Transcript Document

Resonance Dynamics
in Heavy Ion Collisions
22nd Winter Workshop on Nuclear Dynamics
17.03.2006, La Jolla, California
Sascha Vogel, Marcus Bleicher
UrQMD group (Mohammed Abdel-Aziz, Marcus Bleicher, Stephane Haussler,
Quingfeng Li, Hannah Petersen, Diana Schumacher, Sascha Vogel, Xianglei Zhu)
Outline
Outline
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Introduction and motivation (more or less a reminder)
Model
Rescattering of resonances
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Rapidity, transverse momentum, mass spectra
Re-feeding of resonances
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Average cross sections
 Collision rates
 Center of mass energies
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Summary
Motivation
Motivation
We want to …
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learn something about freeze-out dynamics of heavy ion collisions
understand why statistical models cannot describe resonance data
understand quantitatively the effect of rescattering and regeneration of
daughter particles in order to understand the data already measured
learn something about in-medium properties of hadrons
Thanks to Christina Markert, STAR Collaboration
Motivation
Statistical model fitting
• Particle ratios well reproduced
• Resonance ratios not reproduced
(Braun-Munzinger, Schweda QM 2004)
• D++/p too low
• K*/K too high
Braun-Munzinger et al.
Motivation
Motivation
p+p
p+p interactions:
• No extended initial medium
• Chemical freeze-out (no thermalisation)
• Kinetic freeze-out close to the chemical
freeze-out
Hot and dense
medium
A+A
Particle yields
Particle spectra
A+A interactions:
• Extended hot and dense phase
• Kinetic freeze-out separated from
chemical freeze-out
• in medium effects
• Rescattering effects
• Regeneration effects
time
Thanks to Christina Markert, STAR Collaboration
Quick reminder on resonances
Resonances in a hadronic medium
Since they are unstable (decaying) particles with a cross section, they can
 scatter
 decay
Decay products (or daughter particles) can
How does the experiment (reconstruct)
the resonance?
Invariant mass reconstruction
of decay products
counts/(10 MeV/c2)
 escape the collision zone
 (re-)scatter
 build another resonance (“regenerate“)
Au+Au
sNN = 200 GeV
40% to 80%
0
ρ
STAR Preliminary
f0
1.2  pT  1.4 GeV/c
K0S
|y|  0.5
ω
K*0
Statistical error only
Quick reminder on resonances
Resonances in a hadronic medium
 Since hadronic decay products interact with the surrounding medium the
experiment cannot reconstruct all resonances
 The consequence is, that all spectra one observes by reconstructing
hadronic decay products are altered by the hadronic medium
 Interesting effect for resonances which have a hadronic and a dileptonic
decay channel! (e.g. r0  p+p- , r0  e+e-)
hadronic decay channel
dileptonic decay channel
+
ππ-
ρ0
ρ0
-
π+
π-
π+
-
ρ0
π+
ρ0
+
ρ0
+
Quick reminder on resonances
Resonances in a hadronic medium
Differences in observables between the different decay channels depend
on various factors, e.g:
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system (p+p, Au+Au?)
centrality
life time of the resonance (see below)
freeze-out mechanism
life time of the medium
density of the medium
etc…
r0(770)
D++(1232)
f 0(980)
K*0±(892)
S (1385)
L (1520)
F (1020)
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p+ pp p+
p+ ppK
Lp
pK
K+ K-
B.R. ~1
B.R. ~1
B.R. ~ 2/3
B.R. ~ 2/3
B.R. ~ 0.88
B.R. ~ 0.45
B.R. ~ 0.49
 = 1.3 fm
 = 1.6 fm
 = 2.6 fm
 = 4 fm
 = 5.5 fm
 = 12.6 fm
 = 44 fm
Models
Models
What kind of model do we need for our study of resonance
rescattering and refeeding?
