1. PHENIX measurements of reaction plane dependence of high p T photons and pions in Au+Au collisions Vladislav Pantuev, University at Stony Brook for PHENIX collaboration

2. Outline: Part 1. Direct photons Motivation Method Results Part 2. High p T neutral pion suppression and reaction plane angular dependence R AA and Partial energy loss Reaction plane dependence Possible explanation Conclusion 2

3. Part 1. Direct photons. Motivation Why in this case we are looking for any angular dependence? Why in this case we are looking for any angular dependence?  High-p T direct photons produced in initial hard parton-parton scatterings  Low p T thermal photons expected to reflect the initial temperature of the fireball  Photons leave the subsequently produced medium unaltered Leading Particle Direct  Hadrons g q frag. 3

4. There are theoretical predictions with a sizable azimuthal parameter v2 : S.Turbide, C.Gale, R.J.Fries, PRL 96 032303 (2006) R. Chatterjee et al., PRL 96, 202302 (2006) Thermal photons are produced throughout the expansion history and reflect quark anisotropy Jets lose more energy where the medium is thicker - more jet-photon conversions (v2 0) 4

5. How does PHENIX measure direct photons and v 2 ?   Measure inclusive photon yield, N inc   Measure hadron contribution components  0 and    By Monte Carlo calculate   and  decay background in inclusive sample, N bg   Calculate direct photon excess over hadron decays, R   Measure inclusive and hadron v 2 by reaction plane method   Calculate direct photon v 2 as R * v 2 inc – v 2 BG R = N inc / N BG v 2 dir = R – 1 Use large statistics Run 4 Au+Au data set See also poster 86 by Kentaro Miki 5

6. Step by step calculations: PHENIX preliminary 6

7. PHENIX direct photon v2 result Within statistical and systematic errors v 2 is consistent with zero. v 2 =0 or cancellation of different contributions? Systematic errors dominantly from R: Enormous background of decay photons at low p T 7

8. Part 2. Neutral pion suppression and azimuthal anisotropy   Suppression of high pt pions is in favor of energy loss of hard partons   On the other hand, this canonical view is in trouble to describe heavy quark suppression   Clearly, inclusive particle spectra are not sufficient to validate or exclude different models   Centrality dependence of R AA is an effective estimator of path length dependence for energy loss, but   It is more precise to vary path length keeping the same medium conditions: select different angles versus the reaction plane of the event See PHENIX paper nucl-ex/0611007, submitted to PRC 8

9. PHENIX Run 2 final  0 results. Level-2 trigger data – factor 3 more statistics Nuclear modification factor: Systematic error  Factor of ~5  0 suppression at high p T  Approximately constant with p T 9 PHENIX, nucl-ex/0611007, submitted PRC

10. Suppression as spectrum shift: shift in momentum by fractional energy loss of primary parton At high p T spectra are linear and parallel in log-log scale Power law shape, ~P T -n More details in nucl-ex/0611007, submit PRC Can calculate average fractional parton energy loss: Original momentum of parton: 10

11. In agreement with model estimations by: Calculate S loss from R AA : In average, parton loses 15-20% of its original energy in most central Au+Au events 11

12. Pion absorption versus angle w.r.t. reaction plane – other way to control thickness of the medium Centrality 40-50% At fixed centrality change parton path length by varying  and keeping the same : Initial conditions Longitudinal and transverse expansion 12

13. The results: 3 < p T < 5 GeV/c 5 < p T < 8 GeV/c -R AA in plane and out of plane changes by factor ~2 - For peripheral bins no suppression in plane, while a factor ~2 out of plane PHENIX Run2, nucl-ex/0611007, submitted PRC 13

14. We vary path length by centrality and angle , both results should agree Colors represent different centralities R AA is universal function of L  S loss is universal and linear with L  R AA ~1 and no energy loss for L  < 2 fm Variable 1 : simply L , distance from the center of interaction region to the edge 60-70% 10-20% Flow contribution up to 8 GeV/c? Cronin effect? Something else? See D. Winter poster 47, Run4 data, preliminary 14

15. Surprising, variables like  L,  L 2,  L xy (see details in nucl-ex/061107) do not work so well: 15

16. Why no absorption? Alternative explanation: Time matters! Let jets fly in ANY direction: N coll distribution in transverse plane, Glauber + Woods-Saxon Stop jet after some time T. T =2.3 fm/c to fit peripheral data see my poster 32 and hep-ph/0506095 16

17. The result: Results of calculation Describes inclusive R AA and  dependence v 2 =11% at high pT, Simple explanation of lack of absorption in a layer < 2 fm, some other features 17 From V.P. hep-ph/0506095

18. Conclusions:   Within errors direct photon azimuthal asymmetry is consistent with zero   As in previous papers we observe a factor of ~5  0 suppression at high p T, approximately constant with p T   We see factor ~2 suppression for out of plane compared to in plane   R AA ~1 or there is no jet absorption if the medium size is less than 2 fm 18

19. Twice better reaction plane resolution in upcoming Run 7 with new reaction plane detector MC simulation 19

20. backup 20

21. Green line is for Raa extracted with free expansion method. Free streaming is automatically taken into account in the original assumptions 21

22. 22s Can calculate elipticity parameter v2 as jet surviving probability in and out of plane Data are for high pt pi0s, PHENIX, blue cicles – 4.59 GeV/c, green squares – 5-7 GeV/c, preliminary No hydro/collective flow!

23. How to explain rising and falling down v_2 with momentum? 100% 0% pt core+hydro +exponent corona Relative contribution At low momentum hydro scenario produces most of particles and v2 increases with momentum. At high pt, particles are produced from corona with smaller v2. Corona contribution “dilutes” hydro v2 at mid pt to the value of geometry limit. Knowing corona contribution can correct for hydro 23v2 22

24. Compton Annihilation In p+p Hard photons: direct component Photons in A+A Direct PhotonsDecay Photons hardthermalhard+thermal QGPHadron gasdirectfragmentation Preequilibrium photons jet-  - conv. Medium induced  bremsstr. In A+A picture is much more complicated: V2=0 V2<0 V2>0