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Gustavo Conesa Balbastre

High p T direct photon measurement and correlations with hadrons and jets in ALICE. Gustavo Conesa Balbastre. Direct photons as probes for QGP. Direct EM probes convey unperturbed information and their production probe the medium Tag medium-modified jets:

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Gustavo Conesa Balbastre

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  1. High pT direct photon measurement and correlations with hadrons and jets in ALICE Gustavo Conesa Balbastre High pT physics at LHC

  2. Direct photons as probes for QGP Direct EM probes convey unperturbed information and their production probe the medium Tag medium-modified jets: Prompt photons from 2->2 hard process (Eg > 10 GeV) Medium modified production: Fragmentation photons (Eg < Ejet) Medium produced photon: Bremsstrahlung and jet conversion (Eg < Ejet) Thermal photons (Eg < 10 GeV) 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) High pT physics at LHC

  3. Fragmentation g NLO g g High z isolated photons Jet Bremsstrahlung/jet coversion induced by medium thermal radiation g g Direct photon sources prompt LO Isolated photons High pT physics at LHC

  4. Estimates with a thermal model Turbide, Gale, Jeon, and Moore PRC (2004) • Jet bremsstrahlung/fragmentation correlated with hadrons • Jet-plasma & thermal, uncorrelated High pT physics at LHC Gale QM2008

  5. Unambiguous signature of medium produced photons How can we distinguish different direct photon sources? • Prompt : RAA = 1, v2=0 • Fragmentation: RAA<1, v2>0 • Thermal, Bremsstrahlung, Jet Conversion: RAA>1, v2<0 High pT physics at LHC

  6. RHIC News: PHENIX RAA Gale QM2008 • Run 2: No  suppression (PRL 94, 232301 (2005)). • Run 4 (QM06): High pT suppression • Isospin (PDF) effect • Fragmentation photon suppresion? • Something else? High pT physics at LHC

  7. RHIC News : PHENIX v2 Gale QM2008 v2: small! Consistent with zero (within errors) High pT physics at LHC

  8. Jet g Why photon-tagged jets? ^ • Medium effects redistribute (qL) the parton energy, Ejet, inside the hadron jet (multiplicity, jT). A B • Redistribution can be best measured with the Fragmentation Function... If we know Ejet. • HI environment hinders precise reconstruction of Ejet. • If we measure Eg≈ Ejet High pT physics at LHC

  9. RHIC news : PHENIXg-h correlations Nguyen QM2008 Use Near Side peak to determine direct  associated with h, I.e. fragmentation photons. Statistical subtraction of decay photons. High pT physics at LHC

  10. RHIC news : PHENIXh-g correlations I Nguyen QM2008 X10-3 h-inclusive g X10-3 h-decay g X10-6 2.5 < pT,g < 3.5 10-6 h-direct g High pT physics at LHC • Idea: By triggering on a hadron and looking for near-side direct photon partners one can measure the fragmentation photon yield directly • Measure hadron - inclusive g and hadron - decay g correlations • Decay corr’s are made by tagging p0 and h by invariant mass • Must know tagging efficiency and false tagging rate precisely  dominant source of systematic uncertainty Near Side

  11. RHIC news : PHENIXh-g correlations II Nguyen QM2008 • First measurement of it’s kind at RHIC! • Will measure jet shape distributions, e.g. xE, pout • Constrain photon FF? • Nfrag/Ninc ≈ 0.1 at intermediate pT • Measure in Au+Au High pT physics at LHC 11

  12. RHIC news : STARg -h correlations I Hamed QM2008 The -rich sample has lower near-side yield than 0. High pT physics at LHC 12

  13. "X-N Wang & H. Zhang et al (to be published)" "T. Renk and K. Eskola, Phys.Rev.C75:054910,2007" RHIC news : STARg -h correlations II Hamed QM2008 IAA of p0 IAA of direct • Within the current uncertainty in the scaling the IAA of direct  and 0 are similar. High pT physics at LHC

  14. How can we measure direct photons in ALICE? EMCal D =110º || < 0.7 E > 10 GeV DE/E <3 % sx =[3,50] mm TPC D =360º || < 0.9 Dp/p= 2%, =1.1º PHOS ITS D =100º || < 0.12 E > 10 GeV DE/E < 1.5%, sx=[0.5,2.5] mm High pT physics at LHC

