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Gustavo Conesa Balbastre

Review on direct photon measurement and jet correlations with EMCal. Gustavo Conesa Balbastre. Outline. Introduction : Why direct photon measurement and correlation with jets. Photon / p 0 discrimination in EMCal : Shower shape analysis Summary of work done by Cynthia Hadjidakis

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Gustavo Conesa Balbastre

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  1. Review on direct photon measurement and jet correlations with EMCal Gustavo Conesa Balbastre 4 Convegno sulla Fisica di ALICE

  2. 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

  3. Introduction: Why jets and photonsJet-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. • Direct photon tagged-jets are an useful observable.: • EEjet • Back-to-back in azimuth • The measurement: • Particle species spectra • S ( pTh) • RAA = SNN/(Norm x Spp) • Fragmentation function • FF ( z = pTh/Ejet) • RFF = FFNN/(Norm x FFpp) Jet Nucleus A parton parton Nucleus B QGP prompt } Medium coefficient transport 4 Convegno sulla Fisica di ALICE

  4. Prompt Fragmentation q q γ γ γ g q q γ g q g g g g q g q g LO NLO High z isolated photons Isolated photons Jet Bremsstrahlung/jet coversion induced by medium Thermal radiation g g g 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 (Eg > 10 GeV) • Medium modified production: • Fragmentation photons (Eg < Ejet) • Medium produced photon: • Bremsstrahlung and jet conversion (Eg < Ejet) • Thermal photons (Eg < 10 GeV) QGP Pb Pb 4 Convegno sulla Fisica di ALICE

  5. Photons are produced during all stages of the collision. • 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) Photon sources Pre-equilibrium: Prompt photons Equilibrium: Thermal-Bremsstrahlung- jet conversion photons Freeze-out Decay photons 4 Convegno sulla Fisica di ALICE

  6. RHIC : PHENIX RAA • Hadron suppression of factor 5 at high pT. • Leading jet particle suppression • Run 2: No direct  suppression (PRL 94, 232301 (2005)). • Run 4 (QM06): High pT direct  suppression • Isospin (PDF) effect • Fragmentation photon suppression? • Something else? 4 Convegno sulla Fisica di ALICE

  7. 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

  8. 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

  9. … but g/p0 = 0,01-0,1 for pT > 10 GeV/c We need a good g/p0 PID Introduction: 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 … 4 Convegno sulla Fisica di ALICE

  10. merged clusters not spherical  shower shape analysis 5 - 40 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 > 40 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 • < 15 GeV/c in EMCal • < 30 GeV/c in PHOS 4 Convegno sulla Fisica di ALICE

  11. See Cynthia’s H. talk about EMCal PID during PWG4 meeting in 11 December 2007 for more details g/p0discrimination in EMCalShower Shape Analysis p+p 14 TeV Ndirect g / Ng from p0 central Pb+Pb 5.5 TeV factor 5 p0 suppression pT (GeV/c) pT (GeV/c) Photons have0 < 0.25 → p0suppression factor = 5-10 for pT =[10-30] GeV/c in p+p → p0suppression factor > 5 for pT =[15-30] GeV/c in Pb+Pb → Ndirectg / Ng fromp0~ 1 in Pb+Pb for pT =[20-30] GeV/c 4 Convegno sulla Fisica di ALICE

  12. g/p0 discrimination in EMCalShower 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 g/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

  13. Talk in Prague Physics week Direct photon identification in EMCal:Isolation Cut : Prompt photon / jet clusters 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 4 Convegno sulla Fisica di ALICE

  14. Talk in Prague Physics week Direct photon identification in EMCal: Isolated Spectra 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

  15. 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 fragmentation photons do not carry all the parton energy (more in back-up). Talk in Prague, and April PWG4 meeting Direct photon identification in EMCal:Direct & Fragmentation photons Isolation efficiency. Pythia Simulations No detectors, no reconstruction 4 Convegno sulla Fisica di ALICE

  16. Direct photon identification 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. • 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

  17. Leading Jet core max min R EMCal IP g Tagging jet with photon • Search identified prompt photon(PHOS or EMCal)with largest pT(Eg> 20 GeV). • Strategy (event by event): • Search leading particle or jet core: • g-leading180º • Reconstruct the jet in TPC and EMCal (if available): • 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 TPC • MonteCarlo studies: • Pythia pp events to generate gamma and jet signal • Hijing to generate PbPb background EMCal/PHOS 4 Convegno sulla Fisica di ALICE

  18. If signal is quenched If signal is quenched -tagged Fragmentation FunctionF and RFFphoton in PHOS, jets in TPC+EMCal ALICE-INT-2005-014 G. Conesa et al., NIM A 585(2008) 28 Systematic errors due to jet(p0)-jet background PbPb fragmentation function UE background removed statistically No quenching in simulation Sensitive to medium modifications at low z if larger than ~5% in both configurations. 4 Convegno sulla Fisica di ALICE

