The PHENIX Experiment at RHIC: Can We Rewind the Clock to Catch a Glimpse Near the Beginning of Time? Thomas K Hemmick, Stony Brook University PHENIX for the PHENIX Collaboration

T.K. Hemmick 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. 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. While cooling, the material of the universe underwent several phase changes. 2.7 Kelvin is the temperature of most of the universe today. 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. 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 Evolution of the Universe Nucleosynthesis builds nuclei up to He Nuclear Force…Nuclear Physics Universe too hot for electrons to bind E-M…Atomic (Plasma) Physics E/M Plasma Too hot for quarks to bind!!! Standard Model (N/P) Physics Quark- Gluon Plasma?? Too hot for nuclei to bind Nuclear/Particle (N/P) Physics Hadron Gas Solid Liquid Gas 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.  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. Reheating Matter

T.K. Hemmick 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. polarized 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 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. How and whether the collisions express the energy as new phases of matter cannot be controlled by us. The collisions are so fleeting (  t ~ sec) the signals from a single such collision travel only several nuclear diameters before the system breaks apart. The collisions are so fleeting (  t ~ 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 The Medium and the Probe  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 Calibrated Probe “The tail that wags the dog” (M. Gyulassy) “The tail that wags the dog” (M. Gyulassy) p+p->  0 + X Hard Scattering Thermally- shaped Soft Production hep-ex/ S.S. Adler et al. “Well Calibrated”

T.K. Hemmick 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” hadrons q q leading particle leading particle schematic view of jet production Once quenched, the jets could not re-appear since this would violate the 2 nd Law of Thermodynamics

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

T.K. Hemmick Nuclear Modification Factor: R AA  We define the nuclear modification factor as:  R AA is what we get divided by what we expect.  By definition, processes that scale with the number of underlying nucleon-nucleon collisions (aka N binary ) will produce R AA =1. R AA 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. nucl-ex/ S.S. Adler et al. Au+Au->  0 +X

T.K. Hemmick 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. Jets are not quenched, but are a priori made in fewer numbers. Color Glass Condensate hep-ph/ ; Kharzeev, Levin, Nardi, Gribov, Ryshkin, Mueller, Qiu, McLerran, Venugopalan, Balitsky, Kovchegov, Kovner, Iancu  Small + Large distinguishes all initial and final state effects. Nucleus- nucleus collision Proton/deuteron nucleus collision

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

T.K. Hemmick R AA vs. R dA for Identified  0 d+Au Au+Au Initial State Effects Only Initial + Final State Effects d-Au results rule out CGC as the explanation for Jet Suppression at Central Rapidity and high p T

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

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

T.K. Hemmick 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 % PHENIX Preliminary Escaping Jet “Near Side” Lost Jet “Far Side” d+AuAu+Au Near Far Min Bias 0-10% 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 “PHENIX Preliminary” results, consistent with STAR data in submitted paper

T.K. Hemmick 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 Heavy Quark States Dissolution of J/  &  ’, the bound states of charm-anticharm quarks probes quark deconfinement.Dissolution of J/  &  ’, the bound states of charm-anticharm quarks probes quark deconfinement. Electromagnetic Probes (no strong interaction) Electromagnetic Probes (no strong interaction) Lack of strong interaction allows them to penetrate the black medium and see through the hadronic veilLack of strong interaction allows them to penetrate the black medium and see through the hadronic veil Direct Photons, e + e -,  +  -Direct Photons, e + e -,  +  -  PHENIX plans to make these measurements in the next Au+Au run.

T.K. Hemmick 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