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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.

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Direct Photons at RHIC

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  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 Au+Au collisions At RHIC, we are in the fortunate position of having a large (factor~5) suppression of p0, dominant background source of photons * Re-analysis of WA98: d’Enterria nucl-ex/0403055

  5. 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 (q+gq+g) • Annihilation (q+qg+g) • 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

  7. Experimental “Trick” • Excess Above Background Double Ratio: • [g/p]measured / [g/p]background 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

  8. 1 + (g pQCD x Ncoll) / gphenix backgrd Vogelsang NLO 0-10% Central 200 GeV AuAu Central Au+Au Direct Photon Result J. Frantz QM’04 PHENIX Preliminary PbGl / PbSc Combined • [g/p0]measured / [g/p0]background= gmeasured/gbackground • = 1 + (gdirect/gexp. bkgd)

  9. 30-40% Central AuAu 200 GeV 20-30% Central AuAu 200 GeV 50-60% Central AuAu 200 GeV 0-10% Central 200 GeV AuAu 10-20% Central 200 GeV AuAu 60-70% Central AuAu 200 GeV 70-80% Central AuAu 200 GeV 40-50% Central AuAu 200 GeV Centrality Dependence of Direct Photon Signal PHENIX Preliminary • [g/p0]measured / [g/p0]background= gmeasured/gbackground • = 1 + (gdirect/gexp. bkgd)

  10. 1 + (g pQCD direct x Ncoll) / (gphenix pp backgrd x Ncoll) [if there were no p suppression] 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.)

  11. 1 + (g pQCD direct x Ncoll) / gphenix backgrd Vogelsang NLO 1 + (g pQCD direct x Ncoll) / (gphenix pp backgrd x Ncoll) 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.)

  12. 1 + (g pQCD direct x Ncoll) / gphenix backgrd Vogelsang NLO 1 + (g pQCD direct x Ncoll) / gphenix backgrd Vogelsang, mscale = 0.5, 2.0 1 + (g pQCD direct x Ncoll) / (gphenix pp backgrd x Ncoll) 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.)

  13. 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 g-jet correlation studies

  14. Direct Photon Calculations Calculation for p+p scaled by Ncoll – no medium effects Including energy loss of bremsstrahlung photons Including energy loss of bremsstrahlung photons Including “jet quenching” photons Non-thermal direct photon calculations

  15. 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)

  16. Inclusive Photon v2 PHENIX preliminary

  17. PHENIX preliminary Comparison to the v2 probed withp0’s 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

  18. 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 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

  19. PHENIX Preliminary Vogelsang NLO 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

  20. 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.

  21. PHENIX Preliminary p0 from PRL91(2003) 241803 h/p0 ratio in p+p collisions 0.54+/-0.05 (pT range:3.5-9 GeV/c)

  22. p0 h Nuclear Modification Factor for h (compared to p0) PHENIX Preliminary RCP =

  23. Srivastava and Sinha nucl-th/0006018 Hydro calculations : Prompt + QGP  Mixed phase  HG QGP dominates at high pT 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

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