Collisions%20at%20RHIC%20are%20very%20strange

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Collisions at RHIC are very strange
Outline

• Bulk matter
• Equilibrium
• Enhancement
• Beyond the bulk
• Intermediate pT

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RHIC - a strange particle factory
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Are we in thermal/chemical equilibrium?

• Statistical Thermal Model
• 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
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Canonical vs Grand Canonical

• Canonical (small system i.e. p-p)
• Quantum Numbers conserved exactly.
• Computations take into account energy to
  create companion to ensure conservation of
  strangeness.
• Relative yields given by ratios of phase space
  volumes
• Pn/Pn fn(E)/fn(E)
• Grand Canonical limit (large system i.e. central
  AA)
• Quantum Numbers conserved on average via
  chemical potential Just account for creation of
  particle itself.
• The rest of the system picks up the slack.

Not new idea pointed out by Hagedorn in
1960s (and much discussed since)
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Comparison to data
Au-Au vsNN 200 GeV STAR Preliminary
p-p vs 200 GeV STAR Preliminary
??B 45 10 MeV
??S 22 7 MeV
T 168 6 MeV
?s 0.92 0.06
Canonical ensemble
T 171 9 MeV
?s 0.53 0.04
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Centrality and energy dependence
and 62 GeV
STAR preliminary AuAu at vsNN200GeV
TLQCD160-170MeV
Small Nch dependence of gs
Energy dependence of mB
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Centrality dependence
STAR Preliminary
We can describe p-p and Au-Au average
ratios. Can we detail the centrality
evolution? Look at the particle
enhancements. E(i) YieldAA/Npart Yieldpp /2
Au-Au vsNN 200 GeV
Transition described by E(i) behaviour
There is an enhancement E(X) gt E(L)
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Strangeness phase space suppression - gs

• Canonical system p-p
• Small system
• Lack of phase space available
• Strangeness suppressed
• Grand Canonical system
• central A-A
• Large system
• Large phase space available
• Strangeness saturated

Canonical suppression increases with
strangeness decreases with volume
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Model description of centrality dependence
Correlation volume V (ANN) V0 ANN
Npart/2 V0 4/3 pR03 R0 1.1 fm
proton radius/ strong interactions
STAR Preliminary
T 170 MeV
T 165 MeV
Au-Au vsNN 200 GeV
Seems that T170 MeV fits data best but
shape not correct
K. Redlich
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Varying T and R
Au-Au vsNN 200 GeV
Calculation for most central Au-Au
data Correlation volume V0 ? R03 R0
proton radius strong interactions
Rapid increase in E(i) as T decreases SPS data
indicated R 1.1 fm
K. Redlich
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Npart dependence
Correlation volume V (ANN)a V0
ANN Npart/2 V0 4/3 pR03 R0 1.2
fm proton radius/ strong interactions
STAR Preliminary
T 165 MeV a 1/3
T 165 MeV a 1
T 165 MeV a 2/3
Au-Au vsNN 200 GeV
Npart is NOT directly correlated to the
strangeness volume.
K. Redlich
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More on flavour dependence of E(i)
PHOBOS measured E(ch) for 200 and 19.6
GeV Enhancement for all particles?
PHOBOS Phys. Rev. C70, 021902(R) (2004)
Au-Au vsNN 200 GeV
Yes not predicted by model
s quark content determines E
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Moving from the bulk
Compare AuAu with pp Collisions ? RAA
AA yield
Nuclear Modification Factor
pp cross section
ltNbinarygt/sinelpp
R lt 1 at small momenta R 1 baseline expectation
for hard processes R gt 1 Cronin
enhancements (as in pA)R lt 1 Suppression
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Rcp vs RAA
vsNN 200 GeV STAR Preliminary
Canonical suppression in pp
vsNN 200 GeV STAR Preliminary
Rcp ? RAA
Effect increases as strange content of baryon
increases.
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Parton recombination at medium pT

• Parton pT distribution is
• exponentialpower-law
• 7 GeV particle via
• Fragmentation from high pT
• Meson
• - 2 quarks at 3.5 GeV
• Baryon
• - 3 quarks at 2.5 GeV

