal (isolation cut study)

23. 23 PID, shower shape Shower shape 0 : Cluster in EMCal higher energy °°  °° °°      Gustavo Conesa, thesis : University of Nantes and University of Valencia, 2005 ALICE-INT-2005-053 A tower

24. 24 Apport de EMCal dans ALICE : Énergie totale du jet (E’+  E) mesurée avec des biais réduits et une meilleure résolution Meilleure définition de la fonction de fragmentation, à bas z (dynamique de la perte d’énergie) Déclenchement efficace sur les jets d’énergie élevée Trajectographie centrale permet l’étude :  de la réponse du milieu (partie soft)  de la composition des jets atténués L’expérience idéale pour la physique des grands p T : Calorimètres tels ceux d’ATLAS et de CMS  Résolution (détecteur, mesure des jets) meilleure  Acceptance plus grande Trajectographie et Identification de Particules d’ALICE La réalité : des expériences complémentaires Atouts de ATLAS/CMS :  Taux élevés  domaine cinématique étendu (> 350 GeV)  Études  -jet, Z-jet (avec des statistiques limitées) Atouts de ALICE : cf ci-dessus

25. 25 TPC acceptance = 90% ALICE dN/d  =6000 CMS dN/d  = 3200

26. 26

27. 27

28. 28

29. 29

30. 30