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Prompt photon physics in ALICE: g -jet & g -hadron correlations

Prompt photon physics in ALICE: g -jet & g -hadron correlations. A feasibility and performance study. Gustavo Conesa Balbastre. Outlook. Motivation: Photons in heavy-ion collisions Photons sources RHIC measurements ALICE experiment: Calorimeters Prompt photon identification

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Prompt photon physics in ALICE: g -jet & g -hadron correlations

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  1. Prompt photon physics in ALICE: g-jet & g-hadron correlations A feasibility and performance study Gustavo Conesa Balbastre Gustavo Conesa Balbastre @ High pT physics at LHC workshop

  2. Outlook • Motivation: Photons in heavy-ion collisions • Photons sources • RHIC measurements • ALICE experiment: Calorimeters • Prompt photon identification • Prompt photon correlations • Conclusions Gustavo Conesa Balbastre @ High pT physics at LHC workshop

  3. Fragmentation g p0 Why g? Jet Study the properties of matter at high density and temperature with: • Jets • Partons (jets) suffer energy loss traversing the medium Jet multiplicity and energy redistribution  Jet-Quenching • Photons • Production unperturbed by the medium • Prompt photons: Test QCD, g-jet events  Jet-Quenching • Production modified or created in the medium • Fragmentation photons Quenching • Bremstrahlung, Jet-conversion  Enhancement • Decay photons (neutral mesons)  Observation of hadron suppression  Jet-Quenching Promptg Gustavo Conesa Balbastre @ High pT physics at LHC workshop

  4. p0 A B Promptg • Direct thermal photons • Equilibrium: QGP and hadron gas. • Thermal emission from the medium. Photon sources • Direct prompt g [O(aS)]: • g+q g+q (Compton) • q+q g+g (Annihilation) • Parton in-medium-modification imprinted in the final hadronic state (jet-quenching). • Prompt photons are not perturbed by the medium. Colliding ions Pre-equilibrium Equilibrium Freeze-out Gustavo Conesa Balbastre @ High pT physics at LHC workshop

  5. p0 Photon sources Fragmentation g • Fragmentation prompt g [O(a2S)]: • Bremsstrahlung production modified by the medium • Jet re-interaction • q+gmediumg+q • q+qmediumg+g Colliding ions A B Pre-equilibrium Promptg Equilibrium Freeze-out Gustavo Conesa Balbastre @ High pT physics at LHC workshop

  6. Photon sources Rate Hadron Gas Thermal Tf • Photons carry unperturbed information • on the hot and dense medium (direct photon), • and reveal medium induced modifications (decay photons). • Photon sources in the initial stage of the collision when the system is hottest (pQCD prompt, jet re-interaction, QGP thermal). • Hadron gas + decay: later phase of the collision  Background QGP Thermal Ti Jet Re-interaction √(Ti x √s) Bremsstrahlung (jet -quenching) pQCD LO (Compton) + NLO (fragmentation) Eg Gustavo Conesa Balbastre @ High pT physics at LHC workshop

  7. Jet A B Jet • Decay Photons : • Neutral mesons, p0 and , decay mainly into 2 g. • Main photon source in heavy-ion collisions, background for direct g. • Mesons production suppressed (RHIC) by medium effects (jet-quenching). • Only identified hadronic probe measurable up to very high pT g g Photon sources Colliding ions p0 Pre-equilibrium Equilibrium Freeze-out Gustavo Conesa Balbastre @ High pT physics at LHC workshop

  8. Decay g vs Direct g Photon Yellow Report hep-ph/0311131 • p+p collisions: • mainly p0 • A+A collisions: • Jet-Quenching • RHIC: • Ng > Np for pT > 10 GeV/c • LHC: • Ng > Np for pT > 100 GeV/c • PID: Shower shape + Isolation cut. g/p0 = 0,01-0,1 Gustavo Conesa Balbastre @ High pT physics at LHC workshop

  9. Today Yesterday RAA reference: pQCD calculation RAA reference: fit to p+p measurements Jet Jet-quenching at RHIC s = 200A GeV Jet Hadrons suppression factor 5! No suppression of g, as expected? CERN Heavy Ion Forum, March 6, 2007 -- G. David, BNL Gustavo Conesa Balbastre @ High pT physics at LHC workshop

  10. RAA with pQCD RAA with p+p data Direct g RAA in Au+Au – Theory and Experiment S. Turbide, Phys. Rev. C72 (2005) 014906 CERN Heavy Ion Forum, March 6, 2007 -- G. David, BNL W. Vogelsang, NLO pQCD + isospin F. Arleo, JHEP09 (2006) O15 Gustavo Conesa Balbastre @ High pT physics at LHC workshop

