Hot Matter and Cool Results from RHIC

1
Hot Matter and Cool Results from RHIC
Every sentence I utter must be understood not
as an affirmation, but as a question. - Niels
Bohr (1885-1962)
QCD at the Interface between Particle
and Nuclear Physics
2
QCD For Beginners
Quarks confined within hadrons via strong
force v(r) a/r sr At large r -second term
dominates At small r -Coulomb-like part
dominates However a function of q( mtm
transfer) and a -gt 0 faster than q (or 1/r)
-gt infinity (called asymptotic freedom) This
concept of asymptotic freedom among closely
packed coloured objects (q and g) has led to one
of the most exciting predictions of QCD !! The
formation of a new phase of matter where the
colour degrees of freedom are liberated. Quarks
and gluons are no longer confined within colour
singlets.
The Quark-Gluon Plasma!
3
Lattice QCD at Finite Temperature
Recently extended to mBgt 0, order still unclear
(2nd, crossover ?)
Ideal gas (Stefan-Boltzmann limit)
F. Karsch, hep-ph/0103314
Tc 150-170 MeV ec 1 GeV/fm3
4
(QCD) Phase Diagram of Nuclear Matter
TWO different phase transitions at work!
Particles roam freely over a large volume
Masses change Calculations show that these occur
at approximately the same point Two sets of
conditions High Temperature High Baryon Density
Deconfinement transition
Chiral transition
5
Time Scales of a Relativistic Heavy Ion Collisions
soft physics regime
e.m. probes (ll-, g)
hard (high-pT) probes
Chemical freezeout (Tch ? Tc) inelastic
scattering stops Kinetic freeze-out (Tfo ? Tch)
elastic scattering stops
6
RHIC _at_ Brookhaven National Laboratory
Relativistic Heavy Ion Collider
h
Long Island

• 2 concentric rings of 1740 superconducting
  magnets
• 3.8 km circumference
• counter-rotating beams of ions from p to Au

• 2000 run
• AuAu _at_ ?sNN130 GeV
• 2001 run
• AuAu _at_ ?sNN200 GeV
• polarized pp _at_ ?s200 GeV (P 15)

7
Geometry of Heavy Ion Collisions
spectators
Particle production scales with increasing
centrality
peripheral (grazing shot)
central (head-on) collision
Number participants (Npart) number of nucleons
in overlap region
Number binary collisions (Nbin) number of
equivalent inelastic nucleon-nucleon
collisions
Nbin Npart
8
Au-Au Central Events at RHIC
STAR
9
Charged Particle Multiplicity
200 GeV
19.6 GeV
130 GeV
PHOBOS Preliminary
dNch/dh
Central
Peripheral
h
Central at 130 GeV 4200 charged particles !
Total multiplicity per participant pair scales
with Npart
Not just a superposition of pp
10
?B/B Ratios
RHIC Preliminary Au-Au 130 GeV
?B - all from pair production B - pair
production transported ?B/B ratio 1 -
Transparent collision ?B/B ratio 0 - Full
stopping, little pair production

