# Presentation: 506412

## 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
- for the PHENIX Collaboration

## 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

## Evolution of the Universe

**Quark-****Gluon****Plasma??**

- 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.**
**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.
- The collisions are so fleeting (t ~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

## 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
- “The tail that wags the dog” (M. Gyulassy)

**p+p->********0**** + X**

- hep-ex/0305013 S.S. Adler et al.

- 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”

- Once quenched, the jets could not re-appear since this would violate the 2nd 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: RAA

- 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.

- 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.

- nucl-ex/0304022 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.
- Color Glass Condensate hep-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

## d+Au Spectra

- Final spectra for charged hadrons and identified pions.
- Data span 7 orders of magnitude.

- T.K. Hemmick

## RAA vs. RdA for Identified 0

- RAA vs. RdA 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 pT

- T.K. Hemmick

## Charged Hadron Results

- Striking difference of d+Au and Au+Au results.
- Charged Hadrons higher than neutral pions.

- 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.

**Au + Au Experiment**

**d + Au Control Experiment**

- Preliminary Data

- Final 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.

**60-90%**

**PHENIX Preliminary**

**d+Au**

**Au+Au**

**Near**

**Near**

**Far**

**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

- 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
- Dissolution of J/ & ’, 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-, +-
- 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

## Slide 18

- T.K. Hemmick