Haibin Zhang / Brookhaven National Laboratory

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• Haibin Zhang / Brookhaven National Laboratory
• For the STAR Collaboration

Physics Motivations
? Resonance in Heavy-Ion Collisions
Measurement Technique
Results and Discussions
Resonance Signal and Spectra
Particle Ratios
Thermal Model Comparison
Time Scale
Summary
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Resonance in Heavy Ion Collisions - I
Resonances are strongly decaying particles which
have lifetimes a few fm/c
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Motivation - II
Resonance in Heavy Ion Collisions - II

• If resonance decays before kinetic freeze-out ?
  not reconstructed due to rescattering of
  daughters
• K0 (c? 4 fm) survival probability ? time
  between chemical and kinetic freeze-out, source
  size and pT of K0
• Chemical freeze-out ? elastic interactions pK ?
  K0? pK regenerate K0(892) until kinetic
  freeze-out
• K0/K may reveal time between chemical and
  kinetic freeze-out
• ?/?, ?/p, ?0/p, f0/p ?

Chemical freeze-out
Kinetic freeze-out
time
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Measurement Techniques

• Event-Mixing technique (for example K0?K??)
• Select K and ?? tracks from PID by energy loss
  in TPC
• Combine all pairs from same event ?
  SignalBackground (same event spectra)
• Combine pairs from different events ? Background
  (mixed event spectra)
• Signal same event spectra mixed event spectra

• Like-Sign technique (for example ?0????)
• Combine ? ? pairs ? same event spectra
• Combine ? ? pairs and ? ? pairs ?
• background spectra
• Signal ? ? 2 ? ? ? ? ? ? ?

• Features
• Enable us to measure very short lived
  resonances
• with high efficiency ( 80)
• Reconstruction is not done particle by
  particle
• Need lots of statistics in order to
  overcome large
• combinatorial background

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Resonance Signal
K0?K?-
?0? ??-
?? KK-
STAR Preliminary
?? ??
?? pK-
?? p?
Resonance K(892) ?(770) f0(980)
?(1020) ?(1232) ?(1520)
?(1385) Decay channel K ?
? ? ? ? K K
p ? p K ? ?
Branching Ratio 100 100
dominant 49.2 gt99
22.5 88.2 Width MeV
50.7 150 40 to 100
4.46 120 15.6
35.8 Life time fm/c 4
1.3 40
1.6 13 5.6
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Resonance Spectra
?0 in AuAu
K0
?
?0 in pp
f0 in AuAu
STAR Preliminary
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f0 in pp
?(1520)
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Particle Ratios
Statistical and systematic errors added in
quadrature

• K/K and ?/? in AuAu significantly smaller than
  in pp
• ?/p and ?/? in AuAu larger than in pp
• F/K- independent of centrality in AuAu and close
  to pp
• ?/? and f0/? in peripheral AuAu close to pp

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Thermal Models
PLB 518 (2001) 41
Grand canonical ensemble
Quantum conservation laws
Markers measured data Lines model predictions
STAR Preliminary
Stable particle ratios can be successfully
predicted by thermal model
Strangeness enhancement observed in AuAu
Discrepancies exist for resonance ratios between
measured data points and model predictions
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Hadronic Interactions
M. Bleicher et al. J. Phys. G 25 (1999) 1859
?(??) determines the rescattering effect for K,
?, ?
Large lifetime of ? (40 fm/c) ? weak rescattering
s(Kp)
?(K?), ?(??), ?(p?), ?(KK) determine the
regeneration effect for K, ?, ? and ?,
respectively
?(p?) gt ?(??) gtgt ?(K?) gtgt ?(KK)
? K/K suppression, ?/p enhancement, flat ?/? and
?/K
s(pp)
Indication of hadronic interactions between
freeze-outs
Tch freeze-out
Tch from Thermal model, Tkin from
Blast-Wave-Fit to p, K and p
s(pp)
Tkin freeze-out
PRL 92 (2004) 112301
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Time Scale
If we only consider the rescattering process
The time between chemical and kinetic freeze-out
should be ?t 3 fm/c
If regeneration process is included, ?t gt 3 fm/c
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Summary
K, ?, f0, ?, ?, ? and ? resonances measured in
AuAu and pp collisions
Resonance rescattering and regeneration effect in
hadronic phase
Discrepancy between thermal model predictions and
measured data
Indications of late stage hadronic interactions.
Resonance can be used as a clock to measure the
time scale between chemical and thermal
freeze-outs
Thanks!
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