Measurements of thermal photons in heavy ion collisions with PHENIX - Torsten Dahms - Stony Brook University February 8 th, 2008 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  see poster by Y. Yamaguchi (P125) High p T photons  see talk by K. Miki (XV)

Direct Photons 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 hard: thermal: Decay photons (  0 → ,  → , …)

Thermal photons? No significant excess at low p T Conventional method: Measure inclusive photons γ incl = γ decay + γ direct Calculate double ratio: (γ incl /π 0 ) measured / (γ decay / π 0 ) background = γ incl / γ decay =1+ γ direct / γ decay If double ratio > 1  direct photons high p T excess consistent with pQCD Run4: more statistics, but still no conclusive measurement Limited by detector resolution and neutral hadron contamination

Clean Photon Sample 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 p T No detector artifacts –But statistics limited due to small X 0 Combining these photon with others measured in EMCAL with loose PID cut  tag photons coming from π 0 decays Correct for missing π 0 decay partners Subtract η, ω, η’ decay photons Calculate ratio N γ incl /N γ decay Uses very pure photon sample avoid explicit calculation of π 0 spectrum  reduce systematic uncertainties no pair cut with pair cut Dalitz Conversions Conversion pairs from π 0 decays γe + e - triplets

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

Compton q  g q q  g q e+e+ e-e- phase space factorform factor invariant mass of virtual photon invariant mass of Dalitz pair phase space factorform factor invariant mass of Dalitz pair invariant mass of virtual photon Virtual Photons Start from Dalitz decay Calculate inv. mass distribution of Dalitz pairs Now direct photons Any source of real  produces virtual  with very low mass Rate and mass distribution given by same formula –No phase space factor for m ee << p T photon Improved S/B by measuring direct photon signal in mass region in which π 0 are suppressed 00   00  e+e+ e-e- 

The Data 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 π0π0 π0π0 e+e+ e-e- e+e+ e-e- γ γ π0π0 e-e- γ e+e+ p+p at √s = 200GeV arXiv:

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 m ee and high p T Au+Au data agree for mee <50MeV Clear enhancement visible above for all p T 1 < p T < 2 GeV 2 < p T < 3 GeV 3 < p T < 4 GeV 4 < p T < 5 GeV p+pAu+Au (MB) PHENIX Preliminary

Shape Comparison At m=0 Dalitz and internal conversion pairs have indistinguishable shape Shape differs as soon as π 0 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 m ee <30MeV –Fit paramater r is fraction of direct photons –Two component fit in 80 < m ee < 300MeV gives: χ 2 /DOF=11.6/10 It’s not the η: –Independent measurement of η in Au+Au fixes π 0 /η ratio to: 0.48 ± 0.08 –Fit with eta shape gives: χ 2 /DOF = 21.1/10

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 μ = 0.5p T μ = 1.0p T μ = 2.0p T p+pAu+Au (MB)

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: Nγ direct = r · Nγ inclusive

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

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

Backup

Relativistic Heavy Ion Collider

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 meson ρ, ω, φ → e + e - –high p T π 0, π +, π - –direct photons –open charm –hadron physics Two muon arms at forward rapidity (1.2 < |η| < 2.4, p  2 GeV/c) e+e+ ee   Measure rare probes in heavy ion collisions (e.g. Au+Au) as well as in p+p (+spin program)

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 Energy-Momentum All charged tracks Background Net signal Real RICH cut

 0 signal extraction Real events Mixed event combine conversion pairs with all photons in EMCal BG subtraction within p T bins Normalized outside the π 0 peak

In practice 0-30 Material conversion pairs removed by analysis cut Combinatorial background removed by mixed events Calculate ratios of various m ee bins to lowest one: R data 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) ÷ MeV ÷ R data ÷ From conventional measurement

Low p T mass spectra