Gustavo Conesa Balbastre Review on direct photon measurement and jet correlations with EMCal Gustavo Conesa Balbastre EMCal meeting

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 EMCal: Summary of work made by Guénolé Bourdaud EMCal meeting

Introduction: 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) EMCal meeting

Introduction: 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 EMCal meeting

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

-0 discrimination Three regions of analysis increasing pT merged clusters not spherical  shower shape analysis 5 - 40 GeV/c in EMCal 30 - 100 GeV/c in PHOS well separated clusters invariant mass analysis < 15 GeV/c in EMCal < 30 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 EMCal meeting

p0 reconstruction via invariant mass See Cynthia’s talk in 3rd convegno di Fisica di Alice in November 07 simulation: single particles+HI only EMCAL events g1 g2 invariant mass (GeV) only EMCAL single particles + HI p0 efficiency p0: E=9 GeV s/mp= 11 % → effect of the HI environment: efficiency decreases (<10%) → measurement with more than 10% efficiency up to 15 GeV/c EMCal meeting

Particle identification with the calorimeters Different particles produce showers of different shapes. One of the most sentitive parameter is the main axis of the shower ellipse l0 Identification probability of photons in pp collisions close to 90%, in PbPb around 60-70% Overlap of p0 decay photons can be rejected in PHOS for 30<E<100 GeV and in EMCal for 5<E<40 GeV. l0 l1 EMCal PHOS g g g 50 GeV g 70 GeV p0 p0 p0 p0 l20 l20 l20 l20 EMCal meeting ALICE-INT-2005-016, ALICE PPR vol II

-0 discrimination in EMCal: Shower Shape Analysis 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 p+p Pb+Pb 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 EMCal meeting pT (GeV/c) pT (GeV/c)

g/p0 discrimination in EMCal Shower Shape Analysis See Cynthia’s H. talk about EMCal PID during PWG4 meeting in 11 December 2007 for more details g/p0 discrimination in EMCal Shower Shape Analysis p+p 14 TeV Ndirect g / Np0 central Pb+Pb 5.5 TeV factor 5 p0 suppression pT (GeV/c) pT (GeV/c) → p0 suppression factor = 5-10 for pT =[10-30] GeV/c in p+p → p0 suppression factor > 5 for pT =[15-30] GeV/c in Pb+Pb → Ndirect g / Ng from p0 ~ 1 in Pb+Pb for pT =[20-30] GeV/c EMCal meeting

g/p0 discrimination in EMCal Shower Shape Analysis, Summary and conclusions Study presented for the CD2 The PID helps to improve the ratio direct g/p0 in the region from 10 to 40 GeV, but it is still too small. Problem: Revision of parameters in digitization implies that the PID parameterization must change. 1° Priority: Redo PID studies. An additional condition to separate direct photons from the rest of clusters (decay photons, hadrons, etc.)is needed: Isolation Cut EMCal meeting

Direct photon identification in EMCal: The Isolation Cut method 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 TPC IP pT threshold candidate isolated if: no particle in cone with pT > pT thres pT sum in cone, SpT < SpTthres   R candidate PHOS/EMCal EMCal meeting

Direct photon identification in EMCal: Event generation 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 Prompt g is the signal under study. 2 jets in final state (MSEL=1)  jet – jet These events constitute the background: decay g, fragmentation g and hadrons. ~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: if l02 < 0.25 cluster is a photon (Cynthia H., PWG4 meeting, 12/07) Full ALICE simulation with AliRoot 4.07 Release. EMCal meeting

See my talk in Prague ALICE meeting. 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). EMCal meeting 14/40

Fidutial cuts Fidutial cut + PID Fidutial cut + PID See my talk in Prague ALICE meeting. Fidutial cuts About 25% of the calorimeter acceptance after the fidutial cut. Fidutial cut + PID Fidutial cut + PID EMCal meeting 15/40

See my talk in Prague ALICE meeting. Direct photon identification in EMCal: Prompt photon reconstruction + PID efficiency Efficiency = reconstructed / all generated PbPb @ √5.5 TeV pp @ √14 TeV 50 % of g convert in the material before EMCal (5-10% in ITS-TPC). Cluster reconstruction efficiency in pp collisions is almost 95% but with shower shape selection the efficiency descends to 60-80% due to the non photon shape of some converted clusters. In PbPb collisions efficiency goes from 45 to 75 % EMCal meeting

See my talk in Prague ALICE meeting. Direct photon identification in EMCal: jet-jet clusters rejection with photon PID pp @ √14 TeV PbPb @ √5.5 TeV, no quenching Decay g not overlapped Overlapped decay g rejection no more feasible Jet cluster rejection : pp collisions: from 0.3 (large value due to decay g correctly identified as g) to 0.02 PbPb collisions: from 0.5 to 0.05. Increase at 30-40 GeV because PID for larger pT cannot separate effectively overlapped g EMCal meeting

