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