• All data
• mid-rapidity
• ratios from raw yields

2/3 of proton from pair production First time
pair production dominates Still some baryons from
beam
11
Do We Reach the Critical Energy Density?
Bjorken formula for thermalized energy density
PHENIX
EMCAL
time to thermalize the system (t0 1 fm/c)
130 GeV
6.5 fm
pR2
30 times normal nuclear density 5 times above
ecritical from lattice QCD
For Central Events
eBjorken 4.5 GeV/fm3
12
Is There Collective Motion?
Look at Elliptic Flow
SPS, RHIC
AGS
Almond shape overlap region in coordinate space
Anisotropy in momentum space
Interactions
v2 2nd harmonic Fourier coefficient in dN/d?
with respect to the reaction plane
13
Hydro Calculation of Elliptic Flow
A pressure build up -gt Explosion zero for
central events self quenching Elliptic flow
observable sensitive to early evolution of
system Collective motion large energy density
-gtHydrodynamics Assumes continuum matter
with local equilibrium, thermalization
Equal Energy Density lines
P. Kolb, J. Sollfrank, and U. Heinz
Large v2 is an indication of early
thermalization
Heavy-Ion Collisions create a system which
approaches hydrodynamic limit
14
Identified Particle V2
STAR PRL87 (2001)182301
Hydro-inspired model also predicts mass
dependence well
15
Kinetic Freeze-Out and Radial Flow
Want to look at how energy distributed in
system. Look in transverse direction so not
confused by longitudinal expansion
Slope 1/T
Look at pt or mt ?(pt2 m2 ) distribution
A thermal distribution gives a linear
distribution dN/dmt ? e-(mt/T)
If there is radial flow
dN/dmt- Shape depends on mass and size of flow
Heavier particles show curvature
16
Radial Flow and Hydrodynamical Model
PHENIX Preliminary
STAR Preliminary
Models differ slightly in details but same concept
PHENIX Tfo 104 ? 21 MeV, lt ?t gt 0.5 ?
0.1c STAR Tfo 107 ? 8 MeV, lt ?t gt 0.55 ? 0.1c
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Tfo and ltbrgt vs vs

• lt?r gt
• increases continously
• Tfo
• saturates around AGS energy

• Strong collective radial expansion at RHIC
• high pressure
• high rescattering rate
• Thermalization likely

Slightly model dependent here blastwave model
(Kaneta/Xu)
18
Models to Evaluate Tch and ?B

• Statistical Thermal Model
• F. Becattini P. Braun-Munzinger, J. Stachel, D.
  Magestro
• J.Rafelski PLB(1991)333 J.Sollfrank et al.
  PRC59(1999)1637
• Assume
• Ideal hadron resonance gas
• thermally and chemically equilibrated fireball
  at hadro-chemical freeze-out
• Recipe
• grand canonical ensemble to describe partition
  function ? density of particles of species ?i
• fixed by constraints Volume V, , strangeness
  chemical potential ?S, isospin
• input measured particle ratios
• output temperature T and baryo-chemical
  potential ?B

Particle density of each particle
Qi 1 for u and d, -1 for ?u and ?d si 1 for
s, -1 for ?s gi spin-isospin freedom mi
particle mass Tch Chemical freeze-out
temperature mq light-quark chemical
potential ms strangeness chemical
potential gs strangeness saturation factor
Compare particle ratios to experimental data
19
Beautiful Agreement Between Model Data
Does the success of the model tell us we are
dealing indeed with locally chemically
equilibrated systems?
This flow measurements If you ask me Yes!
20
Phase Diagram from AGS to RHIC
Tch MeV mB MeV
AGS ?s 2-4 GeV 125 540
SPS ?s 17 GeV 165 250
RHIC ?s 130-200 GeV 175 30
Again slight variations in the models
QCD on Lattice Tc 1738 MeV, Nf2 Tc 1548
MeV, Nf3
Remember Measure hadrons not partons so cant
measure Tgt Tc with this method
21
Summary on Soft (pT lt 2 GeV/c) Physics

• Particle production is large
• Total Nch 5000 (AuAu ?s 200 GeV) ?
  20 in pp
• Nch/Nparticipant-pair 4 (central
  region) ? 2.5 in pp
• Vanishing anti-baryon/baryon ratio (0.7-0.8)
• close to net baryon-free but not quite
• Energy density is high ? 4-5 GeV/fm3 (model
  dependent)
• lattice phase transition 1 GeV/fm3, cold matter
  0.16 GeV/fm3
• System exhibits collective behavior (radial
  elliptic flow)
• strong internal pressure that builds up very
  early
• explosive expansion
• Particles ratios suggest chemical equilibrium
• Tch?170 MeV, mblt50 MeV ? near lattice phase
  boundary

