Direct Photons at RHIC
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Transcript of Direct Photons at RHIC
Direct Photons at RHIC
Saskia Mioduszewski
for the PHENIX Collaboration
21 July 2004
Direct Photons
• Sources of photons (direct and background)
• Results
• Disentangling sources of Direct Photons
• What can we learn?
• Conclusion
Measuring Direct Photons
• Dominant source of photons is hadronic decays 0, , ’ : background photons
• First step is to measure 0 spectrum• “Cocktail” of background sources of
photons• Measure inclusive photon spectrum• Subtract background photons from
inclusive photon spectrum Direct photon spectrum
“Background” Photons
At RHIC, we are in the fortunate position of having a large (factor~5) suppression of 0, dominant background source of photons
* Re-analysis of WA98: d’Enterria nucl-ex/0403055
RAA of 0 in Central Au+Au collisions
Direct Photons at the SPS
• Evidence for direct photons in central Pb-Pb collisions?
10-20% excess but 1 effect only
WA98 WA98
• Comparison to scaled pA: similar spectrum but factor of ~2 enhanced yield in Pb-Pb, again ~1 effect.• pQCD underpredicts direct photon yield
Sources of Direct Photons
• pQCD (“prompt”) photons– Compton (q+gq+)– Annihilation (q+qg+)– Bremsstrahlung (may be modified by medium)
• “jet quenching” photons QGP (Fries et al)?
• Thermal photons (pT~1-4 GeV) sensitive to initial temperature
} not affected by medium
Experimental “Trick”
• Excess Above Background Double Ratio: – []measured / []background
measured/background
• The excess above 1 is the direct photon signal
• We do this because in the ratio of the actual point by point and inclusive measurements will cancel many systematics
Central Au+Au Direct Photon Result
0-10% Central 200 GeV AuAu
PHENIX Preliminary PbGl / PbSc Combined
[]measured / []background = measured/background
= 1 + (direct/exp. bkgd)
1 + ( pQCD x Ncoll) / phenix backgrd Vogelsang NLO
J. Frantz QM’04
Centrality Dependence of Direct Photon Signal
70-80% Central AuAu 200 GeV60-70% Central AuAu 200 GeV50-60% Central AuAu 200 GeV40-50% Central AuAu 200 GeV30-40% Central AuAu 200 GeV20-30% Central AuAu 200 GeV10-20% Central 200 GeV AuAu0-10% Central 200 GeV AuAu
PHENIX Preliminary
[]measured / []background = measured/background
= 1 + (direct/exp. bkgd)
PHENIX Preliminary
AuAu 200 GeV Central 0-10%
PHENIX Preliminary PbGl / PbSc Combined
[if there were no suppression] 1 + ( pQCD direct x Ncoll) / ( phenix pp backgrd x Ncoll)
Central 0-10% consistent with pQCD scaled by Ncoll
Theory curves include PHENIX expected background calculation based on : ( direct + exp. bkgd.) / exp. bkgd. = 1 + (direct/exp. bkgd.)
1 + ( pQCD direct x Ncoll) / ( phenix pp backgrd x Ncoll)
[w/ the real, suppressed background]
PHENIX Preliminary PbGl / PbSc Combined
AuAu 200 GeV Central 0-10%
1 + ( pQCD direct x Ncoll) / phenix backgrd Vogelsang NLO
Theory curves include PHENIX expected background calculation based on : ( direct + exp. bkgd.) / exp. bkgd. = 1 + (direct/exp. bkgd.)
Central 0-10% consistent with pQCD scaled by Ncoll
1 + ( pQCD direct x Ncoll) / ( phenix pp backgrd x Ncoll)
1 + ( pQCD direct x Ncoll) / phenix backgrd Vogelsang NLO
1 + ( pQCD direct x Ncoll) / phenix backgrd Vogelsang, mscale = 0.5, 2.0
PHENIX Preliminary PbGl / PbSc Combined
AuAu 200 GeV Central 0-10%
Theory curves include PHENIX expected background calculation based on : ( direct + exp. bkgd.) / exp. bkgd. = 1 + (direct/exp. bkgd.)
Central 0-10% consistent with pQCD scaled by Ncoll
What can we learn?
• Errors are still too large to extract thermal photons (pT~1-4 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 -jet correlation studies
Direct Photon Calculations
Calculation for p+p scaled by Ncoll – no medium effectsNon-thermal direct photon calculationsIncluding energy loss of bremsstrahlung photons Including energy loss of bremsstrahlung photons Including “jet quenching” photons
Elliptic Flow and Jet Quenching
Coordinate space: initial asymmetry
• 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
(Fries et al)
PHENIX preliminary
Inclusive Photon v2
Comparison to the v2 probed with 0’s
PHENIX preliminary
Next step: subtract v2 of photons from 0 decays from the v2 of inclusive photons v2 of direct photons. Look forward to run-4 data
Summary
• PHENIX has measured direct photons at high pT • At pT~10 GeV/c, a photon is twice as likely to be
a direct photon than from a hadronic decay • Very promising for -jet correlation studies to
study effect of medium on jet energy• PHENIX has also measured v2 of inclusive
photons and 0
• 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
PHENIX Run2 p-p Direct Photon Measurement
PHENIX Preliminary
Vogelsang NLO
• Vogelsang calculation: different scale factors (0.5,1.0,2.0), using CTEQ6 gluon pdf: JHEP 9903 (1999) 025/ private communication
On Double Ratio (1 in %)Central Periph.
Systematic Errors
Totals: 21 15 17 18 15
Central Periph.
3GeV 7 11 3GeV 7 3GeV 7 11 3GeV 7
3GeV 7GeV 11GeV 3GeV 7GeV
Central Periph.
/0 ratio in p+p collisions
0.54+/-0.05 (pT range:3.5-9 GeV/c)
0 from PRL91(2003) 241803
PHENIX Preliminary
Nuclear Modification Factor for (compared to 0)
peripheralbinaryperipheral
centralbinarycentral
NYield
NYield
//
0
RC
P =
PHENIX Preliminary
23
Direct Photons at the SPS (II)
• Comparison to scaled pA: similar spectrum but factor of ~2 enhanced yield in Pb-Pb, again ~1 effect.• pQCD underpredicts direct photon yield
WA98WA98
Hydro calculations :Prompt + QGP Mixed phase HG QGP dominates at high pT
Srivastava and Sinha nucl-th/0006018Srivastava and Sinha nucl-th/0006018