Relativistic Heavy Ion Physics: Results from AGS to RHIC

1
Relativistic Heavy Ion PhysicsResults from AGS
to RHIC

• Peter Steinberg
• Brookhaven National Laboratory

Thanks to PHOBOS collaboration, W. Zajc, J.
Nagle, M. Belt-Tonjes, F. Videbaek
Also see poster by John Haggerty, BNL
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Studying QCD with Heavy Ions

• QGP
• QCD at high T, high density
• Phase diagram of QCD
• Relevance of lattice calcuations
• Strongly-interacting systems
• Evolution of colliding nuclei
• Can we study early-stage dynamics with
  final-state hadrons?

hadrons ?quark/gluon
Hadrons
Nuclei
Hard Interactions
Parton Cascade (QGP?)
(F. Karsch, hep-lat/0106019)
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Heavy-Ion Colliders
3 orders of magnitude! (by 2009)
However, RHIC data only makes sense in context
We want to understand strong interactions in all
forms
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RHIC Capabilities Run I
Performance Year 1 Year
2 ?snn 130 GeV 200 GeV L cm-2
s -1 2 x 1025 2 x 1026
Interaction rates 100 Hz up to 2500 Hz

• Nucleus-nucleus (AA) collisions up to ?sNN 200
  GeV
• Polarized proton-proton (pp) collisions up to
  ?sNN 450 GeV

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RHIC Experiments _at_ BNL
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Large RHIC Experiments
STAR Hadronic Observables over a Large
Acceptance Event-by-Event Capabilities Solenoidal
magnetic field Large coverage Time-Projection
Chamber Silicon Tracking, RICH, EMC, TOF
PHENIX Electrons, Muons, Photons and Hadrons
Measurement Capabilities Focus on Rare Probes
J/y, high-pT Two central spectrometers with
tracking and electron/photon PID Two forward muon
spectrometers
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Small RHIC Experiments
BRAHMS Hadron PID over broad rapidity
acceptance Two conventional beam line
spectrometers Magnets, Tracking Chambers, TOF,
RICH
PHOBOS Charged Hadrons in Central
Spectrometer Nearly 4p coverage multiplicity
counters Silicon Multiplicity Rings Magnetic
field, Silicon Pad Detectors, TOF
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What have we learned with Heavy Ions at RHIC?
1
2
3
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Colliding Nuclei
Parton Cascade
Freeze-out to Hadrons
HardCollisions
Initial StateDynamics
Properties of Final State
Initial Conditions
Probes ofInitial State
With x10 increase in energy, can study interplay
between
Energy
Geometry
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Why Energy Matters (pp)
RHIC
AGS
Fragmentation Region grows
SPS
Plateau rises

• Low Energy ? Reggeon exchange
• Valence (quark) degrees of freedom
• High Energy ? Pomeron exchange
• Sea (gluon) degrees of freedom

• With increasing energy
• increasing beam rapidity
• Ylog(?s/m)
• probe smaller x at midrapidity

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Why Geometry Matters

• Binary Collisions
• Jet Production
• Heavy Flavor

b
Glauber model of AA
Binary Collisions
Npart, Ncoll

• Color Exchange
• Soft Hadron Production
• Transverse Energy

Participants
wounded nucleon model
b (fm)
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Measuring Centrality

• Cannot directly measure the impact parameter!
• but we can distinguish
• peripheral collisions from
• central collisions!

Nch
Spectators
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Zero-degreeCalorimeter
Spectators
Paddle Counter
Npart

• Spectators measured directly with zero-degree
  calorimeters
• Participants via monotonic relationship with
  produced particles

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Initial Conditions

• What happens when two nucleicollide at high
  energy?
• Proton stopping
• What can we understandabout the initial parton
  densitiesby examination of the final state?
• Energy particle density
• Can we relate this to QCD as studied at HERA?
• Parton saturation models
• Can we relate this to QCD at LEP?
• Universality of particle production

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Proton Stopping

• How does energy get from y 5.4 to y 0 ?
• Studied in pp and pA by measuring leading proton
• ltxgt.5 so half of energy lost per collision
  (corr. to Dy1)

200 GeV
BRAHMS Preliminary
xF
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Charged Particle Density _at_ h0

• Npart 340 (6 most central)
• 5000 total charged particles!

