# Presentation: 645534

## Relativistic Heavy Ion PhysicsWhere Are We and Where Are We Going?

- Jamie Nagle
- University of Colorado at Boulder
- Aspen 2004 Winter Conference

## Quark Gluon Plasma

- Lattice QCD predicts a phase transition to a Quark Gluon Plasma at high temperature where the number of degrees of freedom is significantly increased.

- Phase Transition:
-   T = 170 MeV ~ 1012 0F
-   =  0.8 GeV/fm3
- Assumes thermal system.

- Expect a “weakly” interacting gas of quarks and gluons where the long range confining potential is screened.

## Deconfinement Transition

- _QCD in Vacuum_
-  linear increase in potential with
-     distance from color charge
-  strong attractive force
-  _spontaneous breaking of chiral     _
-     _symmetry_
-  _confinement of quarks _to
-     hadrons baryons (qqq) and
-     mesons (qq)

- _QCD in dense and hot matter_
-  screening of color charges
-  potential vanishes for large
-     distance scales
-  _restoration of approximate chiral    _
-     _symmetry_ as quarks act as nearly
-     massless particles
-  _deconfinement of quarks !_

- r

- V(r)/

- Lattice QCD calculation

## Heavy Ion Collisions

- _Heavy Ion Collisions_

- 10,000 gluons, quarks, and antiquarks
- from the nuclear wavefunctions
- are made physical in the laboratory !
- What is the nature of this ensemble of partons?

## Where Are We?

- _Where Are We?_

-  Relativistic Heavy Ion Collider online since 2000.
-  Design Au+Au energy and luminosity achieved.
-  All experiments successfully taking data

## Experimental Observations

- As an experimentalist, let the data speak for itself.

- At least as much as possible.

## Perturbative QCD and Factorization

-  Proton+Proton 0 + X
-  NLO pQCD calculation agrees with data from 2-13 GeV
-  Extend to nuclei by expected
- scaling with nuclear thickness
- calculated as a function of impact parameter

## What About Heavy Ion Reactions?

-  Peripheral Au+Au = superposition of p+p reactions.
-  Central Au+Au reveals a significant suppression !

**d + Au Control Experiment**

- Preliminary Data

**Au + Au Experiment**

**d + Au Control**

## Jet Correlations

**away-side**

**near-side**

-  Complete jet reconstruction is not available
-  We study back to back jets via angular correlations

## “Disappearance” of Away-Side Jet

-  In central Au+Au reactions, the away-side jet disappears (blue points)
-  No correlation for particles with pT > 2 GeV

**away side**

**near side**

-  Conservation of energy and momentum says that away side jet cannot really disappear

## Where is the Energy?

- High pT trigger hadron selects surface emission.
- Thus, away-side partner has maximum path through the medium.

## Opaque Medium

- Perhaps partons lose energy in medium, and thus fragmentation products are below pT > 2 GeV.
- Perhaps scattering causes broadening of angular distribution.

## Thermalization of Energy !

- PHENIX

-  Away side jet is significantly broadened
-  One recovers energy in hadrons ~ 500 MeV.
-  Not so different from <pT> of the bulk medium.

## Direct Photons Shine!

-  Use measured 0 , and look for photon excess
-  Excess agrees with NLO pQCD direct photons
-    scaled by nuclear thickness
-  Direct photons unaffected by medium as expected

- pT

## Charm and Beauty

- _Charm and Beauty_

- QCD is flavor independent, but heavy quarks at same pT are moving much slower than light quarks.
- Expected “dead-cone” with no induced gluon radiation.
- Recent single lepton and direct
- D reconstruction allow us to
- address this physics.

- c

- D±

## Jet Quenching

- _Jet Quenching_

- Baier, Dokshitzer, Mueller, Schiff, hep-ph/9907267
- Gyulassy, Levai, Vitev, hep-pl/9907461
- Wang, nucl-th/9812021
- and many more.....

-  Partons are expected to lose energy via induced gluon radiation in traversing a dense colored medium.
-  Coherence among these radiated gluons leads to E  L2
-  Effective softening of fragmentation function.

## Data and Theory Collide

- _Data and Theory Collide_

**Au-Au**

**d-Au**

**dAu**

- Induced gluon brehmstrahlung pQCD calculation Agrees with data with initial gluon density:
- dNg/dy ~ 1100 > 100 0

- Lowest energy radiation sensitive to infrared cutoff.

## Collective Motion

-  In non central collisions, large initial spatial anisotropy
-  The degree to which this translated into momentum space is an excellent measure of the pressure

## Large Pressure Felt By All Hadrons

-  Very large rescattering, more than most predicted
-  Multi-strange baryons flow, and maybe charm too !

- PHENIX preliminary

## Hydrodynamics

-  Assume early equilibration, and initial energy density
-   from gluon radiation calculations
-  Equations of Motion
-  Equation of State from lattice QCD

## Data and Theory Collide (II)

-  Agreement between hydrodynamics at mid-rapidity for hadron spectra and elliptic flow (v2) up to           pT ~ 2 GeV
-  First time hydrodynamics without any viscosity describes heavy ion reactions

## Some Puzzles?

- More (anti) baryons than pions at moderate pT.
- Does not look like vacuum jet fragmentation.

- Proton-Proton

- Central Au-Au

## Some Puzzles?

- Hydrodynamic models have trouble describing longitudinal motion
- Many assume boost-invariance (plateau in rapidity)
- Particle correlations indicate shorter lifetime

- BRAHMS

## Quark-Gluon Plasma?

- Many indications of energy density well above phase transition value.
- However, it does not look anything like a “weakly” interacting gas of quarks and gluons.
- More like a liquid with
- zero viscosity.
- New quasi-particles or
- couplings?
- Help needed from lattice,
- effective theory, string theory

## Interactions Above Tc

- Recent lattice QCD calculations indicate
-  Bound states at T ~ 1.5 Tc
-  Quark-Antiquark interactions persist above Tc

- Key measurements of quarkonia and low mass vector meson spectral functions are the next step

## Nuclear Structure Function Physics

- x

**Saturation?**

**shadowing**

**enhancement**

**EMC effect**

**Fermi Effect**

**Color Glass Condensate**

- Proton-Nucleus and Deuteron-Nucleus reactions at RHIC are extending studies from FNAL fixed target program.
- Collider allows one to probe
- both forward and backward
- xF ranges.
- Also, complementary gluon
- saturation physics to HERA.
- Possible future electron-ion
- collider at RHIC.

## Where Are We Going?

_**RHIC has passed the first key test.**_
- Nuclear Physics community is capable of constructing and running world class “high-energy” type experiments and
- re-constructing the physics from the 10,000 particle debris.
_**The second phase has arrived.**_
- First observations of very dense gluonic medium with strong pressure built up.
- Gluon density well above predicted phase transition level and behaving as zero viscosity opaque liquid.
_**The third phase is next.**_
- Quarkonia measurements to test deconfinment.
- Low mass vector mesons for parton correlations in plasma.
- Large Hadron Collider heavy ion program will add many complementary studies.

## Where are We Going?

- For Relativistic Heavy Ions,
- The Future is Now.