Relativistic HeavyIon Collisions: Recent Results from RHIC

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Relativistic Heavy-Ion Collisions Recent Results
from RHIC

• David Hardtke
• LBNL

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Outline

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

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Relativistic Heavy Ion Collider
p?p? to AuAu, vs 20 - 200 (AuAu) - 500 (pp)
GeV/c Today pp, dAu, and AuAu
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The Experiments
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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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Whats 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
time
g
e
? Expansion ?
space
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Geometry of Heavy Ion Collisions
Reaction plane

Number of participants number of incoming
nucleons (participants) in the overlap
region Number of binary collisions number of
equivalent inelastic nucleon-nucleon collisions
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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?
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dN/d? per participant (?s 200 GeV)
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Total Multiplicity vs. ?s
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Why is pp different? Leading
particle effect?
Basile et al., 1980
effective energyavailable forparticle
production
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Universal Particle Production

• Simple relation between ee- 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?

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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

Nuclear Medium
pp
Monojets
Probe energy loss via leading hadrons and
di-hadron correlations
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Energy Loss of Scattered Partons in Dense Matter

• elastic scattering of partons dE/dx 0.1 GeV/fm
  

QCD LPM effect

• nonlinear interaction of gluons in thin plasma

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Mid-rapidity spectra
AuAu
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Nuclear Modification Factor RAA

• Yield (pT) in AuAu and pp collisions
• Number of binary collisions from Glauber
• Ratio

• If no effects
• R lt 1 in regime of soft physics
• R 1 at high-pT where hard scattering dominates
• Suppression ?
• Is R lt 1 at high-pT ?

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Nuclear effects in pA collisions
High pT multiple scattering ?gt1 Cronin Effect
Low pT Coherent interactions and shadowing ??lt1
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dAu results from RHIC
Expected Cronin enhancement observed by all
four RHIC experiments in dAu collisions
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Nuclear modification RAA
Peripheral (b10 fm) Binary Scaling
Central (blt3 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
  qqq ? baryon
• Re-discovery of flavor dependence of Cronin
  effect?

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Jets in AuAu? The Challenge
Find this .in here
pp ?dijet
Central AuAu Event
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Jets in pp collisions at RHIC

• Two-particle azimuthal correlations
• pp events with high pT track (maximally biased
  jet finder)
• ?? distribution of other tracks (pTgt2 GeV/c) in
  these events
• normalize to the triggers

pp ? dijet
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Jets in dAu using ?? correlations

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

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Jets in AuAu collisions at RHIC
Central AuAu
Peripheral AuAu dAu
Monojets
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High pT particle production _at_ RHIC
Surface emission!

• 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 AuAu collisions

Bjorken 82
Theory Calculations dE/dxt0 7 GeV/fm ?t0
30 ?nucleus
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Jet Quenching ? Quark-Gluon Plasma

• Want Equation of State of hot nuclear matter

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Conclusions

• Qualitatively new physics in AuAu collisions at
  RHIC
• Total multiplicity AuAu ee- after
  accounting for geometry
• Suppression of high pT particle production
• monojets
• High pT AuAu data consistent with Jet
  Quenching scenario
• High gluon density ?t0 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