• AuAu collisions produce 50 more particles per
  effective NN collision than proton-proton
  collisions at same ?s

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Energy Density

• Q Does energy density (considered by lattice
  calculations) relate to particle density?

PHENIX finds a constant amount of transverse
energy(ET) per particle vs. impact parameter
A Energy density comes from producing
more particles!
Bjorken Estimate
to
R
Central Events
(iff R1.18A1/3 to 1fm/c)
e(200 GeV) 4.6 x 1.15 5.3 GeV/fm3
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QCD Structure of the proton

• DGLAP evolution good description of F2
• pQCD predicts a large gluon density at low-x

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Parton Saturation

• Gluon distribution rises rapidly at low-x
• Gluons of x1/(2mR) overlap in transverse plane
  with size 1/Q
• Below saturation scale Qs2 gluon recombination
  occurs
• Saturation scale measures density of partons in
  the transverse plane
• Increases with A and/or ?s

Saturation describes HERA data!
Scale depends on thickness
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Saturation vs. Geometry ?s
More paritcles ?
0
100
200
300
400
More central ?
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Consequences of Parton Saturation

• Saturated initial state gives predictions about
  final state.
• N(hadrons) c ? N(gluons) (parton-hadron
  duality)
• Describes energy, rapidity, centrality dependence
  of charged particle distributions
• t 1/Qs .2 fm/c
• e 18-20 GeV/fm3

Kharzeev Levin, nucl-th/0108006
? Does successful description imply large gluon
density?
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Total Multiplicity in pp and ee-
ee-
inclusive
p p
ISR data

• Basile et al (1980) measured
• xpz/pz,max of leading proton
• Nch in same hemisphere

Universality
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Universality in the RHIC era?
1-parameter QCD fit to ee- ? hadrons data
PRELIMINARY

• RHIC data on
• ?Nch ?/?Npart ?/2
• Smoothly approaches ee- data at high energies!
• Proton-proton data has effective energy ?s ?s
  /2

AGS
SPS
RHIC
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Rapidity Distributions at 200 GeV
PRELIMINARY
3 Central
Systematic Error for AA
yT
h
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Initial State Implications

• Implications of ee- comparisons intriguing but
  not well understood
• AA total multiplicity scales as (Npart/2)xNee-
• pQCD describes ee-
• DGLAP vs. Saturation controversy in DIS
• Why is ?s the effective energy in AA, rather than
  ?s/2 as in pp? More stopping?
• ET gives e gt 5 GeV
• RHIC may be in saturation regime
• Describes energy, rapidity, centrality dependence
• Saturation suggests e 18-20 GeV!
• Does this imply a thermalized system?

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Dynamics of Parton Cascade
If sufficient rescattering occurs (e.g.
thermalization)
2
3.
2.
1.
v2 7
reactionplane
v2 0

• Initial state geometry (almond)
• maps onto
• Final state angular distribution
• Elliptic Flow

Hydro Limit v2 e
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Elliptic Flow at 130 GeV
6.5

• Hydrodynamic calculations, with e20 GeV/fm3,
  agree with flow in central events

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Energy Dependence
Maximum v2
H. Appelshauser, QM2001
High EnergyEmission from surface
Low energy Pions interactwith spectators

• Instead of disappearing, maximum elliptic flow
  grows with beam energy above ?s4 GeV

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Flow Systematics vs. Hydro

• Hydro calculations predict many systematics of v2
• Describes low pT vs. particle mass
• Fails at high pT and large rapidity open
  questions!