Recombination - more baryons than mesons at
medium pT
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RCP - an energy scan
vsNN200 GeV
First time differences between L and ?L ?B
absorption?
Baryon meson splitting at all energies
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The Rcp double ratio
NA57 G. Bruno, A. Dainese nucl-ex/0511020
Baryon/meson splitting at SPS and RHIC is the same
62 GeV AuAu data also follows the same trend
STAR Preliminary
Recombination present in all systems?
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Conclusions

• Thermal models give good description of the data
  as function of energy and centrality.
• The enhancement of strangeness as a function of
  centrality CAN be described scales with Npart
  1/3 NOT Npart
• Non-strange particles are enhanced NOT
  predicted by phase space models.
• The phase space effects of p-p extend into high
  pT regime.
• Baryon/meson splitting energy independent. ReCo
  at SPS.

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BACKUP
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Predictions from statistical model
Behavior as expected
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mT scaling
No complete mT scaling
Au-Au Radial flow prevents scaling at low
mT Seems to scale at higher mT
p-p Appears to be scaling at low mT
Baryon/meson splitting at higher mT Gluon jets?
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Gluon vs quark jets in p-p
No absolute mT scaling data scaled to match
at mT1 GeV/c
Quark jets events display mass splitting
Gluon jets events display baryon/meson splitting
Way to explore quark vs gluon dominance
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Recombination and v2
The complicated observed flow pattern in v2(pT)
for hadrons is predicted to be simple at the
quark level pT ? pT /n v2 ? v2 / n , n (2,
3) for (meson, baryon)
Works for p, p, K0s, ?, ? v2s v2u,d 7
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ReCo model and Correlations
R. Hwa, Z. Tan nucl-th/0503060
0-10/40-80
3 lt pTtrigger lt 6
The ratio of near side yields in central to
peripheral collisions is around 3 at 1 GeV/c and
decreases with increasing pTassoc
This is in good qualitative agreement with ReCo
model predictions though there are some
differences to the model (trigger pT, centrality)
Long range d? correlations are visible in the
STAR data and not taken into account in the plot.
This is pT dependent and may reduce any slope.
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Recent ReCo Model Predictions
Premise
The production of F and O particles is almost
exclusively from thermal s quarks even out to 8
GeV/c
Observables
1)The ratio of O/F yields should rise linearly
with pT
2) Any O or F di-hadron correlations are swamped
by the background and not observed
Being actively studied, but no results are
available as yet
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Correlations near side yields
No trigger particle dependence in the near side
yield/trigger in either dAu or AuAu
dAu
AuAu
No definite trigger particle dependence vs
centrality but meson triggers appear to be
systematically below baryon triggers
Reason for increase may be due to longe range
correlations in ?
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Strange Correlations in AuAu

• ?F correlations per trigger particle
• 3 lt pTtrigger lt 3.5 GeV/c
• 1 lt pTassoc lt 2 GeV/c
• ? lt 1

Correlations corrected for TPC acceptance and
efficiency of associated particles
v2 is then subtracted to give final correlations
Near side
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RAA - A mocked upstring picture does well
Are strong color fields the answer?
HIJING/BBar KT 1 GeV Strong Color Field (SCF)
qualitatively describes RAA. SCF - long range
coherent fields SCF behavior mimicked by
doubling the effective string tension
Topor Pop et al. hep-ph/0505210
SCF only produced in nucleus-nucleus collisions
RAA? RCP
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RAA for central and peripheral data
Au-Au vsNN 200 GeV STAR Preliminary
Au-Au vsNN 200 GeV STAR Preliminary
Peripheral and central data both show an
enhancement
Peripheral data is more enhanced Cronin effect?
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Baryons/Mesons
nucl-ex/0601042
The ?/K0S ratio exhibits a peak in the
intermediate pT region. The peak high varies with
centrality.
At higher pT the ratios for all centralities
converge again.
Magnitude and shape of ratio cannot be explained
by flow alone.
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Particle identification
a) dE/dx
c) Topology
?
K
p
d
e
Approx. 10 of a central event
b) RICH
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gluon vs quark jets