Transport model, since we need to keep track of the particles throughout the whole
collision.
initial
final
thermodynamical models
hydrodynamical models
transport models
Model
UrQMD
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Ultra Relativistic Quantum
Molecular Dynamics
Non equilibrium transport model
All hadrons and resonances up
to 2.2 GeV included
String excitation and
fragmentation
pQCD hard scattering at high
energies with PYTHIA
Bratkovskaya, Bleicher et al., Phys.Rev.C69:054907,2004
Model
Fochler, Vogel et al, Physical Review C, in print (arxiv.org/abs/nucl-th/0601062)
UrQMD
Model
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Cross sections are fitted to
available experimental data or
calculated by the principle of
detailed balance and the additive
quark model
Does dynamically account for
canonical suppression
Generates full space-time
dynamics of hadrons and strings
Bleicher et al., J.Phys.G25:1859-1896,1999
Rescattering
Baryon resonances in central AuAu collisions at RHIC
Experimental signal loss due to rescattering of decay products.
All decayed particles
Reconstructable particles
Rescattering
Meson resonances in central AuAu collisions at RHIC
Note: L.h.s. would be visible in a dilepton analysis
(multiplied with the corresponding branching ratio).
All decayed particles
Reconstructable particles
Rescattering
pT spectra
Stronger suppression towards lower transverse momenta
 apparent ‚heating‘ of the spectra
Open symbols: reconstructable particles, filled symbols: all decayed
r meson mass
Mass spectrum of the r meson
AuAu
Ecm=200AGeV
• mass drops towards
central reactions
Note: C+C collisions at 2AGeV
AuAu
Ecm=200AGeV
• mass drops towards
low pT
S.Vogel, M. Bleicher, Physical Review C, in print (arxiv.org/abs/nucl-th/0509105)
Motivation
Some data from STAR
• Increase of the F/Kratio from pp to central
AuAu
• Decrease of the K*/Kand L*/Lratio from pp to
central AuAu
Preliminary
• Rescattering and
Regeneration effects are
to be considered!
Thanks to Christina Markert, STAR Collaboration
Re-feeding of resonances
Rough estimate of the re-feeding probability
Tch freeze-out
• Strong decrease in kinetic freeze-out
temperature from central to peripheral
collisions
• Kinetic freeze-out as low as
80 - 90 MeV
Tkin freeze-out
• Consequences for resonance
re-feeding
Blast Wave Fit by Olga Barranikova, STAR
Re-feeding of resonances
Rough estimate of the re-feeding probability
Tkin
Estimate of available energy for
re-feeding at different reaction stages
with a simple thermal ansatz:
3 
s  m1 + m2 + 2 T 
2 
D
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baryons can re-created
until end of the reaction
• r meson re-creation is only
possible near chemical freezeout
Decay time analysis
Decay time analysis
r mesons are emitted earlier than D baryons
Peak emission times:
D ~ 22 fm/c
r ~ 15 fm/c
Cross sections and collision rates
Cross sections and collision rates
Production channel for measured resonances:
S+p  L(1520)
N+K L(1520)
L+p  S(1385)
K+p  K*
L+X Y
K+X Y
S+X Y
K* and L* show rescattering
S* shows regeneration
Regeneration/Rescattering cross section:
s(L+p) > s (K+p) > s (S+p) ?
S*
K*
L*
Mean center of mass energy
Mean center of mass energy
Blue: Collision rate of the corresponding reactions
Red: Average center of mass energy
Green: Probability to form a resonance
pp
pp
Kp
Mean center of mass energy
Mean center of mass energy (in linear scale)
pp
Kp
pp -> D
pp -> r
pp Kp
Kp -> K*
Summary
Summary
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Resonances provide additional information compared to
stable hadrons and HBT measurements
Thermal models do not describe all resonance yields
When trying to understand resonance data one has to
consider both effects, rescattering and refeeding
The effect of rescattering is huge and can be measured for
example with the r meson
The probability to regenerate a r or K* meson is lower than
the chance to regenerate a D baryon
The cross section for L* production is lower than for S*
production
Thank you!
Backup slides
Backup slides
Model
UrQMD - correlations
Correlations are well described
except for most central reactions
Q. Li, M.Bleicher, H. Stoecker, nucl-th/0602032; Data: STAR