  19. Realistic spectrum simulated: Eg-jet > 30 GeV Jet core is the jet seed 1/10 of a year statistic. x range usable is 0.5 < x < 3.2 G. Bourdaud See July PWG4 meeting Deviation from 1 used to calculate systematical errors Tagging jets with photons in EMCalFinal Fragmentation function and Nuclear Modification Factor No quenching PbPb/pp Bkg not substracted With quenching Bkg substracted PbPb/pp 4 Convegno sulla Fisica di ALICE

  20. Conclusion of gamma tagging jets with gamma in EMCal • The modification of the fragmentation function is usable with 30 GeV for 0.5 < x < 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

  21. Back-up 4 Convegno sulla Fisica di ALICE

  22. 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 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 correlations good up to 30 GeV. 4 Convegno sulla Fisica di ALICE

  23. See Cynthia’s H. talk about EMCal PID during PWG4 meeting in 11 December 2007 for more details -0 discrimination in EMCal: Shower Shape Analysis Uniform g and p0 distributions, weighted to match INCNLO predictions identified g: l02 < 0.25 identified p0: l02 > 0.25 Pb+Pb p+p g identified as g g identified as g PID efficiency PID efficiency g identified as p0 g identified as p0 p0 as p0 p0 as p0 p0 as g p0 as g 4 Convegno sulla Fisica di ALICE pT (GeV/c) pT (GeV/c)

  24. 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

  25. See my talk in Prague ALICE meeting. Direct photon 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 • Promptgis the signal under study. • 2 jets in final state (MSEL=1) jet –jet • These events constitute the background: decayg,fragmentation gand hadrons. • Jet-Jet Event generation with jets containing at least onep0with 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: ifl02< 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

  26. With fidutial cuts With fidutial cuts With PID With PID 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. 4 Convegno sulla Fisica di ALICE

  27. Direct photon identification in EMCal:Generated and reconstructed+PID spectra: g-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. 4 Convegno sulla Fisica di ALICE

  28. 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 ! 4 Convegno sulla Fisica di ALICE

  29. 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 4 Convegno sulla Fisica di ALICE

  30. Tagging jets with photons in PHOSReconstructed jet selection40 GeV jets ALICE-INT-2005-014 G. Conesa et al., NIM A 585(2008) 28 p-p collisions, pT, part > 0.5 GeV/c Pb-Pb collisions, pT, part >0.5 GeV/c TPC alone TPC+EMCAL 4 Convegno sulla Fisica di ALICE

  31. Pb-Pb collisions, pT, part > 2 GeV/c TPC alone TPC+EMCAL Tagging jets with photons in PHOSReconstructed jet selection40 GeV jets ALICE-INT-2005-014 G. Conesa et al., NIM A 585(2008) 28 p-p collisions, pT, part > 0.5 GeV/c TPC alone TPC+EMCAL 4 Convegno sulla Fisica di ALICE

  32. Prompt g identified in PHOS Background If signal is quenched HIC background Signal ALICE-INT-2005-014 G. Conesa et al., NIM A 585(2008) 28 Tagging jets with photons in PHOSFragmentation function z = pT, jet particle /Eg Any neutral signal in PHOS Pb-Pb collisions Eg>20 GeV/c; TPC+EMCal detect jet particles, PHOSg HI Background subtracted statistically, accumulate a fake fragmentation function with particle in cone at opposite f and same h as jet 4 Convegno sulla Fisica di ALICE

  33. 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 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 Direct photon identification in EMCal:Prompt & Fragmentation photons: pT distribution PYTHIA pp collisions √s=14 TeV || < 1 4 Convegno sulla Fisica di ALICE

  34. || < 0.5 Direct photon identification in EMCal:Prompt & Fragmentation photons: pT distribution PYTHIA pp collisions √s=14 TeV NLO calculation thanks to Lamia B. 4 Convegno sulla Fisica di ALICE

  35. 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. 4 Convegno sulla Fisica di ALICE

  36. 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? 4 Convegno sulla Fisica di ALICE

  37. Tagging jets with photons in EMCalJet energy reconstruction Reconstructed jet energy in HI / HI Background fluctuation Fraction of reconstructed jet energy vs R 100 GeV g-jets, no bkg. For Ejet = 30 GeV and R = 0.25 ! Ejet/ fluctuations bkg = 2. 4 Convegno sulla Fisica di ALICE

  38. RHIC : PHENIX RAA • Hadron suppression of factor 5 at high pT. • Leading jet particle suppression • Run 2: No direct  suppression (PRL 94, 232301 (2005)). • Run 4 (QM06): High pT direct  suppression • Isospin (PDF) effect • Fragmentation photon suppression? • Something else? 4 Convegno sulla Fisica di ALICE

  39. 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

  40. 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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