Transcript PPT

Resonance Production at STAR
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
Haibin Zhang
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Resonance in Heavy Ion Collisions - I
Kinetic Freeze-Out
t?
Chemical Freeze-Out
Resonances!!!
Colliding
Resonances are strongly decaying particles which have lifetimes a few fm/c
Unique properties compared to stable particles:
Short lifetime  directly measure particle properties in medium, such as mass
Decayed daughters can undergo a period of re-interaction in the hadron gas
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Resonance inMotivation
Heavy Ion- IICollisions - II
K*
π
K*
K*
K
π
π
K
K
Chemical
freeze-out
• If resonance decays before
kinetic freeze-out  not
π
reconstructed due to
rescattering of daughters
K K*
• K*0 (c = 4 fm) survival
probability  time between
measured
chemical and kinetic freeze-out,
K*
source size and pT of K*0
lost
π
K*
• Chemical freeze-out  elastic
interactions πK  K*0 πK
regenerate K*0(892) until kinetic
freeze-out
K K*
Kinetic
measured •
freeze-out
time
K*0/K may reveal time between
chemical and kinetic freeze-out
• Λ*/Λ, Δ++/p, ρ0/π, f0/π ?
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Measurement Techniques

+
K*
K+
K*0
0
•
Event-Mixing technique (for example: K*0K+)
–
Select K+ and  tracks from PID by energy loss in
TPC
–
Combine all pairs from same event 
Signal+Background (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


*+
p

+
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Resonance Signal
K*0K+-
0 +-
 K+K-
STAR Preliminary
*± ±
++
Resonance
Decay channel
Branching Ratio %
Width [MeV]
Life time [fm/c]
* pK-
p+
K*(892) (770) f0(980)
K
~100
50.7
4

~100
150
1.3
(1020)

KK
dominant
49.2
40 Haibin
to 100 Zhang 4.46
40
(1232)
p
>99
~120
~1.6
(1520) (1385)
pK
22.5
15.6
13

88.2
35.8 5
5.6
Resonance Spectra
K*0

0 in Au+Au
0 in pp
f0 in Au+Au
++
f0 in pp
(1520)
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Particle Ratios
Statistical and systematic
errors added in quadrature
•
•
•
•
K*/K and */ in Au+Au significantly smaller than in p+p
/p and */ in Au+Au larger than in p+p
Φ/K- independent of centrality in Au+Au and close to p+p
/ and f0/ in peripheral Au+Au close to p+p
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Thermal Models
PLB 518 (2001) 41
ni 

g
2
2
p 2 dp
 e( E ( p )   ) / T  1 ,
i
i
p 2  mi2
0
 Si ni  0,  Ci ni  0
i
Ei 
i
4 3
 R  I i ni  I system
3
i
4 3
 R  Bi ni  A system
3
i
Markers: measured data
Lines: model predictions
STAR Preliminary
Stable particle ratios can be successfully predicted by thermal model
Strangeness enhancement observed in Au+Au
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
σ(Kπ)
σ(ππ)
() determines the rescattering effect for K*, , 
Large lifetime of  (40 fm/c)  weak rescattering
(K), (), (p), (KK) determine the regeneration
effect for K*, ,  and , respectively
(p) > () >> (K) >> (KK)
 K*/K suppression, /p enhancement, flat / and /K
Indication of hadronic interactions between freeze-outs
Tch freeze-out
σ(πp)
Tkin freeze-out
Tch from
Thermal model,
Tkin from
Blast-Wave-Fit
to , K and p
PRL 92 (2004) 112301
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Time Scale
If we only consider the rescattering
process
- t
N(t) = N0 e
t
- Δt

e
=
K*0
K- Au+Au 0.205
=
0.385
K*0
K- p+p
The time between chemical and kinetic freeze-out should be Δt ~ 3 fm/c
If regeneration process is included, Δt > 3 fm/c
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Summary
K*, , f0, , , * and * resonances measured in Au+Au and p+p 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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