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Gustavo Conesa Balbastre

Measurement of hard probes in heavy ion collisions at LHC with ALICE: Jets and photons. Gustavo Conesa Balbastre. Introduction. Heavy-Ion collisions at ultrarrelativistic energies: Search of QGP RHIC : Au-Au, √ s=200 GeV. Since year 2000 until now LHC : Pb-Pb, √ s=5.5 TeV.

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Gustavo Conesa Balbastre

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  1. Measurement of hard probes in heavy ion collisions at LHC with ALICE: Jets and photons Gustavo Conesa Balbastre

  2. Introduction • Heavy-Ion collisions at ultrarrelativistic energies: Search of QGP • RHIC: Au-Au, √s=200 GeV. • Since year 2000 until now • LHC: Pb-Pb, √s=5.5 TeV. • 1st HI run hopefully end 2009 • Several probes to study the medium: • Quarkonia • Strangeness • Elliptic flow • Photons • Jets • … Gustavo Conesa Balbastre @ Strings and Strong Interactions Workshop

  3. Why jets and photons: Jet-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 Gustavo Conesa Balbastre @ Strings and Strong Interactions Workshop

  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 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 Gustavo Conesa Balbastre @ Strings and Strong Interactions Workshop

  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 Gustavo Conesa Balbastre @ Strings and Strong Interactions Workshop

  6. Estimates with a thermal model Turbide, Gale, Jeon, and Moore PRC (2004) • Photons abundantly produced. • Jet bremsstrahlung & fragmentation correlated with hadrons. • Jet-plasma & thermal, uncorrelated. • At LHC pQCD photons dominant for pT > 20 GeV In contrast to RHIC, at LHC sources of non prompt direct photons are dominant up to high pT Gustavo Conesa Balbastre @ Strings and Strong Interactions Workshop

  7. 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? Gustavo Conesa Balbastre @ Strings and Strong Interactions Workshop

  8. 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 Gustavo Conesa Balbastre @ Strings and Strong Interactions Workshop

  9. Jet fragmentation function with quenching L. Cunqueiro HP2008 Suppresion of leading particles at low  (as observed with RAA) Increase of soft particles at high  Gustavo Conesa Balbastre @ Strings and Strong Interactions Workshop

  10. Gustavo Conesa Balbastre @ Strings and Strong Interactions Workshop

  11. Gustavo Conesa Balbastre @ Strings and Strong Interactions Workshop

  12. Gustavo Conesa Balbastre @ Strings and Strong Interactions Workshop

  13. 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 Gustavo Conesa Balbastre @ Strings and Strong Interactions Workshop

  14. … but g/p0 = 0,01-0,1 for pT > 10 GeV/c We need a good g/p0 PID How many direct photons and jets? 10k/year Large sample of direct LO g-jet for pT < 30 GeV/c in PHOS and pT < 50 GeV/c in EMCal … Gustavo Conesa Balbastre @ Strings and Strong Interactions Workshop

  15. Jets measurement in ALICE • Jets are measured in : • Tracking system, charged particles • Complete azimuth coverage, -0.9<<0.9 • Calorimeters, neutral particles • Only EMCal, PHOS too small • =110 degrees , -0.7<<0.7 • Not available first runs, and maybe an small fraction first year • Several jet reconstruction algorithms considered: • Cone, kT, Deterministic Annealing, FastJet … • Jet energy resolution: • Only tracking system: E/E0.45 • Tracking system + EMCal: E/E0.3 Gustavo Conesa Balbastre @ Strings and Strong Interactions Workshop

  16. Thanks to Joern P. Jets: RCP() for 125 GeV jets Central to Peripheral Pb-Pb collisions Nuclear modification will be observed with great accuracy combining the Central Tracking System and EMCal Gustavo Conesa Balbastre @ Strings and Strong Interactions Workshop

  17. -0discrimination in the calorimeters Three regions of analysis increasing pT • well separated clusters • invariant mass analysis • < 10 GeV/c in EMCal • < 30 GeV/c in PHOS 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 Gustavo Conesa Balbastre @ Strings and Strong Interactions Workshop PHOS/EMCal

  18. PYTHIA g-jet (signal) and jet-jet (p0 –hadron background) events simulated and reconstructed in ALICE, full material budget. PHOS identified spectrum pp and PbPb annual statistics ALICE-INT-2005-014 G. Conesa et al., NIM A 580 (2007) 1446 Y. Mao, Poster QM2008, ALICE-INT-2007-021 2 PHOS modules IC: R =0.3,S (pT)=2 GeV/c IC: R =0.2, pT>2 GeV/c Gustavo Conesa Balbastre @ Strings and Strong Interactions Workshop

  19. Direct photon identification in EMCal:Isolation Cut : Prompt photon / jet clusters Preliminary Ratio isolated clusters ing-jet / isolated clusters in jet-jet PbPb @ √5.5 TeV, qhat=0 PbPb @ √5.5 TeV, qhat = 50 pp @ √14 TeV Prompt photons signal larger than jet-jet clusters background for pT larger than around 15 GeV/c for pp and quenched PbPb events Gustavo Conesa Balbastre @ Strings and Strong Interactions Workshop

