Photon Measurements in Heavy Ion Collisions

1. PhotonMeasurements in Heavy Ion Collisions Yorito Yamaguchi CNS, University of Tokyo

2. 1/11 Direct photons • Definition: Photons NOT from hadron decays. • Reason why measure direct photons • Leave the medium without a strong interaction • Generated in every stage of the collision • Their pT are characterized by their origins. S. Turbide et al., PRL77,024909(2008)

3. 1/11 g q q g Direct photons • Definition: Photons NOT from hadron decays. • Reason why measure direct photons • Leave the medium without a strong interaction • Generated in every stage of the collision • Their pT are characterized by their origins. • High pT : Initial hard scatterings S. Turbide et al., PRL77,024909(2008)

4. 1/11 g q q g Direct photons • Definition: Photons NOT from hadron decays. • Reason why measure direct photons • Leave the medium without a strong interaction • Generated in every stage of the collision • Their pT are characterized by their origins. • High pT : Initial hard scatterings Jet-QGP photons • Mid pT : Jet-Medium interactions S. Turbide et al., PRL77,024909(2008)

5. 1/11 g q q g p p r g Direct photons • Definition: Photons NOT from hadron decays. • Reason why measure direct photons • Leave the medium without a strong interaction • Generated in every stage of the collision • Their pT are characterized by their origins. • High pT : Initial hard scatterings Thermal photons Jet-QGP photons • Mid pT : Jet-Medium interactions • Low pT : Thermal radiations from QGP and Hadron Gas S. Turbide et al., PRL77,024909(2008)

6. 1/11 g q q g p p r g Direct photons • Definition: Photons NOT from hadron decays. • Reason why measure direct photons • Leave the medium without a strong interaction • Generated in every stage of the collision • Their pT are characterized by their origins. • High pT : Initial hard scatterings Thermal photons Jet-QGP photons • Mid pT : Jet-Medium interactions • Low pT : Thermal radiations from QGP and Hadron Gas • Thermal photons are of keen interest • Provide key inputs (Tinit & t0) to describe evolution of the created matter S. Turbide et al., PRL77,024909(2008)

7. 1/11 g q q g p p r g Direct photons • Definition: Photons NOT from hadron decays. • Reason why measure direct photons • Leave the medium without a strong interaction • Generated in every stage of the collision • Their pT are characterized by their origins. Hadron decay photons • High pT : Initial hard scatterings Thermal photons Jet-QGP photons • Mid pT : Jet-Medium interactions • Low pT : Thermal radiations from QGP and Hadron Gas • Thermal photons are of keen interest • Provide key inputs (Tinit & t0) to describe evolution of the created matter • Thermal photon measurement is very challenging due to a large background from hadron decays. S. Turbide et al., PRL77,024909(2008)

8. 2/11 Measurement methods • Successful direct photon measurement with two different methods at RHIC • Statistical subtraction method by EMCal • Subtract hadron decay g from inclusive g • Remainder = direct g • Large uncertainty on hadron decay g at low pT • Suitable for pT>5GeV/c • Virtual photon method • Measure low mass e+e- from g* • Selecting e+e- in mee>135MeV • Dramatically improvement of S/B ratio for direct g*→e+e- • Suitable for pT<5GeV/c A. Adare et al., PRL104,132301(2010)

9. 3/11 Direct g fractions NLO pQCD expectations are calculated as : μ = 0.5pT μ = 1.0pT μ = 2.0pT • p+p & d+Au : Almost consistent with NLO pQCD expectations • Cu+Cu & Au+Au : System size dependence is likely to be seen.

10. 4/11 Direct photon spectra p+p vs d+Au p+p vs Au+Au A. Adare et al., PRL104,132301(2010) • p+p vs d+Au : Consistent → Little nuclear effects • p+p vs Au+Au : Significant excess over scaled p+p result in pT<3GeV/c • → Exponential fit gives inverse slope of T = 221±19stat±19systMeV (Central).

11. 5/11 Tinit & t0 TAuAu(fit) ~ 220 MeV TC from Lattice QCD ~ 170 MeV A. Adare et al., PRC81,034911(2010) • Hydrodynamic models agree with the data within a factor of 2. • Uncertainty on Tinit (300-600MeV) is still large. • Depending on thermalization time t0 (0.1-0.6fm/c) • → Need sensitive observable to further constrain Tinit

12. 6/11 Elliptic flow: v2 Z • Collective motion from conversion of spatial eccentricity into momentum anisotropy through pressure gradient. • Expected v2 varies depending on a production process: • Initial hard scattering → v2=0 • Thermal radiation → v2>0 Reaction plane Y X Pressure gradient Pz Py Px

13. 6/11 Elliptic flow: v2 Z • Collective motion from conversion of spatial eccentricity into momentum anisotropy through pressure gradient. • Expected v2 varies depending on a production process: • Initial hard scattering → v2=0 • Thermal radiation → v2>0 Reaction plane Y Hydro after t0 X Pressure gradient Pz Py • Direct photon v2 is sensitive to thermalization time t0. • Early thermalization • → Small thermal photon v2 • Late thermalization • → Large thermal photon v2 • Possible to further constrain t0 R. Chatterjee & D. K. Srivastava, PRC 79, 021901 (2009) Px

