The Strange Physics Occurring at RHIC

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The Strange Physics Occurring at RHIC
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Why do we do this research?
To explore the phase diagram of nuclear matter

• How
• By colliding nuclei in lab.
• By varying energy (vs) and size (A).
• By studying spectra and
• particle correlations.

Rajagopal and Wilczek, hep-ph/-0011333
To probe properties of dense nuclear matter

• How
• By colliding most massive and
• highest energy nuclei.
• By comparing to more elementary
• systems.
• Through high pT studies

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Lattice QCD calculations

• Coincident transitions deconfinement and chiral
  symmetry restoration
• Recently extended to mBgt 0, order still unclear
  (1st, 2nd, crossover ?)

TC 170 MeV
F. Karsch, hep-ph/0103314
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A theoretical view of the collision
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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RHIC _at_ Brookhaven National Lab.
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

• Previous Runs
• AuAu _at_ ?sNN130 GeV 200 GeV
• pp _at_ ?sNN 200 GeV
• dAu _at_ ?sNN 200 GeV
• Present Run
• Au-Au ?sNN200 GeV

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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
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Particle creation and distributions
19.6 GeV
130 GeV
200 GeV
PHOBOS Preliminary
dNch/dh
Central
Peripheral
h
Total multiplicity per participant pair scales
with Npart
Not just a superposition of p-p
To get much further need PID
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STAR is a large acceptance detector
X
STAR Preliminary
K0s
f
STAR Preliminary
STAR Preliminary
K
L
STAR Preliminary
W
K
Preliminary
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Strangeness enhancement
General arguments for enhancement 1. Lower
energy threshold TQGP gt TC ms 150
MeV Note that strangeness is conserved in
the strong interaction 2. Larger production
cross-section 3. Pauli blocking (finite chemical
potential)
Strange particles with charged decay modes
Enhancement is expected to be more pronounced for
multi-strange baryons and their anti-particles
Arguments still valid but now use Strange
particles for MUCH MORE
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Strangeness enhancement?

• Canonical (small system)
• Computed taking into account energy to create
  companion to ensure conservation of strangeness.
  Quantum Numbers conserved exactly.
• Grand Canonical limit (large system)
• Just account for creation of particle itself.
  The rest of the system acts as a reservoir and
  picks up the slack. Quantum Numbers conserved
  on average via chemical potential

• Phase space suppression of strangeness in
• small system/low temperature

• canonical suppression
• increases with strangeness
• decreases with volume
• observed enhancements
• Hamieh et al. Phys. Lett. B486 (2000) 61

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Correlation volume

• Grand Canonical description is only valid in a
  system in equilibrium that is large.
• BUT being large is not a sufficient condition for
  being GC!
• if AA were just superposition of pp STILL need
  to treat CANONICALLY
• System must know it is large...
• Must know that an O generated here can be
  compensated by, say, an O- on the other side of
  the fireball!
• what counts is the correlation volume
• How does the system KNOW its big?
• Not from hadronic transport no time
• One natural explanation returning from
  deconfined state

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Grand canonical applicable at RHIC?
130 GeV

• See drop in enhancement at higher energy
• Enhancement values as predicted by model
• Correlation volume not well modeled by Npart

System is in G.C. state for most central data
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A theoretical view of the collision
Chemical freezeout (Tch ? Tc) inelastic
scattering stops
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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
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Thermal model fit to data

• Particle ratios well described
• Tch 160 ? 5 MeV
• mB 24 ? 5 MeV
• ms 1.4 ?1.4 MeV
• gs 0.99 ?0.07

Data Fit (s) Ratio
Created a Large System in Local Chemical
Equilibrium
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Tch systematics

• Hagedorn (1964)
• if the resonance mass spectrum grows
  exponentially
• (and this seems to be the case)
• there is a maximum possible temperature for a
  system of hadrons

