Tkin of multistrange baryons shows no sensitivity to collision centrality and coincides with Tch for

1
Probing collision dynamics at RHICOlga
Barannikova (Purdue University) for the STAR
collaboration
1. Motivation
2. Data Analysis
3. Blast-wave Fit
4. Rapidity Distributions, ltpTgt and Ratios
Tc170 MeV eC0.5 GeV/fm3 F. Karsch, Nucl. Phys.
A698, 199c (2002).
Ludlam, McLerran, Physics Today
All data were studied within the same framework,
Blast Wave model (E. Schnedermann et. al. PRC48
(1993) 2462). The model assumes a boosted thermal
source in transverse and longitudinal directions.
There are three fit parameters ß the flow
velocity, Tkin the kinetic freeze out
temperature, and n that describes the shape of
the flow profile.

• No changes are observed in either shape or
  fiducial yield for any particle species within
  ylt0.5. The pp data and all centrality bins of
  the AuAu data exhibit the same behavior.
• This uniformity indicates the development of a
  boost-invariant region within the measured
  kinematic ranges.

• Quantum Chromodynamics predicts a phase
  transition from hadronic matter to quark-gluon
  plasma (QGP) at extreme conditions of high energy
  density, possibly achieved in relativistic heavy
  ion collisions. Lattice QCD calculations give
  phase transition critical temperature
• Tc 170 MeV
• Signals of QGP may remain in the bulk properties
  of the collision, and simultaneous observations
  of multiple QGP signals in the final state would
  serve as a strong evidence of QGP formation.
• These bulk properties include strangeness and
  baryon production rates, collective transverse
  radial flow, and system temperature. These can be
  studied via particle spectra.

STAR preliminary
The spectra of kaons and pions, and (anti)protons
are fitted together with a single set of
parameters in each centrality bin (pions with
momentum below 0.5 GeV/c are excluded to reduce
effects of resonance feed-downs).

• Measurements of a variety of hadron species (??
  , K? , K0s, K, ?, p, ?, ?, ?) in pp and AuAu
  collisions at 200 GeV by the same experimental
  setup allow a detailed investigation  of the
  final  hadronic state properties of such
  collisions.

Freeze out temperature
Courtesy of S. Bass
STAR preliminary
STAR preliminary
STAR preliminary
Tc170 MeV
Flow velocity
The ltpTgt increases from pp and peripheral AuAu
to central AuAu collisions, especially for
(anti)protons and kaons. This behavior is
consistent with an increase of radial flow with
collision centrality. Little centrality
dependence of the K-/p or p-/p ratios is
observed from mid-central to central AuAu
collisions, indicating a similar freeze-out
condition in these collisions. Similar centrality
behavior has been observed for other particle
ratios measured at 200GeV and 130GeV.
STAR preliminary
Bulk properties of the collision
Spectral shapes kinetic freeze-out
properties transverse radial flow Tkin _at_
kinetic freeze-out
STAR preliminary
STAR preliminary
STAR preliminary
Flavor composition chemical freeze-out
properties strangeness baryon production
Tch _at_ chemical freeze-out
Kaon and pion, and (anti)proton spectra are well
described by a common set of freeze-out
parameters. The obtained fit parameters for the
5 AuAu events are Tkin 8910MeV and ltbgt
0.59 0.05
Spectra shape similar in pp, different in AuAu
5. Systematic
6. Statistical Model Fit
7. Freeze-out Dynamics and Inferred Initial
Conditions
8. Summary and Conclusions
Sequential kinetic freeze-out ?,X,f ? L?
p,K,p,K
STAR preliminary
STAR preliminary

• STAR has measured a wide variety of hadron
  species produced in 200 GeV collisions at RHIC.
  Data were studied within framework of chemical
  and local kinetic equilibrium models to
  investigate final hadronic state properties. The
  major results are
• Statistical model fit Tch 160 MeV TC
• Blast wave model fit (central collisions)
• X,W,f Tkin 160 MeV, b 0.45 c
• p,K,p Tkin 90 MeV, b 0.6 c
• sequential kinetic freeze-out X,W,f ?
  p,K,p,L,K
• Multistrange baryons Tkin Tch ? bch
  0.45 c
• Tch ? Tkin Dt(ch?kin) 6 fm/c
• These results suggest the following picture at
  RHIC collision systems with varying initial
  conditions always evolve towards the same
  chemical freeze-out condition followed by cooling
  and expansion of increasing magnitude with
  centrality, and a sequential decoupling of
  particles dictated by their hadronic
  cross-sections.
• (Ref nucl-ex/0310004)

STAR preliminary
STAR preliminary
Tkin of multistrange baryons shows no sensitivity
to collision centrality and coincides with Tch
for all centralities.
Enhanced kaon and antiproton production compared
to lower energies. Different centrality
dependence of K-/p ratio at RHIC.
STAR preliminary
STAR preliminary
Tch ? Tkin For massless particles in equilibrium
Entropy density T3

• reflect effects of both the collision energy and
  centrality
• related to initial conditions

J.P. Blaizot and A.H. Mueller, Nucl. Phys.
B289, 847 (1987).D. Kharzeev and E. Levin,
Phys. Lett. B523, 79 (2001).
Tch is insensitive to centrality. Tch TC
Chemical freeze-out appears very close to
hadronization. No much interaction from
hadronization to chemical freeze-out.
Emerging picture collision systems with varying
initial conditions always evolve towards the
same chemical freeze-out condition followed by
cooling and expansion of increasing magnitude
with centrality
Strangeness production at low energies depends on
how the collision was initially prepared, but
not at RHIC energies.