T. Hirano
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Description of Final State
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• How do we characterize final hadronic state?
• Equilibrated hadron gas?
• Chemical Equilibrium total yields of hadron
  species
• Thermal Equilibrium spectral shapes
• Does it show any collective behavior, e.g.
  expansion?
• How long does it last?
• RHIC experiments have a wide range of tools to
  address these questions
• Only a subset mentioned here!

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Particle Ratios

• In absence of baryons (e.g. early universe)
• expect symmetry between matter antimatter
• Anti-particle/particle ratios 1
• At RHIC, ratios at y0 are approaching unity
• still see evidence of initial baryon asymmetry
• Closer than ever before to net-baryon-free

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Chemical Equilibrium
M. Kaneta, STAR Collaboration

• Thermal model lets us put data on QCD phase
  diagram
• RHIC energies appear close to Tc
• Data from different energies has simple trend
• ?Eh?/?Nh? 1 GeV (is this a general property of
  hadronization?)

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Strangeness vs. Energy

• Strangeness seen as a QGP signal
• Yields are 2x larger in AA than ee-,pp
• Explained as transition from
• Canonical ensemble
• Grand-canonical
• Volume effect
• Strangeness suppressed in elementary collisions!
• Equilibrated in AA
• At lower energies, increased strange baryon
  production associated with baryon density (mB)

J. Cleymans, hep-ph/0201142
Wroblewski factor
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Thermal Freeze-out Radial Flow
TthltTch
N. Xu, QM2001
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Thermal Fits to Particle Spectra

• Example from Broniowski Floriowski,
  nucl-th/0112043
• Unified framework
• Hubble expansion
• Sudden freezeout
• ? tchemical tthermal
• Fits measured spectra
• PHENIX STAR data at all pT
• Even K(892) (t4fm) fits
• Justifies single time
• Consequences
• System appears to expand and fall apart
• Hadronic phase appears to be short (lt2fm)

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Hadronization Implications

• Particle ratios
• Approaching unity vs. energy
• Mid-rapidity approaches zero net-baryon density
• System appears to be in chemical equilibrium
• T170 and mB?0 (similar to ee-)
• Strangeness is equilibrated in AA, suppressed in
  pp, ee-
• Hadronic system appears to be expanding (v.5c)
• And it falls apart over short duration (lt2 fm)
• Open question is it hadrons which equilibrate?
• Perhaps the prehadronic phase is already
  equilibrated?
• Consistent with presumption that elliptic flow is
  an initial- state effect

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Probing the Early Stages
2
Hard Probes
Saturated gluon densities
Expanding Final State
HydrodynamicFlow

• pQCD jets can be used to study medium
• Partons interact strongly with other partons
• Weakly with colorless bound hadrons

Hadron gas
QGP
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Jet-Quenching

• While jet production can be calculated in pQCD,
  it is hard to see jets like at LEP
• Soft production fills in the gaps

• Fragmentation functions allow prediction of
  hadron spectra from pQCD cross sections
• Quenching leads to a suppression of expected
  leading hadron spectrum

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Evidence for Jet Quenching

• PHENIX results for 130 GeV
• UA1 scaled to AuAu by Ncoll
• Same spectrum as UA1 for peripheral events
• Charged particles in central events fail to scale
  with binary collisions
• Identified pions are even lower
• Nuclear shadowing Cronin effect do not modify
  result

Poster by J. Haggerty
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Centrality Dependence

• NEW STAR 130 GeV data!
• RAA shown vs. centrality of collision
• Results similar to PHENIX for central events
• Violation of scaling not seen in peripheral
  events!
• Interesting question
• Does ANYTHING scale like binary collisions?

