# Presentation: 128123

## Relativistic Heavy-Ion Collisions: Recent Results from RHIC

- David Hardtke
- LBNL

## Outline

- Why do we study high energy nucleus-nucleus collisions?
- Experimental Results from RHIC
- Particle Multiplicities
- “High” pT phenomena and “jet quenching”
- Conclusions

- David Hardtke - LBNL

- Lepton-Photon 2003

## Relativistic Heavy Ion Collider

- p+p to Au+Au, √s = 20 - 200 (Au+Au) - 500 (p+p) GeV/c
- Today: p+p, d+Au, and Au+Au

- David Hardtke - LBNL

- Lepton-Photon 2003

## The Experiments

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## “Colorful” properties of QCD

- “Colorful” properties of QCD

- Running Coupling Constant  “Asymptotic Freedom”
- Perturbation theory works           well at large momentum        transfer (large Q2)
- Small Q2                 “Confinement”
- No free quarks

- em

- New path to weak coupling limit of QCD:
- High Density or Temperature

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## Lattice QCD Calculations

- Massless ideal gas:

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## Heavy-Ion Physics: The essential questions

- At high temperature, does nuclear matter undergo a phase transition?
- What are the properties of the deconfined phase?
- quark mass : constituent mass (≈300 MeV)  current mass (few MeV) ?
- Residual quark interactions?

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## What’s new with RHIC?

- Quark and gluon degrees of freedom shown to be important elsewhere:
- “Evidence for Hadronic Deconfinement in pbar-p collisions at 1.8 TeV”, E735 Collaboration, PLB (2002)
- “New State of Matter Created at CERN”, CERN Press Release (2000)
- RHIC  *bulk limit ?*
- Thermodynamics of                                                    small systems difficult                                                      to understand
- i.e. Tin nanoclusters

- Shvarsburg and Jarrold, PRL 85

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## Space-time Evolution of Collisions

**e**

****

**space**

**time**

******  ****Expansion  ******

- David Hardtke - LBNL

## Geometry of Heavy Ion Collisions

- Number of participants: number of incoming nucleons (participants) in the overlap region
- Number of binary collisions: number of equivalent inelastic nucleon-nucleon collisions

- Reaction plane

- David Hardtke - LBNL

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## Total Multiplicity

- Multiplicity  Entropy
*Ideal* Quark-Gluon Plasma should have high entropy density
- quark and gluon vs. hadronic degrees of freedom
- Coherent nuclear phenomena may *reduce* total multiplicity
- Shadowing of parton distributions
- “Saturation” of nuclear gluon distributions

- Can we see extra entropy production reflected in the particle multiplicity?

- David Hardtke - LBNL

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## dN/d per participant (s = 200 GeV)

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## Total Multiplicity vs. s 

- David Hardtke - LBNL

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## Why is p+p different?               Leading particle effect?

*“**effective energy”**available for**particle production*

- Basile et al., 1980

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## Universal Particle Production

- Simple relation between e+e- and AA collisions:
- (Any) excess entropy created in heavy-ion collisions *not* reflected in total multiplicity
- Will this relationship hold true at LHC?
- Deeper into nuclear shadowing region of Bjorken x
- A mystery: Why is the total multiplicity unaffected by the complex dynamical evolution of a heavy-ion collision?

- David Hardtke - LBNL

- Lepton-Photon 2003

## Jets in heavy-ion collisions

- Partons lose energy due to induced gluon radiation:               “Jet Quenching”
- Energy loss is measure of gluon density  Indirect QGP signature

- Probe energy loss via leading hadrons and di-hadron correlations

- p+p

- Nuclear Medium

**“****Monojets”**

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## Energy Loss of Scattered Partons in Dense Matter

- elastic scattering of partons dE/dx ~ 0.1 GeV/fm

- nonlinear interaction of gluons in thin plasma

*QCD LPM effect*

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- Lepton-Photon 2003

**Notes:**
CR = color Casimir (=Ncolor for gluons)
v ~log (opacity)
L/mfp of gluon = opacity

## Mid-rapidity spectra

- Au+Au

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## Nuclear Modification Factor RAA

- Yield (pT) in Au+Au and p+p collisions
- Number of binary collisions from Glauber
- Ratio:

- If no “effects”:
-  R < 1 in regime of soft physics
-  R = 1 at high-pT where hard scattering dominates
- Suppression ?
-   Is R < 1 at high-pT ?

- David Hardtke - LBNL

- Lepton-Photon 2003

## Nuclear effects in pA collisions

- Low pT: Coherent interactions and shadowing <1

- High pT:
- multiple scattering
- >1
- “Cronin Effect”

- David Hardtke - LBNL

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## d+Au results from RHIC

- Expected “Cronin” enhancement observed by all four RHIC experiments in d+Au collisions

- David Hardtke - LBNL

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## Nuclear modification RAA

- Peripheral (b≈10 fm): Binary Scaling

- Central (b<3 fm): Factor ≈ 5 suppression

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## Flavor Dependence of Suppression

- Baryons show binary collision scaling at intermediate pT (2-4 GeV/c):
- Novel production mechanism? 	Quark Coalescence: q+q+q  baryon
- Re-discovery of flavor dependence of Cronin effect?

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## Jets in Au+Au? The Challenge

- Central Au+Au Event

- Find this .......................................................in here

- David Hardtke - LBNL

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## Jets in p+p collisions at RHIC

- p+p  dijet

- Two-particle azimuthal correlations:
- p+p events with high pT track (*maximally* biased jet finder)
-  distribution of other tracks (pT>2 GeV/c) in these events
- normalize to the # triggers ...

- David Hardtke - LBNL

- Lepton-Photon 2003

## Jets in d+Au using  correlations

- Jets in d+Au using  correlations

- Similar near-angle and back-to-back correlations
- Increased “pedestal” from multiple interactions

- David Hardtke - LBNL

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## Jets in Au+Au collisions at RHIC

- Peripheral Au+Au ≈ d+Au

- Central Au+Au

**“****Monojets”**

- David Hardtke - LBNL

- Lepton-Photon 2003

## High pT particle production @ RHIC

- Suppression of inclusives compared to binary collision scaling
- High pT  charged hadrons dominated by jet fragments
- Suppression of back-to-back correlations in most central Au+Au collisions

- Surface emission!

- Bjorken ‘82

- Theory Calculations
- dE/dx|t=0 = 7 GeV/fm
- t=0 ≥ 30 nucleus

- David Hardtke - LBNL

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## Jet Quenching ≠ Quark-Gluon Plasma

- Want Equation of State of hot nuclear matter:

- David Hardtke - LBNL

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## Conclusions

*Qualitatively* new physics in Au+Au collisions at RHIC:
- Total multiplicity:  Au+Au ≈ e+e- after accounting for geometry
- Suppression of high pT particle production
- “monojets”
- High pT Au+Au data consistent with “Jet Quenching” scenario
- High gluon density: t=0 ≥ 30 nucleus
- Medium opaque to fast partons
- Essential question: Have we seen the Quark-Gluon Plasma at RHIC?
- Density and temperature of system at or above predicted phase transition temperature, but ...
- No direct evidence for excess entropy production

- David Hardtke - LBNL

- Lepton-Photon 2003