Direct Photons in PHENIX

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Direct Photons in PHENIX

• Stefan Bathe (UCR)
• for the PHENIX collaboration

ICPAQGP-2005, Kolkata, India February 8-12, 2005
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Why Direct Photons? (I)

• Direct Photons
• Pragmatic DefinitionPhotons not coming from
  hadron decays
• Difficult measurementLarge background from

• pp
• Test of QCD
• Reduce uncertainty on pQCD photons in AA
• Future
• Constraining gluon distribution functions
• In polarized pp also gluon spin structure
  function

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Why Direct Photons? (II)

• AA collisions
• Photons dont strongly interact with fireball
• Carry information about early stage of collision
• QGP potentially detectable via thermal photon
  radiation
• Thermal photons dominantly from early, hot QGP
  phase? initial temperature
• Direct Photons at high pT
• Allow test of Ncoll scaling for hard processes
• Important for interpretation of high-pT hadron
  suppression at RHIC

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Photon Sources in AA
Photons in AA
Direct Photons
Decay Photons
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Schematic Photon Spectrum in AA

• Advantage in central AA at RHICDecay photon
  background strongly reduced due to p0 suppression
  

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Realistic Calculation
Turbide, Rapp, Gale, Phys. Rev. C 69 (014903),
2004

• Window for thermal photons from QGP in this
  calculationpT 1 - 3 GeV/c

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PHENIX Experimental Setup

• Central spectrometer arms h lt 0.35
• g, p0 via p0 ? gg
• Electromagnetic calorimeter (EMCal)
• Lead scintillator calorimeter (PbSc)
• Lead glass calorimeter (PbGl)

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Measurement of Direct Photons

• Get clean inclusive-photon sample
• e.g. subtraction of charged particle background
• Measure pT spectrum of p0 and h mesons with high
  accuracy
• Calculate number of decay photons per p0
• Usually with Monte-Carlo
• mT scaling for h, w,
• FinallySubtract decay background from inclusive
  photon spectrum

Handy formula
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Cancellation of Systematic Errors
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Direct Photons in pp PHENIX Result

• Data show good agreement with NLO pQCD
  calculation
• Important baseline for interpretation of AuAu
  results

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Influence of Isolation Cut

• No correction for direct photon loss due to
  isolation cut
• Nevertheless, no difference between cross
  sections with and without isolation cut
• Suggests small contribution of fragmentation
  photons, which would be found within jets

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Direct Photons in AuAu at RHIC
final results shown in public for the first time
about to be published

• Direct Photons at high pT follow Ncoll scaling?
  p0 suppression caused by medium created in AA
  collisions

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Direct Photon Spectra

• Ncoll scaling holds for all centrality classes
  (within errors)
• Errors currently too large for statement about
  thermal photon signal

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Comparing to p0s

• p0s suppressed going to central collisions
• Direct photons consistent with Ncoll scaling
• p0 suppression caused by medium created in AA
  collisions

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Model-independent Representation of p0
Suppression

• Standard representation relies on assumption
  about scaling of hard scattering processes in
  AA
• PHENIX result on high pT direct photon production
  confirms this assumption
• Better Avoid model assumptions and use high pT
  direct photons as direct measure of number of
  hard scatterings in AA

• Define
• If direct photons exactly follow Ncoll scaling,
  then

Klaus Reygers
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GAA in central AuAu at RHIC
PHENIX Preliminary data used

• GAA agrees well with the standard RAA
  representation of the neutral pion suppression

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Conclusions

• pp
• Spectrum consistent with pQCD calculations
• AuAu
• For the first time spectrum observed at high pT
• confirms Ncoll scaling for hard processes
• supports explanation of pion suppression as final
  state effect
• no thermal photon signal yet

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Backup Slides
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Thermal Photons as QGP Signature

• Conventional wisdom 1985
• QGP has lots of free quarks
• QGP radiates more than HG at same temperature
  (false!)
• Lots of thermal radiation evidence for QGP
• Conventional wisdom 2005
• QGP has more d.o.f. than HG and therefore lower
  temperature at same energy density e (e g?T4)
• At same energy density QGP radiates less than HG
• Comparing HG and QGP scenariosLack of radiation
  is evidence for QGP!

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Direct Photons at SPS WA98 Result

• 20 direct photon excess at high pT in central
  PbPb collisions at CERN SPS
• No signal within errors in peripheral collisions

Phys.Rev.Lett.853595-3599,2000
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WA98 Result Just Hard Scattering and kT
Broadening?

• High pT part of the spectrum explained by pQCD
  nuclear kT broadening
• pp
• AA
• Intermediate pT range cannot be explained
  regardless of amount of kT

Dumitru et al., Phys. Rev. C 64, 054909 (2001)
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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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What have we learned from SPS?

• Evidence for thermal photons
• Consistent with QGP scenario
• But
• Temperature and kT competing contributions
• Pure hadronic scenario cannot be ruled out

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Direct Photons in pp Tagging Method

• Photon hits that form an invariant mass in the p0
  or h mass range are rejected as direct photon
  candidates
• Signal/background ratio increased in low
  multiplicity environment
• Accidental loss of genuine direct photons
  corrected for by embedding fake photon hits into
  real events
• Remaining background of decay photons subtracted
  on statistical basis
• No isolation cut

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Beyond simple Ncoll ScalingkT Effects and
Photons from Quark-Jets

• Effect of kT strongest where thermal QGP photons
  are expected

• Interaction of fast quarks with QGP significant
  photon source for pT lt 6 GeV/c (Jet-Photons
  and
  )

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Direct Photon Production in pp Hard Scattering

• Processes in perturbative QCD
• Compton
• Annihilation
• Bremsstrahlung
• Typically 20-30 uncertainty in pQCD calculations
  related to choice of scales

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Evidence for kT Broadening

• Systematic pattern of deviation between NLO pQCD
  and data
• Data above pQCD
• Especially at low vs
• Possible explanation kT broadening (which can
  be produced by multiple soft-gluon emission)

E706, hep-ex/0407011
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Closer Look at Jet-Photon Prediction

• Jet photons dominate below 7 GeV
• pQCD calculation just LO
• Compensated by k factor
• Still underpredicts data by factor 2
• Compare
• NLO pQCD describes data in central and peripheral
• No hot medium in peripheral
• No room for jet photons?

Evidence for Direct Photons from Jet-Plasma
Interaction?

• Possible observable consequence Negative v2 for
  direct photons

• No answer with current uncertainties

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What about Photon Bremsstrahlung in AA ?
calculation by W. Vogelsang
direct
Bremsstrahlung

• Bremsstrahlung contribution large
• Modification of Bremsstrahlung contribution
  expected in AA

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Modification of Bremsstrahlung Contribution in AA
Jeon, Jalilian-Marian, Sarcevic,Nucl. Phys. A
715, 795 (2003)

• Quark energy loss in QGP reduces bremsstrahlung
  contribution in AA

• However, this is compensated by induced photon
  bremsstrahlung in QGP(according to Zakharov)

• Net result direct photon RAA ? 1 at high pT