Direct Photons in 200 GeV p p, d Au, Au Au

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Direct Photons in 200 GeV pp, dAu, AuAu

• Stefan Bathe
  UC Riverside
  
  for the PHENIX collaboration

QM 2005, Budapest, August 4-9
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Why Direct Photons?

• AA
• Photons dont strongly interact with produced
  medium
• Hard photons
• Allow test of Ncoll scaling for hard processes
• Important for interpretation of high-pT hadron
  suppression at RHIC
• Thermal photons
• Carry information about early stage of collision
• QGP potentially detectable via thermal photon
  radiation

• pp
• Test of QCD
• Reduce uncertainty on pQCD photons in AA
• dAu
• Study nuclear effects

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Hard Photons

• pp

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Direct Photons in pp

• good agreement with NLO pQCD
• Important baseline for AuAu

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Hard Photons

• dAu

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Direct g in dAu

• pp and dAu spectra compared to NLO pQCD

• ratio to NLO pQCD
• consistent with 1
• No indication for nuclear effects

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Poster H. Torii
Poster D. Peressounko
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Hard Photons

• AuAu

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Direct Photons in AuAu
Recently published
PRL 94, 232301
Expectation for Ncoll scaling of direct photons
holds for all centrality classes

• p0 suppression caused by medium created in AuAu
  collisions

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Thermal Photons

• AuAu

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Schematic Photon Spectrum in AuAu
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Going to low pT

• No significant excess at low pT

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New from Run4

• New data set
• Selection of most stable runs
• Re-evaluation of systematic uncertainties

• Stay tuned for more improvements

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Thermal Photons

• AuAu

A New Approach
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Opening up the phase space
pT
Minv
0
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The Idea

• Start from Dalitz decay
• Calculate invariant 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

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Method

• Material conversion pairs removed by analysis cut
• Combinatorics removed by mixed events

0-30
90-140
200-300
140-200 MeV
Rdata

• Calculate ratios of various Minv bins to lowest
  one Rdata
• If no direct photons ratios correspond to Dalitz
  decays
• If excess direct photons

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S/B1
p0
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S/B1
calculated from Dalitz formula
p0
measured
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S/B1
calculated from Dalitz formula
p0
measured
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S/B1
calculated from Dalitz formula
p0
measured
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Rdata
140-200 MeV 0-20
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gdirect/ginclusive
0-20
Significant 10 excess of
very-low-mass virtual direct photons
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Centrality Dependence
Indication for centrality dependence
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Comparison to Conventional result
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gdirect
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The Spectrum
Compare to published Run2 result PRL94 232301
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The Spectrum
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The Spectrum
Compare to thermal model

• D. dEnterria, D. Perresounko
• nucl-th/0503054

21 hydro T0ave360 MeV(T0max570 MeV) t00.15
fm/c

• data above thermal at high pT

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The Spectrum
Compare to thermal model

• D. dEnterria, D. Perresounko
• nucl-th/0503054

21 hydro T0ave360 MeV(T0max570 MeV) t00.15
fm/c

• data above thermal at high pT

Compare to thermal pQCD

• data consistent with thermal pQCD

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Conclusions

• Hard direct photons pTgt4GeV/c
• pp
• Spectrum consistent with pQCD calculations
• dAu
• No apparent nuclear effects
• AuAu
• Confirms Ncoll scaling for hard processes

• Thermal (?) direct photons 1ltpTlt4GeV/c
• New EMCal measurement with reduced systematics
• Stay tuned for further improvements
• New measurement through very-low-mass virtual
  photons
• Significant 10 direct photon excess above decay
  photons
• Spectrum consistent with thermal model

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Backup
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WA98 Interpretation T or kT ?

• QGP HG rates convoluted with simple fireball
  model plus pQCD hard photons
• Data described with initial temperature Ti205
  MeV some nuclear kT broadening (Cronin-effect)
• Data also described without kT broadening but
  with high initial temperature (Ti270 MeV)

Turbide, Rapp, Gale, Phys. Rev. C 69 (014902),
2004
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WA98 Data Conclusions
Data can be described under a variety of
different assumptions, e.g.

• Data consistent with QGP picture, but also with
  pure HG picture
• Large variations in extracted initial temperature
  Ti (however, most models give Ti gt Tc)

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PHENIX p0 RdA--Final
Cronin effect small!
New for QM to be published
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90-140
90-140 MeV, 20-40
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Only Ncoll scaling?

• What about fragmentation photons?

• fragmentation contribution substantial in pp
• parton energy loss in QGP reduces fragmentation
  contribution in AuAu
• compensated by induced photon bremsstrahlung in
  QGP

fragmentation contribution ()

• Effects cancel?