Experimental Confirmation of the Formation of Dense, Hot Matter in Nuclear Collisions @ RHIC

1. Experimental Confirmation of the Formation of Dense, Hot Matter in Nuclear Collisions @ RHIC Prof. Brian A. Cole Columbia University On Sabbatical leave at KFKI, RMKI Supported by a Fulbright grant from the US State Dept. and the Hungarian-American Commission for Educational Exchange, a NATO Fellowship, and the Hungarian National Science Fund

2. The Relativistic Heavy Ion Collider STAR

4. Experiment @ RHIC

5. PHENIX Proton-Proton Particle Spectra q Theory q Energy Energy • We have a good theoretical understanding of high energy, large-angle scattering. Number of Particles @ 90º

6. Au-Au Particle Spectra From PHENIX Theories without energy loss Expected* Number of Particles @ 90º Ratio of Observed/Expected* Theories with energy loss Observed Energy Energy • We observe only 20% of the expected particles. • Consistent with theories including energy loss. * — includes only trivial increase in number of particles due to the “size” of the nuclei

7. PHENIX: Au-Au Final Results from 2002 q q • The suppression of high energy particles goes away when we create less matter. Submitted to Phys. Rev. Letters Ratio of Observed to Expected* Energy

8. PHENIX d-Au Measurement d-Au 2 independent measurements Au-Au Energy • Control measurement shows no suppression • If anything we see a modest excess • Understood theoretically (was predicted!) • Suppression of high energy particles in Au-Au is apparently caused by the created matter. Ratio of Observed to Expected*

9. STAR Experiment: “Jet” Observations Number of pairs q q 0º 180º Angle between high energy particles Analyze by measuring angles between pairs of particles. proton-proton jet event • In Au-Au collisions we see only one “jet” at a time ! • How can this happen ? • Possibly due to energy loss

10. STAR: Jets in d-Au Number of pairs 0º 180º Angle between high energy particles q q Disappearance of jet pairs in Au-Au collisions is apparently caused by the created matter.

11. Experimental Summary • We have shown two striking experimental observations: • Consistent observations have been obtained by all 4 experiments at RHIC (where applicable) • In both Au-Au and deuteron-gold collisions • currently can only be explained as resulting from the “quenching” of high energy particles in the hot dense medium created in Au-Au collisions. • deuteron-gold control measurements were essential to rule out alternative explanations for the observations. • Triumph of the scientific method !

12. My profound thanks to Hungarian Fulbright Commission, Dr. Bruckner and his staff RMKI, Dept. of Theoretical Physics, and NATO for their outstanding support on behalf of myself and my family.