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Open Charm Yields
Poster by J. Haggerty

• PHENIX measurement
• Electron spectrum
• Hadronic sources
• Contributions from c,b

• Charm cross section calculated per binary
  collision
• 380 mb ? 200 (sys) ? 60 (stat)
• Consistent with binary scaling!
• Interesting baseline for J/Y

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J/Y suppression at the SPS

• Striking legacy of the CERN SPS program
• CERN Press release in Feb. 2000!
• NA50 measures
• Drell-Yan seems to scale like Ncoll
• J/Y suppressed relative to normal nuclear
  suppression seen in pA collisions
• Similar pattern
• Some hard processes seem to scale, others fail

NA50
Centrality
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Hard Probes Implications

• High-pT production should stem from point-like
  processes
• Hard processes test binary scaling
• Data available at pT lt 5 GeV
• Light quarks appear to violate scaling
• Heavy quarks appear to obey it
• More studies needed
• Higher pT for light quarks (does quenching away?)
• Better statistics for heavy quarks (PID!) to test
  scaling
• Unclear if jet quenching requires QGP, or just
  a large parton density (not necessarily
  deconfined)
• Intriguing that high pT yield approach Npart
  scaling

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Coherent Interactions

• b gt 2RA
• no hadronic interactions
• ltbgt 20-60 fermi at RHIC
• Ions are sources of fields
• Photons (Z2)
• Pomerons or mesons (mostly f0)
• A2 (bulk) A4/3 (surface)
• Fields couple coherently to ions
• Photon/Pomeron wavelength l ?/p gt RA
• P? lt ?/RA, 30 MeV/c for heavy ions
• P lt g?/RA 3 GeV/c at RHIC
• Strong couplings ? large cross sections

Au
g, P, or meson
Au
Coupling nuclear form factor
s(r) 350 mb 5 of sAuAu(had.) at 130
GeV/nucleon
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Exclusive r0

• Trigger on low multiplicity events
• veto on cosmic rays
• 2 track vertex w/ charge 0
• reject (coplanar) cosmic rays
• Peak for pT lt 150 MeV/c
• Background shape from pp and p-p-

Signal region pTlt0.15 GeV
r0 PT
pTlt0.15 GeV
M(pp-)
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r0 Nuclear Excitation

• Nuclear excitation tags small b interactions
  neutron signals in both ZDCs
• excitation and r0 are independent
• Normalized to 7.2 b hadronic cross section
• Exclusive r0 bootstrapped from XnXn
• Good agreement
• factorization works!
• Nucl-ex/0206004

Baltz, Klein Nystrand (2002)
SimilartoZEUS!
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Status of RHIC Physics Program

• Evidence at RHIC points toward creation of
• a deconfined state of large gluon density that
  thermalizes early, expands, and freezes out
  suddenly
• Many results so far consistent with this
  interpretation
• Energy Density exceeds lattice QCD expectations
• Initial conditions saturated gluon picture
  describes dN/dh
• Initial state hydrodynamic flow seen by large
  v2
• Final state thermal fits, radial flow, HBT
  measurements
• Hard Probes jet quenching vs. open charm
• Ultra-peripheral collisions studying coherent
  production in strong fields - rs for now, J/Ys
  to come.
• Run II (200 GeV) completed in November 2001
• All experiments with new capabilities, larger
  data sets
• Polarized pp spin program ran in Dec-Jan 2002

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RHIC Run II
L (mb-1)
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The End
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Balance Functions

• Rapidity correlations
• Opposite charges
• Measures time of freezeout, rather than duration
• Early (pp or hadron gas)
• Late (QGP)

Central events narrower than Peripheral!
S. Pratt, et al
M.Belt-Tonjes
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Nuclear Modifications at high pT
shadowing
EMC effect

• Cronin Effect
• Multiple scattering innucleus modifies pT
  spectra

• Nuclear Shadowing
• Low-x gluons see entire nucleus (cf.
  saturation)
• Reduces effective gluon flux

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Persistence of Quenching

• Calculations incorporate known nuclear effects
• Shadowing and the Cronin effect do not have an
  appreciable effect on the calculated spectrum
• So far, only energy loss can explain the PHENIX
  observation

Scaled pp
Shadowing Cronin
EnergyLoss
With expansion!
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Energy Dependence near y0
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Scaling of Total Multiplicity
Derived from PHOBOS data
200 GeV
130 GeV
For pp