Overall picture System appears to be in
equilibrium but explodes and
hadronizes rapidly
22
High-pT Hadrons at RHIC
Now even have own pp measurements so detector
effects cancel
All 4 experiments have an impressive array of
data out to high pT
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Why study high pT physics at RHIC ?
Early production in parton-parton scatterings
with large Q2. Direct probes of partonic phases
of the reaction

• New penetrating probe at RHIC
• attenuation or absorption of jets jet
  quenching
• suppression of high pT hadrons
• modification of angular correlation
• changes of particle composition

24
Nuclear Modification Factor
Hard Physics -
Scales with Nbin Number of binary collisions
number of equivalent inelastic nucleon-nucleon
collisions
Nuclear Modification Factor
25
Hadron Suppression AuAu at 200 GeV
charged hadrons
p0
PHENIX preliminary
Suppression of central yields persists up to
pT10 GeV/c
26
Hadron Suppresion for Identified Particles

• L and p show different behaviour to Ks and
  p

p
Suppression of L sets in at higher pT
p0
Seem to come together at 6GeV/c - standard
fragmentation?
L
K0s
Is this a mass effect or a baryon/meson effect ?
STAR Prelimimary
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Azimuthal Anisotropy (v2)of Particle Emission
low pT high pT
Bulk (Hydrodynamic) Matter
Jet Propagation
Pressure gradient converts position space
anisotropy to momentum space anisotropy
Energy loss results in anisotropy due to
different length of matter passed through by
parton depending on location of hard scattering
28
Elliptic Flow at High-pT
Jet propagation through anisotropic matter
(non-central collisions)
STAR _at_ 200 GeV

• Finite v2 high pT hadron correlated with
  reaction plane from soft part of event (pTlt2
  GeV/c)
• Finite asymmetry at high pT
• ? Significant in-medium interactions even at 10
  GeV/c

29
Jets in Heavy Ion Collisions
ee- ? q q (OPAL_at_LEP)
pp ?jetjet (STAR_at_RHIC)
AuAu ???? (STAR_at_RHIC)
Jets in Au-Au hopeless Task?
No, but a bit tricky
30
Leading Particle Correlations
Leading Particle

• Trigger on high pT leading particle
• Jet core Df Dh 0.5 0.5
• ? study near-side correlations (Df0) of
  high pT hadron pairs
• Complication elliptic flow ? high pT hadrons
  correlated with the reaction plane (v22)
• Solution compare azimuthal correlation
  functions for
• Dhlt0.5 (short range) ?
• particles in jet cone
• background
• Dhgt0.5 (long range) ?
• background only

incoming partons
associated h?
Near-side correlation shows jet-like signal in
central AuAu
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Back-to-Back Jets?

• away-side (back-to-back) jet can be anywhere
  (Dh2.5) - cant use large Dh subtraction
  trick
• Ansatz correlation function high
  pT-triggered AuAu event
• high pT-triggered pp event
• elliptic flow
• background

PHENIX Preliminary
pp
2-4 GeV

• black real
• green mixed event
• purple black-green

A from fit to non-jet region Dfp/2
v2 from reaction plane analysis
32
Away Side Jets are Suppressed
Peripheral Au Au

• Near-side well-described
• Away-side suppression in central
• collisions

STAR Preliminary
Central Au Au
Away side jets are suppressed!
33
Charm at RHIC
Charm decay is expected to be dominant component
of single e- with pT gt 1.5 GeV/c
Large charm production cross section (300-600
mb) which scales roughly with Nbin Suppression
of high pT ps relative to binary scaling
Observe an excess in single e-s over
expectation from light meson decays and g
conversions ? Observation of charm signal at
RHIC
PHENIX PRL 88
Assuming that all single e- signal is from charm
decay and the binary scaling, charm cross
section at 130 GeV
Data are consistent with ?s systematics(within
large uncertainties)!
34
Summary