  15. … but g/p0 = 0,01-0,1 for pT > 10 GeV/c We need a good g/p0 PID How many direct photons? 10k/year Large sample of direct LO g-jet for pT < 30 GeV/c in PHOS and pT < 50 GeV/c in EMCal … High pT physics at LHC

  16. merged clusters not spherical  shower shape analysis 10 - 30 GeV/c in EMCal 30 - 100 GeV/c in PHOS Opening angle << 1 cell all 0’s at this energy are in jets  isolation cut > 30 GeV/c only method in EMCal • Isolated if: • no particle in cone with pT > pTthres • or pTsum in cone, SpT < SpTthres IP TPC   R candidate PHOS/EMCal -0 discrimination Three regions of analysis increasing pT • well separated clusters • invariant mass analysis • < 10 GeV/c in EMCal • < 30 GeV/c in PHOS High pT physics at LHC

  17. 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 High pT physics at LHC

  18. Direct g identification in EMCal:Event generation • pp PYTHIA collisions : • 3 simulation cases: • pp @ √14 TeV • pp @ √5.5 TeV, merged with HIJING, no quenching in PYTHIA. • pp @ √5.5 TeV, merged with HIJING, quenching: qhat = 50. • g+jet in final state (MSEL=10)  – jet • Prompt gis the signal under study. • 2 jets in final state (MSEL=1)  jet –jet • These events constitute the background: decay g,fragmentation g 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. • 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: ifl02 < 0.25 cluster is a photon (Cynthia H., PWG4 meeting, 12/07) • Full ALICE simulation with AliRoot 4.07 Release. High pT physics at LHC

  19. With PID With PID Generated spectrag-jet : pp @ √14 TeV PYTHIA 50 % of g 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. PID rejects 15% of clusters generated by real photons and from 50 to 25% of converted photons. High pT physics at LHC

  20. With PID With PID Generated spectrag-jet : pp+PbPb @ √5.5 TeV Cluster reconstruction efficiency is almost 90%. With shower shape selection the efficiency descends to 45-75% PID rejects from 45% to 10% of clusters generated by photons and from 60 to 30% of converted photons. High pT physics at LHC

  21. jet-jet clusters rejection with photon PID PbPb @ √5.5 TeV pp @ √14 TeV Decay g not overlapped, p0 invariant mass not done Overlapped decay g rejection no more feasible The jet cluster rejection goes from 0.3 (large value due to decay g correctly identified as g, no invariant mass analysis is done) to around 0.02 in pp collisions. There is an increase at 30-40/c GeV because PID for larger pT cannot separate effectively overlapped g from p0 decay. In Pb-Pb rejection worsens, from 0.5 to 0.05. High pT physics at LHC

  22. PbPb @ √5.5 TeV, qhat = 50 Prompt g / jet clusters pp @ √14 TeV PbPb @ √5.5 TeV With PID prompt g to jet clusters ratio increases significatively but it is not enough. High pT physics at LHC

  23. Isolation Cut : Signal/Background Ratio isolated clusters ing-jet / isolated clusters in jet-jet Clusters selected with PIDl20< 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 quenched PbPb events High pT physics at LHC

  24. Isolated Spectra in EMCal 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 No PID No PID No PID With PID With PID With PID High pT physics at LHC

  25. Direct photon in EMCal :Summary & conclusions • jet-jet and g-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 g 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. High pT physics at LHC

  26. Other approach: photon conversionsStudy 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 pTg > 20 GeV/c (very very rough stimations) Loss of efficiency at high pT under investigation Needs to be improved ! High pT physics at LHC

  27. Azimuthal correlation: Direct g converted– charged particlesStudy performed by Ana Marin (GSI) gdetected in Central Barrel Isolation Cut: R=0.2, pT>0.7GeV High pT physics at LHC

  28. Prompt & Fragmentation g in PYTHIA • Have fragmentation and prompt photon similar properties? Looks the correlation with jets the same for both kind of photons? • I have studied the prompt and fragmentation photon production generating several millions of jet-jet and g-jet events in same pT bins as in previous study. • pp collisions √s=14 TeV. • Pure PYTHIA generation. • I have looked to the isolation cut and correlation and fragmentation function with the back to back jet at this generation level. • Only difference with previous simulations now is the Parton Distribution Function (PDF). Before CTEQ4L now CTEQ5L. High pT physics at LHC

