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Measurements of thermal photons in heavy ion collisions with PHENIX

Measurements of thermal photons in heavy ion collisions with PHENIX. Real photons at low p T Production mechanisms Traditional EMCal measurement Tagging Beam pipe conversions. Virtual photons Production mechanisms Background Au+Au and long awaited p+p results. - Torsten Dahms -

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Measurements of thermal photons in heavy ion collisions with PHENIX

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  1. Measurements of thermal photons in heavy ion collisions with PHENIX • Real photons at low pT • Production mechanisms • Traditional EMCal measurement • Tagging • Beam pipe conversions • Virtual photons • Production mechanisms • Background • Au+Au and long awaited p+p results - Torsten Dahms - Stony Brook University February 8th, 2008  see poster by Y. Yamaguchi (P125) High pT photons see talk by K. Miki (XV)

  2. Direct Photons Decay photons(p0→g+g, h→g+g, …) hard: thermal: • Direct photon sources: • QCD Compton scattering • Annihilation • QCD Bremsstrahlung • Hard photons from inelastic scattering of incoming partons • Thermal photons are emitted via same processes but from thermalized medium carry information about the temperature of the medium

  3. Thermal photons? • Conventional method: • Measure inclusive photonsγincl = γdecay + γdirect • Calculate double ratio:(γincl/π0)measured / (γdecay/ π0)background = γincl/ γdecay=1+ γdirect/ γdecay • If double ratio > 1 direct photons • high pT excess consistent with pQCD • Run4: more statistics, but still no conclusive measurement • Limited by detector resolution and neutral hadron contamination No significant excess at low pT

  4. Clean Photon Sample no pair cut with pair cut Dalitz Conversions • Method I: • Only use EMCal clusters which fulfill very strict PID cuts • Method II: • Identify conversion photons in beam pipe using their orientation w.r.t. the magnetic field • Additional advantage: • very good momentum resolution of charged tracks at low pT • No detector artifacts • But statistics limited due to small X0 • Combining these photon with others measured in EMCAL with loose PID cut tag photons coming from π0 decays • Correct for missing π0 decay partners • Subtract η, ω, η’ decay photons • Calculate ratio Nγincl/N γdecay • Uses very pure photon sample • avoid explicit calculation of π0 spectrum  reduce systematic uncertainties γe+e- triplets Conversion pairs from π0 decays

  5. Results in Au+Au • Agreement of all three results within their errors • There seems to be an excess above the decay photons at low pT

  6. Virtual Photons e+ Compton e- g* q g g g q p0 p0 e+ g* g e- Compton g q g q • Start from Dalitz decay • Calculate inv. mass distribution of Dalitz pairs invariant mass of Dalitz pair invariant mass of Dalitz pair invariant mass of virtual photon invariant mass of virtual photon form factor form factor phase space factor phase space factor • Now direct photons • Any source of real g produces virtual gwith very low mass • Rate and mass distribution given by same formula • No phase space factor formee<< pT photon • Improved S/B by measuring direct photon signal in mass region in which π0 are suppressed

  7. The Data p+p at √s = 200GeV γ e- • 800M MinBias Au+Au events • 2.25pb-1 of triggered p+p data as reference • Material conversion pairs removed by analysis cut • Combinatorial background removed by mixed events (0.25% syst. uncertainty in Au+Au) • additional correlated background: • cross pairs from decays with four electrons in the final state • particles in same jet (low mass) • or back-to-back jet (high mass) • well understood from MC e+ e+ π0 e+ e- π0 π0 γ e- γ arXiv:0802.0050

  8. PHENIX Preliminary p+p Au+Au (MB) Cocktail comparison • QM2005 • Results from Au+Au • QM2008 • long awaited result from p+p • important confirmation of method • p+p • Agreement of p+p data and hadronic decay cocktail • Small excess in p+p at large mee and high pT • Au+Au • data agree for mee <50MeV • Clear enhancement visible above for all pT 1 < pT < 2 GeV 2 < pT < 3 GeV 3 < pT < 4 GeV 4 < pT < 5 GeV

  9. Shape Comparison At m=0 Dalitz and internal conversion pairs have indistinguishable shape Shape differs as soon as π0 is suppressed due to phase space limitation Assume internal conversions of direct photons Fix absolute normalization of cocktail and direct photons by normalizing to data in mee<30MeV Fit paramater r is fraction of direct photons Two component fit in80 < mee < 300MeV gives: χ2/DOF=11.6/10 It’s not the η: Independent measurement of η in Au+Au fixes π0/η ratio to: 0.48 ± 0.08 Fit with eta shape gives:χ2/DOF = 21.1/10

  10. Fraction of direct photons Fraction of direct photons Compared to direct photons from pQCD p+p Consistent with NLO pQCD favors small μ Au+Au Clear excess above pQCD p+p Au+Au (MB) μ= 0.5pT μ= 1.0pT μ= 2.0pT

  11. Comparison • Agreement of all three methods within their errors • Internal conversion method observes clear excess above decay photons • Extract direct photon spectrum by multiplying with measured inclusive photon spectrum: Nγdirect = r · Nγinclusive

  12. The spectrum • Compare spectra to NLO pQCD p+p • consistent with pQCD Au+Au • above binary scaled pQCD • If excess of thermal origin:inverse slope is related to initial temperature

  13. Conclusion Various techniques employed to measure direct photons at low pT Excess of real photons above decay background observed at low pT Measured excess in dielectron spectra Shape consistent with internal conversions of virtual photons p+p in agreement with pQCD Au+Au above pQCD

  14. Backup

  15. Relativistic Heavy Ion Collider

  16. The PHENIX Experiment Charged particle tracking: DC, PC1, PC2, PC3 Electron ID: Cherenkov light RICH shower EMCal Photon ID: shower EMCal Lead scintillator calorimeter (PbSc) Lead glass calorimeter (PbGl) charged particle veto Central arm physics(|y|<0.35, p ≥ 0.2 GeV/c): charmonium J/ψ, ψ’→ e+e- vector mesonρ, ω, φ → e+e- high pTπ0, π+, π- direct photons open charm hadron physics Two muon arms at forward rapidity (1.2 < |η| < 2.4, p  2 GeV/c) • Measure rare probes in heavy ion collisions (e.g. Au+Au) as well as in p+p (+spin program) p g e+ e-

  17. Electron Identification Charged particle tracking (δm: 1%) DC, PC1, PC3 PHENIX optimized for Electron ID Cherenkov light RICH + shower EMCAL Emission and measurement of Cherenkov light in the Ring Imaging Cherenkov detector→ measure of min. velocity Production and of em. shower in the Electro-Magnetic Calorimeter  measure of energy E Electrons: E ≈ p Hadrons: E < p RICH All charged tracks RICH cut Real Net signal Background Energy-Momentum

  18. p0 signal extraction Real events Mixed event • combine conversion pairs with all photons in EMCal • BG subtraction within pT bins • Normalized outside the π0 peak

  19. 0-30 90-140 140-200 200-300 MeV Rdata ÷ ÷ ÷ In practice • Material conversion pairs removed by analysis cut • Combinatorial background removed by mixed events • Calculate ratios of various mee bins to lowest one: Rdata • If no direct photons: ratios correspond to Dalitz decays • If excess:direct photons • Fit of virtual photon shape to data in principle also possible(done for d+Au) From conventional measurement

  20. Low pT mass spectra

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