14. 7/11 Direct photon v2 A. Adare et al., arXiv:1105.4126 Au+Au@200 GeV minimum bias • Surprisingly, a large direct photon v2 is observed in pT<3GeV/c • Direct photon v2 →0 indicates prompt photons from initial hard scatterings are dominant in pT>5GeV/c Direct photon v2 preliminary

15. 8/11 Centrality dependence Centrality:20-40% Centrality:0-20% • Low pT (pT<5GeV/c) • Inconclusive centrality dependence due to large errors • High pT (pT>5GeV/c) • Consistent between different experiment results • v2~0 independently of centrality Centrality:10-40% charged p0 direct g

16. 9/11 Comparison with models • Some hydrodynamic models predict direct photon v2 as well as pT spectrum. • Tinit = 580MeV, t0 = 0.17fm/c • Shape is similar to predicted shape, but a magnitude is under-predicted. Au+Au@200 GeV 0-20% Model 1: H. Holopainen et al., arXiv:1104.5371

17. 9/11 Comparison with models • Some hydrodynamic models predict direct photon v2 as well as pT spectrum. • Tinit = 580MeV, t0 = 0.17fm/c • Shape is similar to predicted shape, but a magnitude is under-predicted. Model 1: H. Holopainen et al., arXiv:1104.5371 • Tinit = 450-520MeV, t0 = 0.4-0.6fm/c • Model can describe the data for pT>2GeV/c. Model 2: R. Chatterrjee & D.K. Srivastava, PRC 79, 021901 (2009)

18. 10/11 Future prospects • RHIC energy scan program • Low energy Au+Au collision runs have been done in 2010 & 2011: • √sNN = 62.4, 39, 27, 19.6, 11.5, 7.7GeV • Help to search for Critical Point • Measurement at LHC • Direct photon measurement for wide pT range • Statistical subtraction & virtual photon methods • Positive indication from single electron measurement at ALICE • Excess at low pT increases towards more central collisions. p+p 40-50% 0-10%

19. 11/11 Summary • Direct photons have been successfully measured in low pT region for different collision systems at RHIC. • pT spectrum • Enhanced yield is observed in Au+Au • Exponential fit : T = 221±19stat±19syst MeV for Central collisions • Elliptic flow v2 • Large v2 at low pT → Tinit = 450-520MeV, t0 = 0.4-0.6fm/c? • v2~ 0 at high pT → Dominant source : Initial hard scatterings • More theoretical models are needed to understand the data • Direct photons are still one of “hot” probes • Low energy scan at RHIC & measurements at LHC • Systematic study in wide collision energy

20. Backup

21. How to measure low pT photons • Hard to measure by EMCals due to a finite energy resolution • Alternative method has been developed : “Virtual photon method” e+ • Virtual photon method • Basic idea : Any source of g can emit g*, convert to low mass e+e- • How to identify direct g*→e+e- : q e- g* g q Relation between g and associated g*→e+e- emission rates Process dependent factor • Direct g* : If pT2»mee2, S(mee)~1 • Dalitz decay : • Extraction of direct g*→e+e- can be made by utilizing mee shape difference between direct g* and hadrons.

22. Determination of direct g fraction r : direct g/inclusive g Hadrons Direct g* • Determination of direct g fractions in 0.1-0.3GeV/c2 for pT>1GeV/c • Negligible contribution of p0→ge+e- • Satisfy an important assumption of pT2»mee2 → S(mee)~1 • No contribution of p+p-→e+e- • Enhanced e+e- yield over known hadron contributions is clearly seen due to direct g*→e+e-. • Extended fit result can also describe the data well in mee>0.3GeV/c2. A. Adare et al., PRL104,132301(2010)

23. How to obtain direct photon v2 Statistical subtraction method A. Adare et al., arXiv:1105.4126 Au+Au@200 GeV minimum bias preliminary Measure inclusive gv2 using EMCals inclusive photon v2

24. How to obtain direct photon v2 Statistical subtraction method A. Adare et al., arXiv:1105.4126 Au+Au@200 GeV minimum bias preliminary Measure inclusive gv2 using EMCals Measure p0v2, and then evaluate other hadron v2 (h, w, …) using MC calculation p0v2 inclusive photon v2

25. How to obtain direct photon v2 Statistical subtraction method A. Adare et al., arXiv:1105.4126 Au+Au@200 GeV minimum bias preliminary • Measure inclusive gv2 using EMCals • Measure p0v2, and then evaluate other hadron v2 (h, w, …) using MC calculation • Subtract hadron v2 from inclusive gv2 • pT<5GeV/c : rg from virtual photon method • pT>5GeV/c : rg from statistical subtraction method p0v2 inclusive photon v2

26. How to obtain direct photon v2 Statistical subtraction method A. Adare et al., arXiv:1105.4126 Au+Au@200 GeV minimum bias preliminary • Measure inclusive g v2 using EMCals • Confirm NO charged hadron contamination by external conversion method inclusive photon v2