Thomas K Hemmick, Stony Brook University for the PHENIX Collaboration

1. The PHENIX Experiment at RHIC:Can We Rewind the Clock to Catch aGlimpse Near the Beginning of Time? Thomas K Hemmick, Stony Brook University for thePHENIXCollaboration

2. The Beginning of Time • Time began with the Big Bang: • All energy and matter of the universe was in a state of intense heat and compression. • Since then the Universe has cooled • While cooling, the material of the universe underwent several phase changes. • 2.7 Kelvin is the temperature of most of the universe today. • However, there exist a few “hot spots” where the expanding matter has collapsed back in upon itself. • What do we know and what can we learn from laboratory experiments about this past history? T.K. Hemmick

3. Too hot for quarks to bind!!! Standard Model (N/P) Physics • Collisions of “Large” nuclei convert beam energy to temperatures above 200 MeV or 1,500,000,000,000 K • ~100,000 times higher temperature than the center of our sun. • “Large” as compared to mean-free path of produced particles. Too hot for nuclei to bind Nuclear/Particle (N/P) Physics HadronGas Nucleosynthesis builds nuclei up to He Nuclear Force…Nuclear Physics E/M Plasma Universe too hot for electrons to bind E-M…Atomic (Plasma) Physics SolidLiquidGas Today’s Cold Universe Gravity…Newtonian/General Relativity Stars convert gravitational energy to temperature. They “replay” and finish nucleosynthesis ~15,000,000 K in the center of our sun. Reheating Matter Evolution of the Universe Quark-GluonPlasma?? T.K. Hemmick

4. Relativistic Heavy Ion Collider (RHIC)Pioneering High Energy Nuclear Interaction eXperiment (PHENIX) • 2 counter-circulating rings, 3.8 km circumference • Any nucleus on any other. • Top energies (each beam): • 100 GeV/nucleon Au-Au. • 250 GeV polarized p-p. • Maximal Set of Observables • Photons, Electrons, Muons, ID-hadrons • Highly Selective Triggering • High Rate Capability. • Rare Processes. T.K. Hemmick

5. Nature is in Charge • RHIC provides the energy to reheat matter. • PHENIX observes the debris of the collision. • Creation of a primordial medium is out of our hands: • How and whether the collisions express the energy as new phases of matter cannot be controlled by us. • The collisions are so fleeting (Dt ~10-22 sec) the signals from a single such collision travel only several nuclear diameters before the system breaks apart. • Nature must create both the medium and its diagnostic signatures. • We set the stage and fill the audience, Nature puts on the show. T.K. Hemmick

6. Thermally-shaped Soft Production “Well Calibrated” Hard Scattering The Medium and the Probe p+p->p0 + X • At RHIC energies different mechanisms are responsible for different regions of particle production. • The rare process (Hard Scattering or “Jets”) is the probe of whether the soft production products form a medium. • Calibrated Probe • “The tail that wags the dog” (M. Gyulassy) T.K. Hemmick hep-ex/0305013 S.S. Adler et al.

7. schematic view of jet production leading particle hadrons q q hadrons leading particle Fate of Hard Scattered Partons • Hard scatterings in nucleon collisions produce jets of particles. • In the presence of a color-deconfined medium, the partons strongly interact (~GeV/fm) losing much of their energy. • “Jet Quenching” Once quenched, the jets could not re-appear since this would violate the 2nd Law of Thermodynamics T.K. Hemmick

8. Particle Spectra Evolution “Central” Nuclear Physics Particle Physics K. Adcox et al, Phys Lett B561 (2003) 82-92 “Peripheral” T.K. Hemmick

9. Nuclear Modification Factor: RAA • We define the nuclear modification factor as: • RAA is what we get divided by what we expect. • By definition, processes that scale with the number of underlying nucleon-nucleon collisions (aka Nbinary) will produce RAA=1. Au+Au->p0+X RAA is well below 1 for both charged hadrons and neutral pions. The neutral pions fall below the charged hadrons since they do not contain contributions from protons and kaons. T.K. Hemmick nucl-ex/0304022 S.S. Adler et al.

10. Proton/deuteron nucleus collision Nucleus- nucleus collision d+Au Control Experiment • Collisions of small with large nuclei were always foreseen as necessary to quantify cold nuclear matter effects. • Recent theoretical work on the “Color Glass Condensate” model provides alternative explanation of data: • Jets are not quenched, but are a priori made in fewer numbers. • Color Glass Condensatehep-ph/0212316; Kharzeev, Levin, Nardi, Gribov, Ryshkin, Mueller, Qiu, McLerran, Venugopalan, Balitsky, Kovchegov, Kovner, Iancu • Small + Large distinguishes all initial and final state effects. T.K. Hemmick

11. d+Au Spectra • Final spectra for charged hadrons and identified pions. • Data span 7 orders of magnitude. T.K. Hemmick

12. RAA vs. RdA for Identified p0 Initial State Effects Only d+Au Initial + Final State Effects Au+Au d-Au results rule out CGC as the explanation for Jet Suppression at Central Rapidity and high pT T.K. Hemmick

13. Cronin Effect: Multiple Collisions broaden high PT spectrum Charged Hadron Results • Striking difference of d+Au and Au+Au results. • Charged Hadrons higher than neutral pions. T.K. Hemmick

14. “PHENIX Preliminary” results, consistent with PHOBOS data in submitted paper Centrality Dependence Au + Au Experiment d + Au Control Experiment • Dramatically different and opposite centrality evolution of Au+Au experiment from d+Au control. • Jet Suppression is clearly a final state effect. Final Data Preliminary Data T.K. Hemmick

15. Escaping Jet “Near Side” Lost Jet “Far Side” “PHENIX Preliminary” results, consistent with STAR data in submitted paper The “Away-Side” Jet • Jets produced on the periphery of the collision zone coming out should survive. • However, their partner jet will necessarily be pointed into the collision zone and be absorbed. d+Au Au+Au 60-90% Min Bias 0-10% Near Far Near Far PHENIX Preliminary PHENIX Preliminary • Peripheral Au+Au similar to d+Au • Central Au+Au shows distinct reduction in far side correlation. • Away-side Jet is missing in Au+Au T.K. Hemmick

16. What’s Next • We must investigate other probes that look deeply into the medium to characterize it. • Same paradigm, The Rare Processes Probe the Medium: • Heavy Quark States • Dissolution of J/Y & Y’, the bound states of charm-anticharm quarks probes quark deconfinement. • Electromagnetic Probes (no strong interaction) • Lack of strong interaction allows them to penetrate the black medium and see through the hadronic veil • Direct Photons, e+e-, m+m- • PHENIX plans to make these measurements in the next Au+Au run. T.K. Hemmick

17. Summary • We have seen via Au+Au Jet Quenching and the d+Au control experiment that a medium with strong final state effects is formed in Au+Au collisions at RHIC. • Our announcement today is that we indeed have the opportunity to learn about the conditions of our universe soon after the Big Bang. • We have set the stage and Nature has granted us a show. We will measure the properties of the medium and will learn whether or not the quarks are confined. • It would be presumptuous without having measured the additional medium probes to now label the medium in accordance with our preconceptions as being the Quark-Gluon Plasma. • Nature has been known to include surprise endings, the observation and understanding of which represent the real progress in science. T.K. Hemmick

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