See my talk in Prague ALICE meeting. Direct photon identification in EMCal: Prompt photon / jet clusters with and without PID pp @ √14 TeV PbPb @ √5.5 TeV, qhat=0 PbPb @ √5.5 TeV, qhat = 50 With PID prompt g to jet clusters (decay g, pi0, hadrons) ratio increases significantly but it is not enough. EMCal meeting

See my talk in Prague ALICE meeting. Direct photon identification in EMCal: Isolation Cut : Prompt photon / jet clusters Ratio isolated clusters in g-jet / isolated clusters in jet-jet Clusters selected with PID l20< 0.25 pp @ √14 TeV PbPb @ √5.5 TeV, qhat=0 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 EMCal meeting

See my talk in Prague ALICE meeting. Direct photon identification in EMCal: Prompt & Fragmentation photons: pT distribution PYTHIA pp collisions √s=14 TeV 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 Amount of fragmentation photons in previous simulations small. Pure Pythia Generation, millions of events in several pT hard bins. Isolation done on the pure PYTHIA particles, R=0.4, pTth = 1 GeV/c. || < 1 EMCal meeting

See my talk in Prague ALICE meeting. Direct photon identification in EMCal: Prompt & Fragmentation photons: pT distribution PYTHIA pp collisions √s=14 TeV || < 0.5 NLO calculation thanks to Lamia B. EMCal meeting

See my talk in Prague ALICE meeting. Direct photon identification in EMCal: Direct & Fragmentation photons Isolation efficiency FSR photons Prompt g are isolated, FSR at most 50% are not isolated, high z. Are isolated fragmentation photons similar to prompt photons? Isolated prompt and fragmentation g correlate with particles in the away side, not in the near side. But fragmentation function of both do not follow exactly the same trend because z for isolated fragmentation is not 1. EMCal meeting

Direct photon identification in EMCal : Summary & conclusions 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. EMCal meeting

Tagging jets with photons Old approach ALICE-INT-2005-014 G. Conesa et al., NIM A 585(2008) 28 Strategy (event by event): Search identified prompt photon (PHOS or EMCal) with largest pT (E g > 20 GeV). Search leading particle : g-leading180º Eleading > 0.1 Eg Reconstruct the jet in TPC alone or TPC+EMCal: With Standard jet algorithms, only high pT, not enough statistics. Used method: Particles around the leading with pT > 0.5 GeV/c ,inside a cone of R = 0.3 Main conclusion of this study, prompt g in PHOS: Measurement feasible for 20 < p Tg < 30 GeV/c Sensitive to RAA of FF modifications larger than 5% for 0.1<z<0.5 where z = p T jet hadron /Eg Study must be redone with new simulations and newer approach. min max EMCal R leading IP TPC g EMCal/PHOS EMCal meeting

Tagging jets with photons in EMCal Study done by Guénolé Bourdaud See Guénolé’s talk in last PWG4 meeting. Tagging jets with photons in EMCal Study done by Guénolé Bourdaud Generation of gamma-jet events, with Eg>30 GeV, pp, pp+HI and quenched pp + HI. PID and isolation cut applied to the photons. New approach: Search jet core in all event UA1 like, core defined as Df = Dh = 0.01 Jet axis must be in Df=p  0.5 Construct fragmentation function with x EMCal meeting

Subtract HI background See Guénolé’s talk in last PWG4 meeting. Tagging jets with photons in EMCal Fragmentation function calculation Jet selected, collect all charged particles around jet axis in a cone of R=0.7 Subtract HI background FFin = signal + bkg. FFout = bkg only. FFcorr : = FFsignal . ! Available x range. EMCal meeting

See Guénolé’s talk in last PWG4 meeting. Tagging jets with photons in EMCal Fragmentation function, Ejet = 100 GeV Ratio pp / PbPb EMCal meeting

See Guénolé’s talk in last PWG4 meeting. Tagging jets with photons in EMCal Final Fragmentation function and Nuclear Modification Factor Realistic spectrum simulated: Eg-jet > 30 GeV pp events with PYTHIA. Pb-Pb events with PYTHIA+ »PYQUENafterburner«  merged in HIJING events. 1/10 of a year statistic. x range usable is 0.5 < x < 3.2 No quenching PbPb/pp Bkg not substracted With quenching Bkg substracted Deviation from 1 used to calculate systematical errors PbPb/pp EMCal meeting

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. EMCal meeting

PPR: To Do, Who Task Sub Task Who PID Re-evaluate PID parameters with last AliRoot version SUBATECH, 1 person Re-study the g/p0 discrimination EMCAL (gamma) - Central barrel (conversion) combination for Inv, Mass of p0 (not for PPR?) 1 person Direct photons Isolation cut Re-evaluate IC with larger amount of events, specially jet-jet, PYTHIA Gustavo Compare predictions for different generators and theory Decay photon subtraction with Invariant Mass Gamma-jet Improve HI background rejection Guenole? Estimate effect of jet-jet contamination after Isolation Cut Code implementation in Analysis Frame EMCal meeting

