Direct Photons at RHIC

1
Direct Photons at RHIC

• Saskia Mioduszewski
• for the PHENIX Collaboration
• 21 July 2004

2
Direct Photons

• Sources of photons (direct and background)
• Results
• Disentangling sources of Direct Photons
• What can we learn?
• Conclusion

3
Measuring Direct Photons

• Dominant source of photons is hadronic decays p0,
  h, w, h background photons
• First step is to measure p0 spectrum
• Cocktail of background sources of photons
• Measure inclusive photon spectrum
• Subtract background photons from inclusive photon
  spectrum
• ? Direct photon spectrum

4
Background Photons
RAA of p0 in Central AuAu collisions

• At RHIC, we are in the fortunate position of
  having a large (factor5) suppression of p0,
  dominant background source of photons

Re-analysis of WA98 dEnterria nucl-ex/0403055
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Direct Photons at the SPS
WA98

• Evidence for direct photons in
• central Pb-Pb collisions?
• 10-20 excess but 1? effect only

• Comparison to scaled pA similar spectrum
• but factor of 2 enhanced yield in Pb-Pb,
• again 1? effect.
• pQCD underpredicts direct photon yield

6
Sources of Direct Photons

• pQCD (prompt) photons
• Compton (qg?qg)
• Annihilation (qq?gg)
• Bremsstrahlung (may be modified by medium)
• jet quenching photons ? QGP (Fries et al)?
• Thermal photons (pT1-4 GeV) ? sensitive to
  initial temperature

not affected by medium
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Experimental Trick

• Excess Above Background Double Ratio
• g/pmeasured / g/pbackground ?
  gmeasured/gbackground
• The excess above 1 is the direct photon signal
• We do this because in the ratio of the actual
  point by point p0 and inclusive g measurements
  will cancel many systematics

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Central AuAu Direct Photon Result
J. Frantz QM04
PHENIX Preliminary PbGl / PbSc Combined
g/p0measured / g/p0background
gmeasured/gbackground
1 (gdirect/gexp. bkgd)
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Centrality Dependence of Direct Photon Signal
PHENIX Preliminary
g/p0measured / g/p0background
gmeasured/gbackground
1 (gdirect/gexp. bkgd)
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Central 0-10
consistent with pQCD scaled by Ncoll
PHENIX Preliminary
PHENIX Preliminary PbGl / PbSc Combined
AuAu 200 GeV Central 0-10
Theory curves include PHENIX gexpected
background calculation based on p0 (g direct
gexp. bkgd.) / gexp. bkgd. 1
(gdirect/gexp. bkgd.)
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Central 0-10
consistent with pQCD scaled by Ncoll
PHENIX Preliminary PbGl / PbSc Combined
w/ the real, suppressed background
AuAu 200 GeV Central 0-10
Theory curves include PHENIX gexpected
background calculation based on p0 (g direct
gexp. bkgd.) / gexp. bkgd. 1
(gdirect/gexp. bkgd.)
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Central 0-10
consistent with pQCD scaled by Ncoll
PHENIX Preliminary PbGl / PbSc Combined
AuAu 200 GeV Central 0-10
Theory curves include PHENIX gexpected
background calculation based on p0 (g direct
gexp. bkgd.) / gexp. bkgd. 1
(gdirect/gexp. bkgd.)
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What can we learn?

• Errors are still too large to extract thermal
  photons (pT1-4 GeV/c)
• Direct photons agree with pQCD scaled by Ncoll
• One possible conclusion we are seeing hard
  photons and medium modification to Bremsstrahlung
  photons is small
• Or Bremsstrahlung photons lose energy and are
  suppressed, compensated by jet quenching
  photons
• How does one disentangle the sources of photons?
• Also important for g-jet correlation studies

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Direct Photon Calculations
Calculation for pp scaled by Ncoll no medium
effects
Non-thermal direct photon calculations
Including energy loss of bremsstrahlung photons

Including energy loss of bremsstrahlung photons

Including jet quenching photons
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Elliptic Flow and Jet Quenching

• Azimuthal asymmetry in hadrons at high pT
  generated by jet quenching
• more particles emitted at high pT in-plane than
  out-of-plane (less material to traverse)
• If high pT hadron suppression mechanism produces
  (direct) photons, then more of these photons
  produced out-of-plane than in-plane
• ? Negative v2 for these jet quenching photons

Coordinate space initial asymmetry
(Fries et al)
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Inclusive Photon v2
PHENIX preliminary
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Comparison to the v2 probed with p0s
Next step subtract v2 of photons from p0 decays
from the v2 of inclusive photons ? v2 of direct
photons. Look forward to run-4 data
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Summary

• PHENIX has measured direct photons at high pT
• At pT10 GeV/c, a photon is twice as likely to be
  a direct photon than from a hadronic decay
• Very promising for g-jet correlation studies to
  study effect of medium on jet energy
• PHENIX has also measured v2 of inclusive photons
  and p0
• Goal is to extract v2 of direct photons and
  disentangle prompt photons from jet quenching
  (QGP) photons
• Ultimate goal is to extract thermal photons
• Look forward to run-4 results

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PHENIX Run2 p-p Direct Photon Measurement

• Vogelsang calculation different scale factors
  (0.5,1.0,2.0), using CTEQ6 gluon pdf JHEP 9903
  (1999) 025/ private communication

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Systematic Errors
On g/p Double Ratio (1s in )
Central
Periph.
Central
Periph.
3GeV 7 11 3GeV 7
3GeV 7 11 3GeV 7
Totals 21 15 17 18 15
3GeV 7GeV 11GeV 3GeV 7GeV
Central
Periph.
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h/p0 ratio in pp collisions

• 0.54/-0.05 (pT range3.5-9 GeV/c)

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Nuclear Modification Factor for h (compared to p0)
PHENIX Preliminary
RCP
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Direct Photons at the SPS (II)
WA98

• Comparison to scaled pA similar spectrum
• but factor of 2 enhanced yield in Pb-Pb,
• again 1? effect.
• pQCD underpredicts direct photon yield