direct photons at rhic
DESCRIPTION
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. - PowerPoint PPT PresentationTRANSCRIPT
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