Blue Exp. fit Tc 158 MeV
r(m) (GeV-1)
filled AA open elementary
Green - 1411 states of 1967 Red 4627 states of
1996
m
Satz Nucl.Phys. A715 (2003) 3c
Seems he was correct cant seem to get above
Tch 170MeV
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A theoretical view of the collision
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Chemical freezeout (Tch ) 170 MeV Time between
Tch and Tfo
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Thermal model reproduced data
Created a Large System in Local Chemical
Equilibrium
Data Fit (s) Ratio
Do resonances destroy the hypothesis?
Used in fit
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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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Resonance ratios
Life time fm/c ? (1020) 40 L(1520)
13 K(892) 4 ?
1.7 r 1.3
Thermal model 1 Tch 177 MeV mB 29 MeV
UrQMD 2
Nch
1 P. Braun-Munzinger et.al., PLB 518(2001) 41
D.Magestro, private communication 2 Marcus
Bleicher and Jörg Aichelin Phys. Lett.
B530 (2002) 81-87. M. Bleicher, private
communication
Need gt4fm between Tch and Tfo
Small centrality dependence little difference
in lifetime!
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A theoretical view of the collision
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Chemical freezeout (Tch ) 170 MeV Time between
Tch and Tfo ? 4fm Kinetic freeze-out (Tfo ? Tch)
elastic scattering stops
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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 p? or m? ?(p?2 m2 ) distribution
A thermal distribution gives a linear
distribution dN/dm? ? e-(m?/T)
If there is radial flow
dN/dm?- Shape depends on mass and size of flow
Heavier particles show curvature
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Radial flow and hydro dynamical model
Shape of the m? spectrum depends on particle
mass Two Parameters Tfo and b
p,K,p fit
E.Schnedermann et al, PRC48 (1993) 2462
?r ?s (r/R)n
Tfo 90 ? 10 MeV, lt ?? gt 0.59 0.05c
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Flow of multi-strange baryons

• ?, K, p Common thermal freeze-out at Tfo 90
  MeV
• lt??gt 0.60 c
• ? Shows different thermal freeze-out behavior
• Tfo 160 MeV
• lt??gt 0.45 c

Higher temperature Lower transverse flow Probe
earlier stage of collision?
But Already some radial flow!
Tfo Tch Instantaneous Freeze-out of
multi-strange particles? Early Collective Motion?
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A theoretical view of the collision
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Chemical freezeout (Tch ) 170 MeV Time between
Tch and Tfo ? 4fm Kinetic freeze-out (Tfo) 90
MeV (light particles) Very Early Times
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Early 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
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v2 of strange particles
Equal Energy Density lines
P. Kolb, J. Sollfrank, and U. Heinz

• Seems to saturate at v220 for p?3.0 GeV/c
• ? v2(p?) follows ? evolution
• ? v2(p?) consistent with ? and ? v2(p?)

• Multi-strange particles show sizeable elliptic
  flow!
• Reach hydro. limit

Hydro P. Huovinen et al.
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Why high p? 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 p? hadrons
• modification of angular correlation
• changes of particle composition

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The control experiment d-Au
Medium?
Nucleus-nucleus collision

• Collisions of small with large nuclei quantify
  all cold nuclear effects.
• Small Large distinguishes all initial and final
  state effects.

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Jet suppression
Hard scatter back-to-back jet Angular
correlation at 0 and p

• Central Au-Au backwards jet suppressed
• d-Au backwards jet is visible

Jet suppression is a final state effect
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Energy loss creates anisotropy?
Jet Propagation
STAR Preliminary
Energy loss results in anisotropy due to
different length of matter passed through by
parton depending on location of hard scattering
Hypothesis seems verified
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Identified particle correlations
Why To gain insight on possible different
fragmentation function of different parton. To
probe further differences in mesons and baryons
at high p?
Correlation for K0s, L and L, in both cases,
there is an absence of a back-to-back partner
correlation.
Need more statistics for further studies
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Nuclear modification factor
Hard Physics -
Scales with Nbin Number of binary collisions
number of equivalent inelastic nucleon-nucleon
collisions
Nuclear Modification Factor
Can replace p-p with peripheral Rcp
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Suppression of identified particles
Two groups (2ltp? lt6GeV/c) - K0s, K?, K, f ?
mesons - L, X, W ? baryons
Mass or meson/baryon effect?
PHENIX PRL 91, 172301
L
L show different behaviour to K Suppression
of K sets in at lower p?
Rcp
K
Come together again at p? 6 GeV? standard
fragmentation?
Clearly not mass dependence
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d-Au control experiment
Enhancement is the well known Cronin Effect
Au Au, RAA ltlt 1 dAu, RdAu gt 1
RAA results confirm there are final state effects
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Parton coalescence and medium p?