  11. Outlook • Motivation: Photons in heavy-ion collisions • ALICE experiment: Calorimeters • Prompt photon identification • Prompt photon correlations • Conclusions Gustavo Conesa Balbastre @ High pT physics at LHC workshop

  12. ALICE: ALarge Ion Collider Experiment Solenoid magnet 0.5 T HMPID PHOS • ITS • TPC • TRD • TOF • Central tracking system MUON Spectrometer Gustavo Conesa Balbastre @ High pT physics at LHC workshop

  13. Detectors to be used EMCal D =120º || < 0.7 TPC D =360º || < 0.9 Dp/p= 2%, =1.1º PHOS D =100º || < 0.12 E > 10 GeV DE/E < 1.5%, sx=[0.5,2.5] mm Gustavo Conesa Balbastre @ High pT physics at LHC workshop

  14. PHOton Spectrometer: PHOS ALICE PPR chapter 5 High Resolution spectrometer • High granularity detector: • 17,920lead-tungstate crystals (PbWO4), 5 modules (5664) • crystal size:22  22  180 mm3 • depth in radiation length:20 • Distance to IP:4.4 m • Acceptance: • pseudo-rapidity [-0.12,0.12] • azimuthal angle100o • Charged Particle Veto,CPV • multi-wire particle gas chamber CPV Crystals EMC Gustavo Conesa Balbastre @ High pT physics at LHC workshop

  15. ElectroMagnetic Calorimeter: EMCal http://rhic23.physics.wayne.edu/twiki/pub/Alice/ReviewDocs/MIE_Proposal_12-5-05.pdf • Description: • 12,672 towers (scintilator-Pb), 10 modules (24  48) + 2 half size modules (12  48) • tower size:60  60  250 mm3 • depth in radiation length:22 • Distance to IP:4.28 m • Acceptance: • pseudo-rapidity [-0.7,0.7] • azimuthal angle110o • EMCal is 7 times larger than PHOS but it is a moderate energy resolution calorimeter Gustavo Conesa Balbastre @ High pT physics at LHC workshop

  16. PHOS beam-test and simulations ALICE PPR chapters 5 Energy resolution Position resolution Heavy-ion environment worsens the resolution by less than 2% E > 10 GeV DE/E < 1.5% E > 10 GeV sx=[0.5,2.5] mm Gustavo Conesa Balbastre @ High pT physics at LHC workshop

  17. PHOS beam-test and simulations ALICE PPR chapters 5 Inv. Mass Resolution is 3-5% in 0.5 < E < 30 GeV in PHOS simulations Fixed target experiment p-+12Cp0()+X, Ep- = 6 GeV Gustavo Conesa Balbastre @ High pT physics at LHC workshop

  18. EMCal beam-test E > 10 GeV sx< 3.5 mm E > 10 GeV DE/E < 3% Gustavo Conesa Balbastre @ High pT physics at LHC workshop

  19. Outlook • Motivation: Photons in heavy-ion collisions • ALICE experiment: Calorimeters • Prompt photon identification • Isolation cut method • Prompt photon correlations • Conclusions Gustavo Conesa Balbastre @ High pT physics at LHC workshop

  20. Photon identification ALICE PPR chapters 5 and 6, photon sections • We can discriminate g , e and p0from anything else : “PCA” or “Bayesian”, based on: • CPV : Charged particle rejection • TOF : Rejection of massive low pT particles • EMC: Hadron rejection via shower topology • Algorithms tuned with simulations: • Few % hadron contamination in HI environment. • Designed to distinguish high-energy g and p0. • p0 decay g overlaps in PHOS from 30 GeV (15 GeV in EMCal) • High g identification efficiency, ~ 60%, and misidentification smaller than 10 % for 30 GeV < Ep < 100 GeV. • Not enough for Prompt g identification: We need Isolation Cut Method Gustavo Conesa Balbastre @ High pT physics at LHC workshop

  21. Prompt g identification:Event generation ALICE-INT-2005-014 • p-p collisions: PYTHIA 6.2 as event generator, p-p collisions @ 5.5 TeV: • g+jet in final state  – jet. • Prompt gis the signal under study: 20 GeV < E g < 100 GeV. • 2 jets in final state  jet –jet. • These events constitute the background: high-pTp0[O(S)] and bremsstrahlung[O(a2S)]: 30 GeV < E jet < 300 GeV. • Pb-Pb collisions: p-p collisions+underlying event for Pb-Pb collisions @ 5.5 TeV,HIJING, dN/dy~6000. • Binary scaling from p-p collisions,minimum bias. • No medium effects ! Gustavo Conesa Balbastre @ High pT physics at LHC workshop