  20. p0 Why g-jetcorrelations? Jet • Medium effects redistribute (qL) the parton energy, Eparton, inside the hadron jet(multiplicity, kT). • Study medium modification in fragmentation function (RAA of FF) from isolatedg-jet and isolatedg-hadron correlations. ^ • Hadron redistribution can be best measured in the Fragmentation Function... If we know Eparton. • HI environment limits the precision on the energy of the reconstructed jet/parton: Promptg • Measure Eprompt g Eparton Gustavo Conesa Balbastre @ Strings and Strong Interactions Workshop

  21. 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 Gustavo Conesa Balbastre @ Strings and Strong Interactions Workshop

  22. Pb-Pb collisions, pT, part > 2 GeV/c Pb-Pb collisions, pT, part >0.5 GeV/c TPC alone TPC+EMCAL Reconstructed jet selection40 GeV jets, photon in PHOS, leading particle is seed, R=0.3 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 Gustavo Conesa Balbastre @ Strings and Strong Interactions Workshop

  23. 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. Gustavo Conesa Balbastre @ Strings and Strong Interactions Workshop

  24. 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 Deviation from 1 used to calculate systematical errors G. Bourdaud preliminary 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 Gustavo Conesa Balbastre @ Strings and Strong Interactions Workshop

  25. Summary • Heavy Ion collisions at LHC will produce large amounts of direct photons and jets. • Probe properties of QGP with jets and photons studying the jet-quenching effect. • Calculate the coefficient transport. • RHIC has measured jets and photons and observed the quenching effect: • Hadrons are suppressed • Direct Photons are suppressed at high pT, not understood completely. • First measurements of jets at RHIC: no quenching observed … measurement biased. • ALICE is prepared to measure direct photons and jets • Jets can be reconstructed with the tracking system alone or together with the calorimeter. Quenching effects can be observed. • Identification of direct prompt photons feasible with the calorimeters with the Isolation cut technique. • Photon-tagged jet algorithms can be used also to study the quenching effect for the lower jet energies where standard jet reconstruction is not possible due to large background. Gustavo Conesa Balbastre @ Strings and Strong Interactions Workshop

  26. Back-up Gustavo Conesa Balbastre @ Strings and Strong Interactions Workshop

  27. hadron IP g EMCal/PHOS g-hadron correlation in ALICE TPC+ITS • Strategy following François Arleo studies (event by event): • Search identified prompt photon(PHOS or EMCal)with largest pT(Eg > 20 GeV). • Search for all charged hadrons (TPC) or neutral p0(EMCal or PHOS): • 90º< g-hadron < 280º • pT hadron> 2 GeV/c PHOS/EMCal Y. Mao, Poster QM2008, ALICE-INT-2007-021 Gustavo Conesa Balbastre @ Strings and Strong Interactions Workshop

  28. How can we distinguish different direct photon sources? • Prompt : RAA = 1, v2=0 (not considering isospin effects) • Fragmentation: RAA<1, v2>0 • Thermal, Bremsstrahlung, Jet Conversion: RAA>1, v2<0 (v2>0 for thermal) • Unambiguous signal of medium production Gustavo Conesa Balbastre @ Strings and Strong Interactions Workshop

  29. RHIC : PHENIX v2 Gale QM2008 v2: small! Consistent with zero (within errors) Gustavo Conesa Balbastre @ Strings and Strong Interactions Workshop

  30. 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 ! Gustavo Conesa Balbastre @ Strings and Strong Interactions Workshop

  31. Azimuthal correlation: Direct g converted– charged particlesStudy performed by Ana Marin (GSI) gdetected in Central Barrel Isolation Cut: R=0.2, pT>0.7GeV Gustavo Conesa Balbastre @ Strings and Strong Interactions Workshop

  32. M. Estienne. Jets: Resolution |h| < 0.3 TPC+EMCal 100 GeV Jets Full simulation R=0.4 TPC only For a jet of R=0.4: limit for a jet to be totally included in the calorimeter limit at which the leading (here center) of the jet is still in the detector acceptance Jet reconstruction resolution from 45% to 30% Gustavo Conesa Balbastre @ Strings and Strong Interactions Workshop

  33. Particle identification with the calorimeters • Different particles produce showers of different shapes. • 7 parameters used to define the shower topology : • Shower ellipse axis l0 , l1; lateral dispersion; core energy; sphericity; maximal deposited energy; multiplicity. • PHOS can also identify particles with TOF (slow nucleons) and CPV (charged vs neutral) • Bayesian approach used to give to the measured cluster an identification probability l0 l1 EMCal PHOS 50 GeV 70 GeV g g g g p0 p0 p0 p0 l20 l20 l20 l20 Gustavo Conesa Balbastre @ Strings and Strong Interactions Workshop

  34. Prompt g identified in PHOS Background If signal is quenched HIC background Signal Fragmentation function ALICE-INT-2005-014 G. Conesa et al., NIM A 585(2008) 28 z = pT, jet particle /Eg Any neutral signal in PHOS Pb-Pb collisions PHOSg : Eg>20 GeV/c;TPC+EMCal detect jet particles, R=0.3 Gustavo Conesa Balbastre @ Strings and Strong Interactions Workshop

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