• When slope exponential
• coalescence wins
• When slope power law
• fragmentation wins

recombining partons p1p2ph
Fries et al. QM2004
fragmenting parton ph z p, zlt1

• Recombination
• p?(baryons) gt p?(mesons) gt p?(quarks)
• (coalescence from thermal quark distribution
  ...)
• Pushes soft physics for baryons out to 4-5 GeV/c
• Reduces effect of jet quenching

Do soft and hard partons recombine or just
softsoft ? Explore
correlations with leading baryons and mesons
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v2 and coalescence model
STAR Preliminary
Hadronization via quark coalescence v2 of a
hadron at a given p? is the partonic v2 at p?/n
scaled by the of quarks (n).
AuAu ?sNN200 GeV
MinBias 0-80

• Works for K0s, ? ?
• v2s v2u,d 7

D. Molnar, S.A. Voloshin Phys. Rev. Lett. 91,
092301 (2003) V. Greco, C.M. Ko, P. Levai Phys.
Rev. C68, 034904 (2003) R.J. Fries, B. Muller,
C. Nonaka, S.A. Bass Phys. Rev. C68, 044902
(2003) Z. Lin, C.M. Ko Phys. Rev. Lett. 89,
202302 (2002)
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Exotica searches (pentaquarks)
Particle Data Group 1986 reviewing evidence for
exotic baryons states The general prejudice
against baryons not made of three quarks and the
lack of any experimental activity in this area
make it likely that it will be another 15 years
before the issue is decided. PDG dropped the
discussion on pentaquark searches after 1988.

• Constituent quark model of the1960s has been
  very successful in
• describing known baryons as 3-quark states
• QCD and quark model do not forbid composites of
  more quarks
• But early searches were unsuccessful and
  finally given up

• Minimum quark content is 4 quarks and 1
  antiquark
• Exotic pentaquarks are those where the
  antiquark has a different flavor than the other 4
  quarks
• Quantum numbers cannot be defined by 3 quarks
  alone.

Chiral Soliton Model Ns and Ds rotational
states of same soliton field
The mass splittings are predicted to be equally
spaced
Diakonov et al. Z phys A 359 (1997) 305
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Early evidence for pentaquarks
Q results Highest? Significance (CLAS)
7.8 (hep-ex/0311046) X5 results NA49 X--
(1860) ? X- p- X0 (1860) ? X- p Width limits
are experimental resolution
Counts
significance5.6
Need strong confirmation of second member of
anti-decuplet
Mass (Xp) GeV/c2
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RHIC - ideal place for pentaquarks
?B/B ratio 1 should see anti-pentaquark If
form QGP should coalesce into pentaquarks? Look
at Q? K0s p
STAR Preliminary
p-p
Q /event (stat. model calc.) ?0.5 1.5 1.5
Million events ? 0.8 2.3
M Efficiency 3 ? 25 70
K Branching Ratio 50 ? 10 25
K BR 50 from K0s ? 5 18 K
BG in mass range/event ?
2 BG in sample ? 3 M
Significance ?
Signal/v(2 X BGSignal) ? 2-7 s
STAR Preliminary
d-Au
Similar calc. for p-p ? 0.25-3 s
d-Au ? 1-16
s
No Clear Signal Yet.
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Other pentaquarks at RHIC
?Q-??n K-
PHENIX Preliminary
????nK-
??pK0
X--?X-p-
??ppp
N5?LK
S5?L p
S5?p??K0
d-Au
Au-Au Minbias
Possible peaks need more investigation
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Q at the AGS
Really need to determine properties spin, parity
etc
Use AGS Kaon beam (D. Ashery, E. Piasetzky, R.
Chrien, P.Pile)
Q
K d ? Q p ( Ethresh 400 MeV) K p ? Q p
(Ethresh 760 MeV)
d
K
p

• Why
• Large production cross-section
• compared to electro magnetic processes
• (Liu and Ko) 1041
• Only measure single particle mtm to determine
  mass
• Angular distribution determines spin

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Determining spin and parity
K d ? Q p Intrinsic parity -
?
K p ? Q p Intrinsic parity -
? -
Parity Conserved ? 1 (-1)DL n1n2 DL If Ii
DL Odd
DL Even
K d ? Q p spin
0 1 ½(?) ½
K p ? Q p spin 0
1 ½(?) 0
DL 0
DL 1
DL determines the decay angular distribution
Determination of spin and parity will help select
between theories
Correlated quark Chiral soliton models predicts
Jpc½ (p-wave) Quark model naïve expectation is
Jpc½- (s-wave)
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Summary
Different physics for different scales
Hydro
ReCo
pQCD
Strange particles are useful probes for each scale

• All evidence suggest RHIC creates a hot and
  dense
• medium with partonic degrees of freedom
• Only just beginning to understand the rich
  physics of RHIC

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