  22. Signal Factor 5suppression Prompt g identification:Generated signal and background ALICE-INT-2005-014 • Direct results from PYTHIA, no detector response function, corrected for detector acceptance. • Background = • g decay + bremsstrahlung • PHOS identifies gefficientlythrough Shower Topology: Not enough to tag them as prompt. Background Gustavo Conesa Balbastre @ High pT physics at LHC workshop

  23. Overlapped Clusters Rejection ALICE-INT-2005-014 Pb-Pb collisions • Signal/Background: • g as g • p0as g • Too much p0 background ! • New ID criteria to be found 1-d shower shape analysis Gustavo Conesa Balbastre @ High pT physics at LHC workshop

  24. Two parameters define g isolation: • Cone size IP TPC Bremsstrahlung (Background!) Our signal   R candidate • pp collisions R = 0.2, Tthres = 0.7 GeV/c • Identification Probability100 % • Misidentification4.5 % • Signal/Background 13 • Pb-Pb collisions R = 0.2, pTthres = 2 GeV/c • Identification Probability50 % • Misidentification7 % • Signal/Background 4.2 PHOS Prompt g identification:Isolation cut method ALICE-INT-2005-014 • Prompt g are likely to be produced isolated. • pT thresholdcandidate isolated if: • no particle in cone with pT> pTthres • pT sum in cone, SpT < SpTthres Gustavo Conesa Balbastre @ High pT physics at LHC workshop

  25. Final identified prompt gspectrumAnnual statistics ALICE-INT-2005-014 Particles identified as g(medium purity) Corrected spectrum, systematic errors IC: R =0.2, pT>2 GeV/c Factor 5 suppression Signal Background Statistics limits to ~100 GeV Gustavo Conesa Balbastre @ High pT physics at LHC workshop

  26. Systematic error suppressed a factor 5 by quenching Quality of data ALICE-INT-2005-014 Gustavo Conesa Balbastre @ High pT physics at LHC workshop

  27. Outlook • Motivation: Photons in heavy-ion collisions • ALICE experiment: PHOS • Prompt photon identification • Prompt photon correlations • g-jet • g-hadron • Conclusions Gustavo Conesa Balbastre @ High pT physics at LHC workshop

  28. Fragmentation g • Medium effects redistribute (qL) the parton energy, Eparton, inside the hadron jet(multiplicity, kT). ^ p0 Why g-hadron/jet correlations? Jet • Study medium modification in fragmentation function (RAA of FF) from isolated g-jet and isolated g-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 @ High pT physics at LHC workshop

  29. leading • Search identified prompt photon(PHOS)with largest pT(E g > 20 GeV). max min R • Search leading particle: • g-leading180º • Eleading> 0.1 Eg EMCal TPC • Reconstruct the jet: • Particles around the leading with pT > 0.5 GeV/c, inside a cone of R = 0.3. • 2 configurations: charged and neutral hadrons (TPC+EMCAL) and charged only (TPC). IP g PHOS Tagging jet with photon ALICE-INT-2005-014 • Strategy (event by event): Gustavo Conesa Balbastre @ High pT physics at LHC workshop

  30. 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 ALICE-INT-2005-014 p-p collisions, pT, part > 0.5 GeV/c TPC alone TPC+EMCAL Gustavo Conesa Balbastre @ High pT physics at LHC workshop

  31. Prompt g identified in PHOS Background If signal is quenched HIC background Signal Fragmentation function z = pT, jet particle /Eg ALICE-INT-2005-014 Any neutral signal in PHOS Pb-Pb collisions • E g >20 GeV/c; TPC+EMCal detect jet particles, PHOS g Gustavo Conesa Balbastre @ High pT physics at LHC workshop

  32. With quenchedp0 Sensitive to medium modifications at low z if larger than ~5% in both configurations. g-tagged FF RAA ALICE-INT-2005-014 Systematic errors due to jet(p0)-jet background Charged + EM Gustavo Conesa Balbastre @ High pT physics at LHC workshop

  33. g-hadron correlations • We could do the same study in a simpler way: tagging hadrons opposite to the isolated g. • Suggested by F. Arleo et al. in : • JHEP 0609:015,2006, hep-ph/0601075 • JHEP 0411:009,2004, hep-ph/0410088 • hep-ph/0701207 Gustavo Conesa Balbastre @ High pT physics at LHC workshop

