Energy Dependence of Strangeness Production

1
Energy Dependence of Strangeness Production

• Marcelo G. Munhoz
• Universidade de São Paulo
• for the STAR Collaboration

2
Why energy scan?

• STAR 62.4 GeV data
• How is the transition from SPS to top RHIC
  energies?
• Measurement at intermediate energy
• Any surprise?
• Different production mechanisms as a function of
  collision energy?

3
Why Strangeness?

• Production mechanism at early stage
• J. Rafelski, B. Müller, PRL 48 (1982), 1066
• Hadron Gas ? more energy is necessary to produce
  strange hadrons
• ? N ? ? K (Q530 MeV)

• QGP ? gluons contribute to s quark production

(Q?2ms ?300 MeV)
Strangeness enhancement !
4
Why Strangeness?

• Medium interaction - strangeness is abundantly
  produced in relativistic heavy ion collisions
• It can probe the thermalization of the system
• It can bring important information about the
  hadronization
• It plays an important role determining the
  freeze-out properties.

5
Why Strangeness?
V main vertex V0 neutral decaying
particle dca distance of closest approach b
closest approach of reconstructed
V0 to main vertex d decay length
6
Experimental Programs in Relativistic Heavy Ions
SPS
RHIC
AGS
SPS
?sNN (GeV)
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The STAR experiment at RHIC
STAR Experiment
8
STAR Strange Particles Spectra
STAR Preliminary
STAR Preliminary
STAR Preliminary
9
Yields (y0)

• General trend
• Baryon yields approximately constant as a
  function of energy
• Anti-baryon yields increase with energy
• Can we explain it?

STAR Preliminary
AGS E896 SPS K0S,? (NA49) ?, O (NA57) RHIC
STAR
10
Thermal Models

• The basic assumption of a thermal model is the
  existence of a thermalized hadron gas and
  conservation laws
• No assumption on how the system was
  thermalized...

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Thermal Models

• A word of caution!
• Different ensembles
• Canonical and Grand-canonical
• Different parameters
• T, ?B, ?S , ?q, ?C, ?I3, ?q , ?s , ?C , ?s, ?q ,
  ?C
• Different feed-down treatment.

S. Salur, 22nd Winter Workshop (2006)
12
STAR, AuAu 62.4 GeV Data
T 161 ? 7 MeV ?B 87 ? 12 MeV ?s 1.01 ?
0.10

• Data well reproduced by model
• M. Kaneta and N. Xu, nucl-th/0405068
• Free parameters T, ?B, ?q, ?s, ?s, feed-down
  included.
• How does it compare to other energies?

STAR Preliminary
13
Thermal ModelParameters

• Systematic study from A. Andronic et al,
  nucl-th/0511071
• T and µB at 62 GeV where they were expected to be

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Anti-particle/Particle ratios

• Systematic study from A. Andronic et al,
  nucl-th/0511071
• Anti-particle/Particle ratios at 62 GeV where
  they were expected to be

15
Strange Hadron Yields relative to pions

• Systematic study from A. Andronic et al,
  nucl-th/0511071
• Strange Hadron Yields relative to pions at 62 GeV
  where they were expected to be

16
Chemical Freeze-out

• What does it mean to have the data described by
  thermal models?
• What can we learn from the values of the
  parameters and the observed trend of the data?
• Is it a demonstration of chemical equilibrium and
  phase transition?

A. Andronic et al, nucl-th/0511071
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How does the system evolve after chemical
freeze-out?

• Transverse Momentum Distribution
• Information on the dynamics of the fireball
• Separate freeze-outs? How long does it take? Do
  we understand it?
• Collective motion?
• Higher energy density ? higher pressure

18
Blast Wave Source Expansion
where
E.Schnedermann et al, PRC48 (1993) 2462
?r ?s (r/R)n
STAR Preliminary
19
Kinetic Freeze-out

• Temperature Tkinetic is higher for baryons with
  higher strange quark content for Blast-wave fits.
• What does it mean to have different kinetic
  freeze-out temperatures for multi-strange
  particles?

STAR Preliminary
20
Kinetic Freeze-out

• Temperature Tkinetic is higher for baryons with
  higher strange quark content for Blast-wave fits.
• What does it mean to have different kinetic
  freeze-out temperatures for multi-strange
  particles?
• 200 GeV?Tkin Tch (?)

21
Azimuthal Anisotropy of Emission Elliptic Flow
Almond shape overlap region in coordinate space
Anisotropy in momentum space
Interactions/ Rescattering
Quantifying this effect ? v2 2nd harmonic
Fourier coefficient in dN/d? with respect to the
reaction plane
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Azimuthal Anisotropy of Emission Elliptic Flow

• Elliptic flow is sensitive to early
  thermalization
• Mass hierarchy at low pT
• Qualitative agreement with hydrodynamics
  calculations until pT 2 GeV/c
• pT gt 2 GeV/c data deviates from hydro
• Multi-strange baryons v2 ? partonic collectivity
  (?)
• Interesting meson-baryon difference!

J. Adams et al., Phys. Rev. Lett. 95 (2005)
122301
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Azimuthal Anisotropy of Emission Elliptic Flow

• Elliptic flow is sensitive to early
  thermalization
• Mass hierarchy at low pT
• Qualitative agreement with hydrodynamics
  calculations until pT 2 GeV/c
• pT gt 2 GeV/c data deviates from hydro
• Multi-strange baryons v2 ? partonic collectivity
  (?)
• Interesting meson-baryon difference!
• The same behavior is seen in 62 GeV!

M. Oldenburg, QM 2005
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Summary

• Excitation functions of strange baryon production
  dont show any surprises at 62.4 GeV
• Statistical models show good agreement with data
  for basically all energies
• Particle ratios
• Collective expansion of the fireball
• Transverse momentum distribution
• Indication of early thermalization
• Elliptic flow (v2)
• Evidence for partonic degrees of freedom
• Multi-strange baryons present similar chemical
  and kinetic freeze-out temperatures and a
  measurable elliptic flow (v2).