• Soft physics
• System appears to be in equilibrium
  (hydrodynamic behaviour)
• Low baryon density
• Explosive expansion, rapid hadronization
• Hard physics
• Jet fragmentation observed
• Strong suppression of inclusive yields
• Azimuthal anisotropy at high pT
• Suppression of back-to-back hadron pairs
• large parton energy loss and surface emission?
• Open charm cross section scales with Nbin
• Coming Attractions
• dAu disentangle initial state effects in jet
  production
• (shadowing, Cronin enhancement) ? resolution of
  jet quenching picture
• J/? and open charm direct signature of
  deconfinement?
• Polarized protons DG (gluon contribution to
  proton spin)
• Surprises

35
Additional Slides
36
Leading Charged Particle Correlations

• Jet core Df Dh 0.5 0.5 ? study
  near-side correlations (Df0) of high pT hadron
  pairs
• Complication elliptic flow ? high pT hadrons
  correlated with the reaction plane (v22)
• Solution compare azimuthal correlation
  functions for
• Dhlt0.5 (short range) ? particles in jet cone
  background
• Dhgt0.5 (long range) ? background only
• Azimuthal correlation function
• Trigger particle pT triggt 4 GeV/c
• Associate tracks 2 lt pT lt pTtrig
• Caveat Away-side jet contribution
• subtracted by construction,
• needs different method

Near-side correlation shows jet-like signal in
central AuAu
37
Charm and single electron at RHIC
Simulation before RHIC
PHENIX data (PRL88)

• At RHIC, it is expected that charm decay can be
  the dominant component of single electron in pt gt
  1.5 GeV/c
• Large production cross section of charm ( 300-600
  ub)
• Production of the high pt pions is strongly
  suppressed relative to binary scaling
• Production of charm quark roughly scale with
  binary collisions.
• PHENIX observed excess in single electron yield
  over expectation from light meson decays and
  photon conversions ? Observation of charm signal
  at RHIC

38
PHENIX single electron data

• PHENIX observed excess of single electron yield
  over the contribution from light meson decays and
  photon conversoins
• Spectra of single electron signal is compared
  with the calculated charm contribution.
• Charm contribution calculated as
• EdNe/dp3 TAAEds/dp3
• TAA nuclear overlap integral
• Eds/dp3 electron spectrum from charm decay
  calculated using PYTHIA
• The agreement is reasonably good.

PHENIX PRL88 192303
Assuming that all single electron signal is from
charm decay and the binary scaling, charm cross
section at 130 GeV is obtained as
39
Comparison with other experiments

• PHENIX single electron cross section is compared
  with the ISR data single electron data
• Charm cross section derived from the electron
  data is compared with fixed target charm data
• Single electron cross sections and charm cross
  sections are compared with
• Solid curves PYTHIA
• Shaded band NLO QCD

Assuming binary scaling, PHENIX data are
consistent with ?s systematics o (within large
uncertainties)!
40
Leading Photon Correlations
trigger ?
Select events with a photon of pt gt 2.5 GeV/c.
Mostly ?s from decay of a high pt ?? (leading
particle) Build distributions in delta ? -space
of the charged hadrons relative to the trigger
photons.
pp
AuAu
PHENIX Preliminary
2-4 GeV

• black pair distribution
• green mixed event pair distribution
• purple bkg subtracted distribution

In AuAu add v2 component
41
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Parton recombination and high pT

• The buzz word in the last few months quark
  recombination/coallescence

Hwa Yang nucl-th/0211010
Greco, Ko, Levai nucl-th/0301093

• Recombination
• pT(baryons) gt pT(mesons) gt pT(quarks)
• (coalescence from thermal quark distribution
  ...)
• Pushes soft physics for baryons out to 4-5 GeV/c
• Some exotic explanations (e.g. gluon junctions)

43
The Two Large Detectors at RHIC
STAR Solenoidal field Large-? Tracking TPCs,
Si-Vertex Tracking RICH, EM Cal, TOF 420
Participants
Silicon Vertex             Tracker
Coils
Magnet
E-M Calorimeter
Time Projection           Chamber
Time of    Flight
Electronics Platforms
Forward Time Projection Chamber

