Azimuthal Anisotropy at high p T in Au+Au Collisions at PHENIX

1. Azimuthal Anisotropy at high pT in Au+Au Collisions at PHENIX Rui Wei Nuclear Chemistry Group Stony Brook University

2. Outline • Motivation: • Why we are interested in high pT • Why azimuthal anisotropy • Experimental approach: • How to measure the anisotropy • Results discussion • v2(pT, centrality) • RAA(Df, pT, centrality) • Comparisons to model calculations • Summary

3. Domain of hard scattering process Large momentum transfer Q2 (~pT2); Cross-sections are factorizable. p+p as a good reference Fragment into QCD-vacuum; pQCD is applicable; Data and calculation agree well; Au+Au Occurs early in the collisions; Probe the hot and dense medium; PHENIX, PRD76(2007)051006(R) Why high pT Observed deviations from the reference measurements can be attributed to the medium.

4. Au+Au p0 + X (central) p0 1 RAA 0 20 10 0 pT (GeV/c) RAA – nuclear modification factor • RAA ~ 0.2 for pT > 5.0 GeV/c Strong suppression is observed.

5. S.Bass arXiv:0808.0908 arXiv:0903.4886 Why azimuthal anisotropy • Source of energy loss • Radiative • Collisional • Study the path length dependence • Discriminating power of RAA is not enough: • All jet quenching models work well. • But with large discrepancy of extracted transport coefficient q-hat: • HT: 2.3 GeV2/fm • AMY: 4.1 GeV2/fm • ASW: 10 GeV2/fm • Differential angular measurements of RAA: • Run4 results • Help to discriminate between models • High pT: limited by statistics

6. Relative yields corrected by R.P resolution PHENIX Preliminary 30-40% Experimental Measurements • Azimuthal anisotropy (v2): • Particle yields w.r.t. the reaction plane • Corrected for R.P. resolution • p0s in this analysis; • RAA(Df): Multiply By inclusive RAA

7. Reaction Plane Detectors • Run4: BBC (3<|h|<4); • Run7: • MPC (3.1<|h|<3.9) • RXNin (1.5<|h|<2.8) • RXNout (1.0<|h|<1.5) • Provide better R.P. resolution • RXNout is biased by jets; • Closer to central arm. • MPC is used: • Same rapidity window as BBC; • ~40% better resolution; • In addition with 4 times more statistics!

8. Run4 Results Submitted for publication: arXiv:0903.4886

9. Preliminary Run7 p0 v2 results • We extended pT range up to 13GeV/c in each centrality bin; • Sizeable v2 at high pT is observed, and is relatively flat;

10. p/2 0 RAA(Df, pT) results In-plane Out-of-plane Grey bands: Error in RAA

11. p/2 0 Comparisons to model calculations Implication: large q-hat for the medium? Calculation from S.Bass et al arXiv:0808.0908

12. Challenge for theoretical calculations • The models fail, yet reproduce RAA vs pt • Need stronger variation of Eloss on paths length, or • Sharper initial spatial distribution of energy density, or • More rapid variation of q with e, or …… Comparisons in other centralities needed.

13. v2 Comparisons to Geometric Models • E.Shuryak: PRC 66 027902 (2002) • Geometric limit: v2(high pT) < v2max(b) • Too large for a pure “jet quenching” • A.Drees, H.D.Feng, J.Jia: Phys.Rev.C71:034909,2005 • Jet absorption proportional to matter density; • Can’t reproduce the large v2. • V. Pantuev: arXiv:hep-ph/0506095 • Corona effect, L ~ 2fm; • J.Liao and E.Shuryak: arXiv:0810.4116 • Stronger jet quenching at near-Tc region; physics beyond pQCD?...

14. OUT-OF-PLANE IN-PLANE MID-PLANE E-loss: not limited to single particle observable • Two particle correlations • Gamma-jet • R.P. dependent of jet correlations W. Holzmann, QM09

15. p/2 0 in-plane out-of-plane Out-of-plane vs. in-plane pT • Out-of-plane nearly constant for Npart>100; • Geometric dependences are different for two orientations. RAA NPart

16. MinBias 0-10% 40-50% 20-30% 60-70% 80-92% Similar RAA(pT) and v2(pT) PRL 101, 232301 (2008) • No pT dependence at high pT; • Centrality dependence is also similar; • Imply a correlation.

17. RAA(pT) vs. v2 Look at 0-60%, pT>1 GeV/c Transition from soft to hard regimes? v2 RAA Cu+Cu Acta Phys.Hung.A27 (2006): Horowitz, QM05 • Different behavior. • At low pT • Flow carries initial geometry info. • At high pT • In central, the asymmetry is small. • In peripheral, the jet quenching is small.

18. pT dependence of v2(Npart) Charged hadron results • How is v2 related to the initial geometry? • Low pT: saturate; • High pT: more linear;

19. Summary • Presented detailed measurements of RAA and v2: • With enhanced statistics and improved reaction plane resolution; • Measurements indicate: • v2 is sizeable and relatively flat at high pT; • RAA show strong angular dependence relative to the reaction plane. • Initial additional constraints obtained via RAA and anisotropy using 20-30% centrality data; • Implication: large q-hat value for the medium? • Comparisons with geometrically inspired models.

20. Backup

21. Reaction Plane Measurement with PHENIX Reaction Plane Detector plastic scintillators @ 38<|z|<40cm 12 segments in  2 segments in  1.0 < || < 1.5 1.5 < || < 2.8 Pb converter Run 7+ • Beam-Beam Counters • Quartz Cherenkov radiators • 64 elements in 3 rings • 3.0 < || < 4.0 • All Runs • Muon Piston Calorimeter • PbWO4 PHOS crystals • 192 towers • 3.1 < || < 3.7 • Run 6+ Multiple overlapping and complementary measurements

22. PRL. 91, 182301 (2003) φ=Φ-ΨR y ψR x High pT v2 Decompose into Fourier basis: • v2 measurement: • Low pT: hydrodynamics; • Intermediate pT: recombination + hydro; • High pT: jet suppression? • Study their relations with the initial geometry.