  34. g-hadron correlations F.Arleo et al. hep-ph/0701207 = Eparton Gustavo Conesa Balbastre @ High pT physics at LHC workshop

  35. g-hadron correlations F.Arleo et al. hep-ph/0701207 • Requirements for good measurement: • Perturbative direct photons. • At LHC pTg > 20-30 GeV/c • Perturbative hadron, no medium residues. • At LHC pTp > 10 GeV/c • Wide z range, ideally 0  zgp  1 • Thus ideally : minimumpTg>> minimum pTp • Reasonable counting rates • At LHC pTg < 100 GeV/c • Study made for pTg>20 GeV/c and pTg>70 GeV/c. Gustavo Conesa Balbastre @ High pT physics at LHC workshop

  36. g-hadron correlationpTg > 70 GeV/c – pTp>10 GeV/c F.Arleo et al. hep-ph/0701207 Theoretical FF Most of the z interval ... but limited counting rate. Gustavo Conesa Balbastre @ High pT physics at LHC workshop

  37. g-hadron correlationpTg > 20 GeV/c – pTp>10 GeV/c F.Arleo et al. hep-ph/0701207 Theoretical FF No match with real FF ... but good counting rate. Gustavo Conesa Balbastre @ High pT physics at LHC workshop

  38. g-hadron correlationfragmentation g contribution Much larger fragmentation g component with pTg>20 GeV/c. Decay photons will contribute much more. Isolation of direct photons will reject both. Gustavo Conesa Balbastre @ High pT physics at LHC workshop

  39. g-hadron correlationQuenching Weaker energy loss, but see larger z region. Don’t see expected suppression in all region. Gustavo Conesa Balbastre @ High pT physics at LHC workshop

  40. Basic yields for 5.5A TeV Pb+Pb collisions • The study claims that better concentrate inpTg>70 GeV/c direct g, and pTp >10 GeV/c, • but our calorimeters acceptance reduce g counting rate: • PHOS • 20< pTg< 40 GeV/c • EMCal • 20< pTg< 60 GeV/c • We have to investigate results at lower pT cuts. 1k/year Gustavo Conesa Balbastre @ High pT physics at LHC workshop

  41. hadron • Search identified prompt photon(PHOS or EMCal)with largest pT(Eg > 20 GeV). • Search for all charged hadrons (TPC+ITS) or neutral p0(EMCal or PHOS): • 90º< g-hadron < 280º • pT hadron> 10 GeV/c IP g g-hadron correlation in ALICE • Strategy following François Arleo studies (event by event): EMCal/PHOS TPC+ITS PHOS/EMCal Gustavo Conesa Balbastre @ High pT physics at LHC workshop

  42. Outlook • Motivation: Photons in heavy-ion collisions • ALICE experiment: Calorimeters • Prompt photon identification • Prompt photon correlations • Conclusions Gustavo Conesa Balbastre @ High pT physics at LHC workshop

  43. Conclusions 1/3 • Identification of 22 pQCD processes with g. • Prompt photon identification: Isolation cut method. • Efficiently rejects background. • 20% of systematic error from left over background. • Assuming quenching, systematic errors dramatically reduced in Pb-Pb collisions. • Statistics (PHOS acceptance) limits the measurement to energies below 100 GeV . • Photon-tagged algorithm to measure jet properties. • To measure the redistribution of fragmentation hadrons inside the jet (jet multiplicity, jet heating). • EMCal helps to improve the background rejection. • RFF shows a sensitivity of medium induced modification at the level of 5%. Gustavo Conesa Balbastre @ High pT physics at LHC workshop

  44. Conclusions 2/3 • Within its present configuration and the developed methods, ALICE can measure photon (PHOS) tagged jets (TPC) with energy ~20 GeV. • Adding EMCal, due to the increased acceptance, measurements of photon (EMCal) tagged jets (TPC) extended to ~ 40 GeV. • Study of the jet-quenching over a broader energy range. • In both cases the sensitivity to medium effects is of about 5 %. Gustavo Conesa Balbastre @ High pT physics at LHC workshop

  45. Conclusions 3/3 • g-hadron correlation provides new insight in the study of medium modifications • Optimum conditions would be at pTg > 70 GeV/c and pTp>10 GeV/c, but small statistics, pTg > 40-50 GeV/c and smaller pTpcut might be OK, to be investigated. • Event production with the GRID started. Study g-jet/hadron correlations with new more realistic simulations. Gustavo Conesa Balbastre @ High pT physics at LHC workshop

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