• Measurements of Hadronic Observables
• using a Large Acceptance
• Event-by-Event Analyses of Hadrons and
• Jets

44
The Two Small Experiments at RHIC
BRAHMS 2 Conventional Spectrometers
Magnets, Tracking Chambers, TOF, RICH 40
Participants
PHOBOS Table-top 2 Arm Spectrometer
Magnet, Si ?-Strips, Si Multiplicity Rings, TOF
80 Participants
Paddle Trigger Counter
TOF
Spectrometer
OctagonVertex
Ring Counters

• Charged Hadrons in Select Solid Angle
• Multiplicity in 4?
• Particle Correlations

• Inclusive Particle Production Over Large
• Rapidity Range

45
Phase transition in high (energy-) density
matter?

• Hagedorn (1960s)
• Spectrum of excited hadronic states
  exponentially increasing level density
• Heat a hadron gas ? excite more massive
  resonances
• Hadronic gas has limiting temperature T 170 MeV

But cannot continue to arbitrary energy density
hadrons have finite size ? transition to phase
of hadronic constituents at T ?170 MeV?
46
Exploring the Phases of Nuclear Matter

• Can we explore the phase diagram of nuclear
  matter ?
• We think so !
• by colliding nuclei in the lab
• by varying the nuclei size (A) and colliding
  energy (?s)
• by studying spectra and correlation of the
  produced particles
• Requirements
• system must be at equilibrium (for a short time)
• ? system must be dense and large
• Can we find and explore the Quark Gluon Plasma ?
• We hope so!
• by colliding large nuclei at the highest
  possible energy

47
Experimental Determination of Geometry
48
RHIC Runs Machine Parameters
Performance Au Au p p Max ?snn 200 GeV
500 GeV L cm-2 s -1 2 x 1026 1.4 x
1031 Interaction rates 1.4 x 103 s -1 3 x 105 s
-1

• 2000 run
• AuAu _at_ ?sNN130 GeV
• 2001 run
• AuAu _at_ ?sNN200 GeV (80 mb-1)
• polarized pp _at_ ?s200 GeV
• (P 15, 1 pb-1)

49
Midrapidity Centrality Dependence at RHIC
PHOBOS AuAu hlt1
200 GeV
130 GeV
19.6 GeV preliminary
Kharzeev and Nardi PLB 507, 121 (2001)
50
Nch(?sNN) Universality of Total Multiplicity?
Total charged particle multiplicity / participant
pair
Same for all systems at same ?s(?seff for pp)
pQCD ee- Calculation
(A. Mueller, 1983)
Accidental, trivial?
51
?pT? of Charged Hadrons
increase only 2
STAR preliminary
Saturation model J. Schaffner-Bielich, et al.
nucl-th/0108048 D. Kharzeev, et al. hep-ph/0111315
Many models predict similar scaling (incl.
hydrodynamic models)
52
?ET?/ ?Nch ? from SPS to RHIC
A. Bazilevsky (PHENIX)
PHENIX preliminary
PHENIX preliminary
Independent of centrality
Independent of energy
Surprising fact SPS ? RHIC increased flow, all
particles higher ?pT? still ?ET?/ ?Nch? changes
very little Does different composition
(chemistry) account for that?
53
Fireball dynamics Collective expansion
Shape of the mT spectrum depends on particle
mass Inverse-slope depends on mT-range
where
and
Description of freeze-out inspired by
hydrodynamics
Flow profile used ?r ?s (r/R)0.5
The model is from E.Schenedermann et al. PRC48
(1993) 2462 and based on Blast wave model
54
Blastwave Fits at 130 200 GeV
Results depend slightly on pT coverage STAR Tfo
100 MeV ?bT? 0.55c (130) 0.6c
(200) PHENIX Tfo 110 MeV (200) ?bT? 0.5c
(200)
200 GeV
55
p0 suppression comparison to theory

• --- Wang dE/dx 0
• --- dE/dx 0.25 GeV/fm
• Wang X.N. Wang, Phys. Rev. C61, 064910 (2000).
• --- Levai L/l 0
• --- L/l 4
• Gyulassy, Levai, Vitev P.Levai, Nuclear Physics
  A698 (2002) 631.
• --- Vitev dNg/dy 900
• GLV, Nucl. Phys. B 594, p. 371 (2001) work in
  preparation.