• Has been studied in ee- collisions at higher
  energies
• Quark jets tend to fragment harder than gluon
  jets
• We can study this with identified strange hadrons
  in pp collisions in STAR

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Identified Particle RAA
PRC 72 054901

• pT reach constrained by pp data
• Some hint of splitting in the baryons - RAA ? RCP
• HIJING BB predicts such a splitting using Strong
  Colour Fields...
• See also the Corona effect in EPOS

(TOF)
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Strange particles at intermediate pT
The statistics from Run 4 allow us to go much
higher in pT than previously and to study the
intermediate pT region in detail
?
K0S
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Strange Di-hadron Correlations
Charged Hadrons

• Observed suppression of single particle spectra
  compared to pp and dAu
• Disappearance of back-to-back jets

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Multiplicity scaling with log(vs)
If I can describe dNch/dh as function of vs
dNch/d? - strongly correlated to the entropy of
the system!
Can we describe other observables in terms of
dNch/d? ?
PHOBOS White Paper Nucl. Phys. A 757, 28,
nucl-ex/0410022
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HBT and dNch/dh
HBT radii linear as a function Npart1/3 Even
better in (dNch/dh)1/3 power 1/3 gives approx.
linear scale
Scaling works across a large energy range
nucl-ex/0505014 M.Lisa et al.
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First make a consistency check

• Require the models to, in principle, be the
  same.
• Only allow the least common multiple of
  parameters T, ?q, ?s, ?s
• Use Grand Canonical Ensemble.
• Fix weak feed-down estimates to be the same
  (i.e. at 100 or 0).

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The results
Au-Au vsNN 200 GeV
Ratio STAR Preliminary
p-/p K-/K ?p/p K-/p- ?p/p- L/p- ?L/p- X-/p- ?X/p- W/p- ?W/W 1.010.02 0.960.03 0.770.04 0.150.02 0.0820.009 0.0540.006 0.0410.005 (7.81) 10-3 (6.30.8) 10-3 (9.51) 10-4 1.010.08
after feed-down increase ?s decrease T
1 ? error
Similar T and ?s Significantly different errors.

Not identical and feed-down really matters
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Centrality dependence
Solid STAR Au-Au vsNN 200 GeV Hollow - NA57
Pb-Pb vsNN 17.3 GeV
STAR Preliminary
We can describe p-p and Au-Au average
ratios. Can we detail the centrality
evolution? Look at the particle
enhancements. E(i) YieldAA/Npart Yieldpp /2
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Motivation Chemistry Dynamics Summary
O central collisions
Ideal Hydrodynamics

• Data best reproduced with
• Tdec 100 MeV
• Same as for p-, K-, p
• Agreement holds for entire spectra!
• Same results at both energies!

P.F. Kolb and U. Heinz, nucl-th/0305084

• Tdec 164 MeV (decoupling at hadronization) not
  enough radial flow

pT 2 GeV/c
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Blast wave fits to data
Strong centrality dependence on freeze out
parameters for light hadrons Multi-strange
hadrons freeze out earlier, with a lower
ltßTgt Indicative of smaller cross-section for
interactions of multiply strange hadrons with
lighter species. Is this a signature of partonic
collectivity?
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Microscopic picture
What interactions can lead to equilibration in lt
1 fm/c? Need to be REALLY strong
Perturbative calculations of gluon scattering
lead to long equilibration times (gt 2.6 fm/c) and
small v2.
R. Baier, A.H. Mueller, D. Schiff, D. Son, Phys.
Lett. B539, 46 (2002). MPC 1.6.0, D. Molnar, M.
Gyulassy, Nucl. Phys. A 697 (2002).
v2
Clearly this is not the weakly coupled
perturbative QGP we started looking
for. s(trong)QGP
2-2 processes with pQCD s 3 mb
pT (GeV/c)
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Relativistic Heavy-Ion Collider (RHIC)
1 km
v 0.99995?c
AuAu _at_ ?sNN200 GeV
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Runs so far
Run Year Species vs GeV
?Ldt 01 2000 AuAu
130 1 ?b-1 02
2001/2 AuAu 200 24
?b-1
pp 200 0.15 pb-1 03
2002/3 dAu 200 2.74 nb-1
pp
200 0.35 pb-1 04 2003/4
AuAu 200 241 ?b-1
AuAu 62
9 ?b-1 05 2004/5
CuCu 200 3 nb-1
CuCu
62 0.19 nb-1
CuCu 22.5 2.7 ?b-1
pp
200 3.8 pb-1
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A theoretical view of the collision
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• Hadronic ratios.
• Resonance production.
• p? spectra.
• Partonic collectivity.
• High p? measurements.

