Studying the Medium Response by Two Particle Correlations

1. Studying the Medium Response by Two Particle Correlations John Chin-Hao Chen (for PHENIX Collaboration) Department of Physics and Astronomy Stony Brook University RHIC & AGS Annual Users' Meeting 05/27/2008 RHIC & AGS User Meeting

2. Outline • Why do we use two particle correlations? • What do we learn from two particle Df correlations? • What do we learn from two particle Dh-Df correlations? RHIC & AGS User Meeting

3. Is there a jet? • In heavy ion collisions, due to huge background of soft particles, it is difficult to find the jet. • So we turn to two particle correlations RHIC & AGS User Meeting

4. What is two particle correlation? • We select a high pT particle as the trigger particle. • Select another lower pT particle, calculate the angular Df distribution. • Use ZYAM (Zero Yield At Minimum) method to remove the flow background. RHIC & AGS User Meeting

5. The jet has disappeared! • In p+p and d+Au collisions, we see a clear awayside peak • In central Au+Au collision, the awayside peak is disappeared. Phys. Rev. Lett. 91, 072304 RHIC & AGS User Meeting

6. The jet is modified! The awayside peak moved to ~ p +/- 1.1 in central collisions! Phys. Rev. Lett. 98, 232302 (2007) RHIC & AGS User Meeting

7. The nearside is also modified! • The nearside width is wider than pp • The yield at low partner pT is also larger than pp   arXiv:0801.4545 RHIC & AGS User Meeting

8. 2-D Dh-Df correlations Peripheral Au+Au Central Au+Au Dh Dh shoulderridge Df Df rad Both near and away side are modified! RHIC & AGS User Meeting

9. Yield in Dh slices: peripheral 0<|Dh|<0.1 0.1<|Dh|<0.3 Au+Au ~ p+p 0.5<|Dh|<0.7 0.3<|Dh|<0.5 RHIC & AGS User Meeting

10. ridge Yield in Dh slices: 0-20% central 0<|Dh|<0.1 0.1<|Dh|<0.3 0.3<|Dh|<0.5 0.5<|Dh|<0.7 RHIC & AGS User Meeting

11. Jet induced medium response • Nearside: ridge • Awayside: shoulder • Do they correlate with each other? RHIC & AGS User Meeting

12. Decomposition method • Fit away side jet with sum of three Gaussians to decompose components: • Head: punch through jet • Shoulder: new peak either side of p (medium response ?!?!) • Treat all components as Gaussian in shape • Use ZYAM method to fix background level RHIC & AGS User Meeting

13. nearside enhancement vs centrality Enhancement! pp RHIC & AGS User Meeting

14. shoulder & ridge increase with centrality Yields are very similar! RHIC & AGS User Meeting

15. Shoulder & ridge pT spectra vs. p+p • Both are softer than hard scattering. • Ridge harder than shoulder? • Shoulder not quite as soft as inclusive hadrons RIDGE SHOULDER RHIC & AGS User Meeting

16. Where does the momentum go? Phys. Rev. C 77, 011901(R) (2008) • Compare to pp, the awayside per trigger yield at high pT is suppressed. • At lower pT, the awayside yield is enhanced. • During the collision, the total transverse momentum is conserved • How does the jet momentum redistribute into the medium? RHIC & AGS User Meeting

17. Momentum flow • Weight the per trigger yield of each partner pT bins with <pTassociated> •  Ensemble averaged vector sum of associated particles. Vector sum is along the trigger direction • e.g. partner pT trigger pT projected on trigger pT direction RHIC & AGS User Meeting

18. Near & away increase with centrality Number of particles pT Weighted yield RHIC & AGS User Meeting

19. Near = ridge 0.5< Dh <0.7 Near = jet 0 < Dh < 0.1 away/near to ~ cancel acceptance pT is transferred from head  shoulder shoulder to ridge ratio is roughly constant RHIC & AGS User Meeting

20. Summary • In heavy ion collisions, both the nearside and awayside of the jet are modified • Nearside->ridge • Awayside->shoulder • Both ridge and shoulder are softer than hard scattering and slightly harder than inclusive hadron • The momentum sum of head and shoulder scales with nearside in central Dh region RHIC & AGS User Meeting