  29. 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 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 except neutrinos enter in the cone. || < 0.5 NLO calculation thanks to Lamia B. Prompt & Fragmentation g: pT distribution. PYTHIA pp collisions √s=14 TeV || < 1 High pT physics at LHC

  30. Prompt and ISR are isolated, FSR at most 50% are not isolated. Fragmentation photon (FSR) yield is larger than prompt photon for pT < 50 GeV/c but with isolation at pT < 20 GeV/c ISR yield is small, I will not consider it in next slides. No away side correlation. Direct & Fragmentation gIsolation efficiency. PYTHIA pp collisions √s=14 TeV Direct / Fragmentation ||<1 Isolation efficiency High pT physics at LHC

  31. Fraction of parton energy carried by the fragmentation photon PYTHIA pp collisions √s=14 TeV pTg > 5 GeV/c Obviously, fragmentation photon isolation is more efficient when it has an small part of the parton (near side jet) energy. Is hadron correlation with fragmentation photons and prompt photons the same? Could we reduce further the amount of low z fragmentation photons correlating with the away jet and putting a threshold on the energy? I will check. High pT physics at LHC

  32. Photon and back jet type PYTHIA pp collisions √s=14 TeV • Probability that a photon is back to a gluon or a quark • Prompt photons are mainly back to quark jets (Compton). • Fragmentation photons are mainly back to gluon jets High pT physics at LHC

  33. pTg/pTjet > 0.5 pTg/pTjet > 0.9 g-jet energy/phi smearing PYTHIA pp collisions √s=14 TeV Jet energy reconstructed with the PYTHIA jet finder. We have to be careful when we say that g is back and has the energy of the jet. There is an smearing to take into account. High pT physics at LHC

  34. Correlation Function: Df 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 g have a correlation in the near and away sides, direct g only in the away side. Isolated fragmentation g only correlate in the away side To understand: why away side descends a bit in isolated FSR photons. High pT physics at LHC

  35. pT g FSR / pT jet > 0.5 PYTHIA pp collisions √s=14 TeV Fragmentation Function: zT g All charged hadrons with pT > 0.1 GeV/c inside cone of size R=1 around jet axis Gluon Jet Quark Jet Difference in FF for fragmentation and prompt due to z<1 for fragmentation photons? High pT physics at LHC

  36. Prompt and Fragmentation g in PYTHIA: Summary & conclusions • PYTHIA predicts that prompt and fragmentation photons are produced in pp collisions at 14 TeV, being their ratio 0.6 at 10 GeV and increasing linearly to 1.4 at 90 GeV. • Isolation cut rejects few of the fragmentation photons, at best around 50%. • Remaining fragmentation photons carry a significant amount of the original parton/jet energy. • 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 the same trend due to the fact that isolated fragmentation photons do not carry all the parton energy, and something else? High pT physics at LHC

  37. Back-up High pT physics at LHC

  38. -0 discrimination: Shower Shape Analysis Low particle environment EMCal identified g: l02 < 0.25 identified p0: l02 > 0.25 PHOS (PPR Vol2) Bayesian g identified as g g identified as p0 PID efficiency p+p p0 as p0 p0 as g see Cynthia’s H. talk about EMCal PID during PWG4 meeting in December for more details High pT physics at LHC pT (GeV/c)

  39. PHOS identified spectrumPb+Pb annual statistics Reminder ALICE-INT-2005-014 -G. Conesa et al, NIM A 580 (2007) 1446 Particles identified as g Corrected spectrum, systematic errors Factor 5 suppression Signal Background IC: R =0.2, pT>2 GeV/c Statistics limits to ~100 GeV High pT physics at LHC

  40. Direct g identification in EMCal:Event generation and full reconstruction II New • pp PYTHIA collisions: • kT default value (1 GeV/c). • 3 simulation cases: • pp @ √14 TeV • pp @ √5.5 TeV, merged with HIJING, no quenching in PYTHIA. • pp @ √5.5 TeV, merged with HIJING, quenching: qhat = 50. • 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: ifl02 < 0.25 cluster is a photon (see talk from Cynthia in December PWG4) • Full ALICE simulation with AliRoot 4.07 Release. High pT physics at LHC