Tc Critical temperature for transition to
QGP Tch Chemical freeze-out (Tch ? Tc)
inelastic scattering stops Tfo Kinetic
freeze-out (Tfo ? Tch) elastic scattering
stops
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Comparison between p-p and Au-Au
Au-Au vsNN 200 GeV STAR Preliminary
p-p vs 200 GeV STAR Preliminary
Canonical ensemble
T 171 9 MeV
?s 0.53 0.04
r 3.49 0.97 fm
T 168 6 MeV
?s 0.92 0.06
r 15 10 fm
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Resonances and survival probability

• Initial yield established at chemical
• freeze-out
• Decays in fireball mean daughter
• tracks can rescatter destroying part of
• signal
• Rescattering also causes regeneration
• which partially compensates
• Two effects compete Dominance
• depends on decay products and
• lifetime

?
lost
K
K
measured
Chemical freeze-out
Kinetic freeze-out
time
Ratio to stable particle reveals information on
behaviour and timescale between chemical and
kinetic freeze-out
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Chemical to kinetic freeze-out
P. Braun-Munzinger et.al.,PLB 518(2001) 41,
priv. communication Marcus Bleicher and Jörg
Aichelin Phys. Lett. B530 (2002) 81.
M. Bleicher and Horst Stöcker J.
Phys.G30 (2004) 111.
Life-time fm/c K(892) 4.0 S(1385)
5.7 L(1520) 13 ? (1020)
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Finite time span from Tch to Tfo
If only rescattering K(892) most suppressed
Need rescattering and regeneration to fix the
picture.
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Strong collective radial expansion
Model (plot) from P.F. Kolb and R. Rapp, Phys.
Rev. C 67 (2003) 044903
T
AuAu central , vs 200 GeV
pure thermal source
explosive source
light
T,b
1/mT dN/dmT
heavy
mT
mT (pT2 m2)½
Tdec 165 MeV Tdec 100 MeV

• Different spectral shapes for particles of
  differing mass? strong collective radial flow

• Tfo 100 MeV
• bT ? 0.55 c
• Needs initial kick

Good agreement with hydrodynamic prediction for
soft EOS (QGPHG)
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Anisotropic/Elliptic flow
Elliptic flow observable sensitive to early
evolution of system Mechanism is
self-quenching Large v2 is an indication of early
thermalization
dN/df 12 v2(pT)cos(2f) . fatan(py/px)
v2 ?cos2f? v2 2nd harmonic Fourier
coefficient in dN/d? with respect to the reaction
plane

• M. Gehm, S. Granade, S. Hemmer, K, OHara, J.
  Thomas - Science 298 2179 (2002)

Time
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Strong elliptic flow observed
v2(K) gt v2(L) gt v2(X)
Hydrodynamical models with soft Equation-of-State
describe data well for pT (lt 2.5 GeV/c)
Compatible with early equilibration
Although poor statistics even W flows - low
hadronic cross-section.
Evidence v2 built up in partonic phase
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The perfect fluid
First time hydrodynamics quantitatively
describes heavy ion reactions at low pT. Prefers
a QGP EOS
Thermalization time t0.6 fm/c and e20 GeV/fm3
Hydro small mean free path, lots of interactions
NOT plasma-like
Hydro without any viscosity. An ideal (perfect)
fluid
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Moving away from the bulk ...
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Strangeness in pp

• Large statistics data-set allows for the detailed
  analysis of data

Using Pythia (LO) requires changing the K factor
to match the data
NLO calculations show good agreement with
non-strange hadrons
Agreement with strange hadrons is not as
apparent, better for AKK than for Vogelsang
EPOS has very good agreement with all particles,
even Xi