PPR: Data sets available and needed Available (if the grid works, the SE is not down, God is kind, …) Check http://pcalimonitor.cern.ch/job_details.jsp pp events LHC08u: gamma (PHOS) – jet, 15 < Eg < -1 GeV, 3M events LHC08p: gamma (PHOS) - jet quenched, qhat=50, 15 < Eg < -1, 4M events LHC08v: jet-jet, 15<Ejet<50 GeV, 4M events LHC08r: jet-jet, 50<Ejet<-1 GeV, 3M events LHC08q: jet-jet, 100<Ejet<-1 GeV, 800k events Events stored in directory: /alice/sim/PDC_08/LHC08* Need to check if this amount of jet-jet events are enough for gamma isolation. PbPb events: HIJING central, 100k events Desirable Gamma (EMCAL) – jet pp+PbPb events: Currently testing the grid scripts Generate new pp and merge immediately with the available PbPb Gamma (EMCAL/PHOS) – jet (quenched, not quenched) jet-jet (quenched/not quenched) A couple of quenching scenarios? q= 20 and q=50, models PQM, PYQUEN? EMCal meeting

Back-up EMCal meeting

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 EMCal meeting

jet-jet clusters: effect of the p0 trigger 1 p0 with pT > 5 GeV/c in EMCal per event f(pT<30) =0.86+1.55 e -0.072 pT Number of clusters found in p0 triggered pp jet-jet events divided by clusters found in non triggered events. From now on pp p0 triggered 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) Reconstructed clusters Pythia generated particles (more events than in rec. clusters) Where the correction should be more or less EMCal meeting

Fidutial cuts With fidutial cuts With PID 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. With fidutial cuts With PID EMCal meeting

Other approach: photon conversions Study 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) pp @ √s=14 TeV PbPb @ √s=5.5A TeV EMCal 20000 PHOS 3000 Tracking 4000 Loss of efficiency at high pT under investigation Needs to be improved ! EMCal meeting

Azimuthal correlation: Direct g converted– charged particles Study performed by Ana Marin (GSI) g detected in Central Barrel Isolation Cut: R=0.2, pT>0.7GeV EMCal meeting

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. EMCal meeting

Direct photon identification in EMCal: Generated and reconstructed+PID spectra: g-jet : pp+PbPb @ √5.5 TeV PYTHIA Cluster reconstruction efficiency is almost 90%. With shower shape selection the efficiency descends to 45-75% EMCal meeting

Generated 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. PID rejects 15% of clusters generated by real photons and from 50 to 25% of converted photons. EMCal meeting

Generated spectra g-jet : pp+PbPb @ √5.5 TeV With PID 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. EMCal meeting

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 EMCal meeting

Isolation Cut: Efficiency pp @ √14 TeV Ratio isolated clusters / total clusters No PID R=0.3 pTth = 0.5 GeV/c g-jet g-jet jet-jet jet-jet EMCal meeting

Isolation Cut: Efficiency pp+PbPb @ √5.5 TeV, no quenching Ratio isolated clusters / total clusters No PID R=0.3 pTth = 2 GeV/c g-jet g-jet jet-jet jet-jet EMCal meeting

Isolation Cut: Efficiency pp+PbPb @ √5.5 TeV, qhat = 50 Ratio isolated clusters / total clusters No PID R=0.3 pTth = 2 GeV/c g-jet g-jet jet-jet jet-jet EMCal meeting

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 Isolation + PID seem 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 EMCal meeting

Direct & Fragmentation g Isolation efficiency. PYTHIA pp collisions √s=14 TeV Direct / Fragmentation ||<1 Isolation efficiency 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. EMCal meeting

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. EMCal meeting

g-jet energy/phi smearing PYTHIA pp collisions √s=14 TeV pTg/pTjet > 0.9 pTg/pTjet > 0.5 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. EMCal meeting

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. EMCal meeting

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

Tagging jets with photons in PHOS Reconstructed jet selection 40 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 EMCal meeting

Tagging jets with photons in PHOS Reconstructed jet selection 40 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 Pb-Pb collisions, pT, part > 2 GeV/c TPC alone TPC+EMCAL EMCal meeting

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

Tagging jets with photons in PHOS Fragmentation function RFF ALICE-INT-2005-014 G. Conesa et al., NIM A 585(2008) 28 Systematic errors due to jet(p0)-jet background With quenched p0 Charged + EM Sensitive to medium modifications at low z if larger than ~5% in both configurations. Same result using TPC alone to reconstruct the jet EMCal meeting

Tagging jets with photons in PHOS Conclusions and Summary Enough counting rate to do the measurement only in the pT range 20-30 GeV. Good sensitivity to modifications in the jet, being the jet inside or outside EMCal. 0.1 < z < 0.5, region where measurement is sensitive. For z < 0.1, too large HI background fluctuations For z > 0.5, too large statistical fluctuations. Study was done with fast reconstruction of the jet: pT smearing of generated particles EMCal resolution similar to PHOS … Method selects jets as a function of pT of the leading, applying a cut depending on the photon energy. Possible bias, use better a grid in the detectors as done in normal jet finders and avoid the pT cut. Should be repeated with fully reconstructed events and a fine tuned method. EMCal meeting

Tagging jets with photons in EMCal Jet 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. EMCal meeting