  41. Direct g identification in EMCal:Event generation and full reconstruction I New • pp PYTHIA collisions: • g+jet in final state (MSEL=10)  – jet • Prompt gis the signal under study. • Generated pT hard bins (GeV):[5-10], [10-20], [20-30], [30-40], [40-50], [50-60], [60-70], [70-80], [80-90], [90-100] and[>100] with ~10k events per bin. • 2 jets in final state (MSEL=1)  jet –jet • These events constitute the background: decay g,fragmentation g and hadrons. • Generated pT hard bins (GeV): [12-16], [16-20], [20-24],[24-29], [29-35], [35-41], [41-50], [50-60], [60-72], [72-86], [86-104], [104-124], [124-149], [149-179], [179-215], [215-258] and [>258] with ~10k events per bin. • Jet-Jet Event generation with jets containing at least one p0 with pT>5 GeV/c in the acceptance of EMCAL. • Also did a pp simulation without trigger in the same bins, similar number of events. High pT physics at LHC

  42. Prompt g / p0At generation level PYTHIA Theory Ratios for pp collisions at √s=14 and 5.5 TeV agree with theoretical predictions. With qhat=50 ratio a factor ~2 larger than prediction. High pT physics at LHC

  43. Where the correction should be more or less jet-jet clusters: effect of the p0 trigger 1 p0 with pT > 5 GeV/c in EMCal per event Number of clusters found in p0 triggered pp jet-jet events divided by clusters found in non triggered events. From now on pp p0triggered pp jet-jet events will be multiplied by the function fitted in the figure to apply correction for the triggering bias on the hadron contribution. The fragmentation photon yield is much more suppressed (not considered in the cluster points and next plots) f(pT<30) =0.86+1.55 e -0.072 pT Reconstructed clusters Pythia generated particles (more events than in rec. clusters) High pT physics at LHC

  44. IP TPC   R candidate PHOS/EMCal Isolation cut method Reminder ALICE-INT-2005-014 -G. Conesa et al, NIM A 580 (2007) 1446 • Prompt g are likely to be produced isolated. • Two parameters define g isolation: • Cone size • pT thresholdcandidate isolated if: • no particle in cone with pT> pTthres • pT sum in cone, SpT < SpTthres • p Tmin 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 High pT physics at LHC

  45. Isolation Cut: Efficiency Ratio isolated clusters / total clusters No PID pp @ √14 TeV PbPb @ √5.5 TeV PbPb @ √5.5 TeV, qhat = 50 g-jet jet-jet Isolation rejection for jet clusters can be better than 99 % in pp and quenched PbPb events and pT > 20 GeV/c High pT physics at LHC

  46. Isolation Cut: Efficiencypp @ √14 TeV Ratio isolated clusters / total clusters No PID pTth = 0.5 GeV/c R=0.3 g-jet g-jet jet-jet jet-jet High pT physics at LHC

  47. Isolation Cut: Efficiencypp+PbPb @ √5.5 TeV, no quenching Ratio isolated clusters / total clusters No PID pTth = 2 GeV/c R=0.3 g-jet g-jet jet-jet jet-jet High pT physics at LHC

  48. Isolation Cut: Efficiencypp+PbPb @ √5.5 TeV, qhat = 50 Ratio isolated clusters / total clusters No PID pTth = 2 GeV/c R=0.3 g-jet g-jet jet-jet jet-jet High pT physics at LHC

  49. Prompt photon at parton level Partonic photons and back-to-back quarks / gluons do not have the same momentum distribution, there is an energy smearing, and they are not exactly back-to-back. There is also a difference in pT (not in azimuth) between the partonic photon (status 21) and the final state photon (status 1). Where does it come from?, kT? This difference is usually small but not always. High pT physics at LHC

  50. Direct & Fragmentation g pT distribution and ratio pT spectra (full f and , and ||<0.7) Direct/Fragmentation ratio (full f and , and ||<0.7) High pT physics at LHC

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