Measurements of thermal photons in heavy ion collisions with PHENIX

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Measurements of thermal photons in heavy ion
collisions with PHENIX

• - Torsten Dahms -
• Stony Brook University
• February 8th, 2008

• Real photons at low pT
• Production mechanisms
• Traditional EMCal measurement
• Tagging
• Beam pipe conversions

• Virtual photons
• Production mechanisms
• Background
• AuAu and long awaited pp results

? see poster by Y. Yamaguchi (P125)
High pT photons? see talk by K. Miki (XV)
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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

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Thermal photons?

• Conventional method
• Measure inclusive photons?incl ?decay
  ?direct
• Calculate double ratio(?incl/p0)measured /
  (?decay/ p0)background ?incl/ ?decay 1
  ?direct/ ?decay
• If double ratio gt 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
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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 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 p0 decays
• Correct for missing p0 decay partners
• Subtract ?, ?, ? decay photons
• Calculate ratio N?incl/N ?decay
• Uses very pure photon sample
• avoid explicit calculation of p0 spectrum
• ? reduce systematic uncertainties

?ee- triplets
Conversion pairs from p0 decays
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Results in AuAu

• Agreement of all three results within their
  errors
• There seems to be an excess above the decay
  photons at low pT

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Virtual Photons

• Start from Dalitz decay
• Calculate inv. mass distribution of Dalitz pairs

invariant mass of virtual photon
invariant mass of Dalitz pair
invariant mass of Dalitz pair
invariant mass of virtual photon
phase space factor
form factor
phase space factor
form factor

• Now direct photons
• Any source of real g produces virtual g with
  very low mass
• Rate and mass distribution given by same formula
• No phase space factor for meeltlt pT photon
• Improved S/B by measuring direct photon signal in
  mass region in which p0 are suppressed

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The Data
pp at vs 200GeV

• 800M MinBias AuAu events
• 2.25pb-1 of triggered pp data as reference
• Material conversion pairs removed by analysis cut
• Combinatorial background removed by mixed events
  (0.25 syst. uncertainty in AuAu)
• 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

arXiv0802.0050
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Cocktail comparison

• QM2005
• Results from AuAu
• QM2008
• long awaited result from pp
• important confirmation of method
• pp
• Agreement of pp data and hadronic decay cocktail
  
• Small excess in pp at large mee and high pT
• AuAu
• data agree for mee lt50MeV
• Clear enhancement visible above for all pT

1 lt pT lt 2 GeV 2 lt pT lt 3 GeV 3 lt pT lt 4 GeV 4 lt
pT lt 5 GeV
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Shape Comparison

• At m0 Dalitz and internal conversion pairs have
  indistinguishable shape
• Shape differs as soon as p0 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 meelt30MeV
• Fit paramater r is fraction of direct photons
• Two component fit in80 lt mee lt 300MeV gives
  ?2/DOF11.6/10
• Its not the ?
• Independent measurement of ? in AuAu fixes p0/?
  ratio to 0.48 0.08
• Fit with eta shape gives ?2/DOF 21.1/10

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Fraction of direct photons

• Fraction of direct photons
• Compared to direct photons from pQCD
• pp
• Consistent with NLO pQCD
• favors small µ
• AuAu
• Clear excess above pQCD

pp
AuAu (MB)
µ 0.5pT µ 1.0pT µ 2.0pT
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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 N?direct
  r N?inclusive

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The spectrum

• Compare spectra to NLO pQCD
• pp
• consistent with pQCD
• AuAu
• above binary scaled pQCD
• If excess of thermal origininverse slope is
  related to initial temperature

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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
• pp in agreement with pQCD
• AuAu above pQCD

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Backup
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Relativistic Heavy Ion Collider
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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(ylt0.35, p 0.2 GeV/c)
• charmonium J/?, ?? ee-
• vector meson ?, ?, f ? ee-
• high pT p0, p, p-
• direct photons
• open charm
• hadron physics
• Two muon arms at forward rapidity (1.2 lt ? lt
  2.4, p ? 2 GeV/c)

• Measure rare probes in heavy ion collisions (e.g.
  AuAu) as well as in pp (spin program)

p
g
e
e-
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Electron Identification

• Charged particle tracking (dm 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 lt p

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p0 signal extraction

• combine conversion pairs with all photons in
  EMCal
• BG subtraction within pT bins
• Normalized outside the p0 peak

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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 dAu)

From conventional measurement
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Low pT mass spectra