PHENIX preliminary
56
2 Particle Correlations at High-pT Direct
Evidence for Jets?

• Jet core Df Dh 0.5 0.5
• look at near-side correlations (Df0) of high pT
  hadron pairs
• Complication elliptic flow
• high pT hadrons correlated with the reaction
  plane orientation also correlated with each other
  (v22)
• but elliptic flow has long range correlation (Dh
  gtgt 0.5)
• Solution compare azimuthal correlation
  functions for
• Dhlt0.5 (short range) and
• Dhgt0.5 (long range)

57
Reality Check Charge-Sign Dependence

• Compare same-sign (, --) and opposite-sign
  (-) pairs
• Known jet physics charge ordering in
  fragmentation

DELPHI, PL B407, 174 (1997)
STAR preliminary
System ( -)/( - -)
pp 2.7 ? 0.6
0-10 AuAu 2.4 ? 0.6
Jetset 2.6 ? 0.7
Opposite/same correlation strength similar in
AuAu, pp, JETSET ? pT3-4 GeV are jet fragments
58
Particle Composition at pT ? 2 - 4 GeV/c
PHENIX large excess of protons in central
collisions relative to pp at ISR and standard
jet fragmentation (p/p0.3) Phys. Rev. Lett. 88,
242301 (2002)
STAR different behaviour of strange mesons vs.
strange baryons for pT lt 5 GeV/c
p/p
ISR

• Exotic explanation baryon junction interactions
  enhanced in AA (Vitev and Gyulassy)
• Mundane explanation transverse radial flow
  (common velocity)

59
Consider two particles (1 and 2) with azimuthal
angles ?. Then, the standard way to extract v2 is
via the equation
where ? is the angle of the reaction plane. 
Likewise, the same can be written for particle 2,
as well.  Then, we can write the pair
distribution as averaged over  as
We can expand this as
The middle two terms integrate to zero, leaving
us with
We can then write this as
Once again, the last term integrates to zero,
leaving us with
60
Reality Check Charge-Sign Dependence

• Compare same-sign (, --) and opposite-sign
  (-) pairs
• Known jet physics charge ordering in
  fragmentation

DELPHI, PL B407, 174 (1997)
STAR preliminary
System ( -)/( - -)
pp 2.7 ? 0.6
0-10 AuAu 2.4 ? 0.6
Jetset 2.6 ? 0.7
Opposite/same correlation strength similar in
AuAu, pp, JETSET ? pT3-4 GeV are jet fragments
61
Single Particle Spectra and Radial Flow
AuAu _at_ 130 GeV, central and peripheral (STAR,
PHENIX)
Hydrodynamics even works for peripheral collisions
up to b 10 fm! (Heinz Kolb hep-ph/0204061)
Problem with pions at low pT ? mp gt 0 required
p
p
p
p
p
p
p
p
K
t 0.6 fm/c, emax (b0) 24.6 GeV/fm3,
ltegt(t 1 fm/c) 5.4 GeV/fm3 Tmax(b0) 340 MeV,
Tch 165 MeV, Tfo 130 MeV
62
Hydrodynamics Modeling High-Densities

• Such high Energy Densities should make
  Hydrodynamics become applicable
• ? Assume local thermal equilibrium (zero
  mean-free-path limit) and solve equations of
  motion for fluid elements (not particles)
• Equations given by continuity, conservation laws,
  and Equation of State (EOS)
• EOS relates quantities like pressure,
  temperature, chemical potential, volume
• direct access to underlying
• physics

Works qualitatively at lower energy but always
overpredicts collective effects - infinite
scattering limit not valid there
lattice QCD input