What have we learned from transport models?
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Marcus Bleicher, TBS Berkeley 2005
What have we learned from transport models?
Marcus Bleicher
Institut für Theoretische Physik
Goethe Universität Frankfurt
Germany
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Marcus Bleicher, TBS Berkeley 2005
In collaboration with
• Elena Bratkovskaya
• Sascha Vogel
• Xianglei Zhu
• Stephane Haussler
• Hannah Petersen
• Diana Schumacher
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Marcus Bleicher, TBS Berkeley 2005
Todays transport/cascade models
• RQMD (the grandfather of relativistic transport models) development stopped around 2000
• UrQMD (development started 1996 at Frankfurt)
• HSD (Giessen group)
• Parton cascades (ZPC, MPC, GPC, SPC aka VNI/B, ….)
NOT transport/cascade models:• HIJING• PYTHIA/FRITIOF• NEXUS, VENUS• DPM
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Marcus Bleicher, TBS Berkeley 2005
The tool: UrQMDv2.2
• Non-equilibrium transport model• Hadrons and resonances• String excitation and fragmentation• Cross sections are parameterized
via AQM or calculated by detailed balance• pQCD hard scattering at high energies
• Generates full space-time dynamics of hadrons and strings
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Marcus Bleicher, TBS Berkeley 2005
Included Particles
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Marcus Bleicher, TBS Berkeley 2005
Resonance cross sections
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Marcus Bleicher, TBS Berkeley 2005
• Initialization of projectile and target (Lorentz contracted Woods-Saxon)• Generate table with collision/decay sequence with
•Propagate to next collision•Perform collision according to cross sections - elastic scattering - inelastic scattering - resonance production - soft string formation and fragmentation - pQCD hard scattering / fragmentation• Update particle arrays, update collision table, perform next collisions
Reaction stages
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Marcus Bleicher, TBS Berkeley 2005
Basic checks (I)
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Marcus Bleicher, TBS Berkeley 2005
Basic Checks (II)
Unfortunately the data has poor quality
One has to rely on the extrapolation
This leads to ~10% systematic uncertainty
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Marcus Bleicher, TBS Berkeley 2005
Baryon Stopping
Energy deposition is OK
Anything special here?
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Marcus Bleicher, TBS Berkeley 2005
Particle Production
Extrapolation from pp to AA is OK
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Marcus Bleicher, TBS Berkeley 2005
Collision Spectrum
• Initial stage scattering before 1.5 fm/c:Baryon stopping, meson production, may be QGP formation
• Thermalization stage (1.5 – 6 fm/c):Cooking QCD matter
• Hadronic freeze-out stage (6 – 10 fm/c):Elastic and pseudo-elastic hadron scatterings
Pb+Pb @ 160 AGeV
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Marcus Bleicher, TBS Berkeley 2005
What can be studied:
• Kinetic observables: longitudinal pressure (Landau or Bjorken?)transverse pressure (radial flow & elliptic flow)
• Chemical observables: Strangeness enhancementFluctuationsResonances
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Marcus Bleicher, TBS Berkeley 2005
• 1st Order phase transition at high
•No P.T. at low
• Search for irregularities around Ebeam = 10-40 GeV:
Flow, strangeness, E-by-E
Where do we expect interesting effects?
B
B
Plot adapted from L. Bravina
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Marcus Bleicher, TBS Berkeley 2005
AA Excitation functions
• 4 and mid-y abundancies: OK
• Energy dependence: OK
• Hadron-string models work well
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Marcus Bleicher, TBS Berkeley 2005
Check for strangeness enhancement compared to pp
•Strangeness enhancement is strongest at low energies
•Apparent Lambda enhancement from stopping
•Disappearance of canonical suppression
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Marcus Bleicher, TBS Berkeley 2005
Excitation functions: ratios
• ‘Horn’ in the ratio not reproduced
• well reproduced
• relative strange baryon enhancement reproduced
/K
/K
/
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Marcus Bleicher, TBS Berkeley 2005
Transverse Pressure:Proton-Proton
• PP works well
• pQCD needed at RHIC
• PYTHIA included in
UrQMD 2.x and HSD
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Marcus Bleicher, TBS Berkeley 2005
Proton-Nucleus
• pA is well under control
• CC and SiSi are also under control
• What about AA?
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Marcus Bleicher, TBS Berkeley 2005
Transverse mass spectra
• Standard UrQMD and HSD underestimate the data
• Additional resonances of 2-3 GeV mass may improve the description
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Marcus Bleicher, TBS Berkeley 2005
Inverse slope systematics
• High mass resonances improve the description at low and high energies
• Cronin effect at high energies improves RHIC results
• How can we test those scenarios?
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Marcus Bleicher, TBS Berkeley 2005
Hints from elliptic flow
• High mass resonances can not explain scaled v2 above 40 AGeV
• Data shows saturation of scaled v2
• Strong hint for large pressure and short mean free paths in the early stage of the reaction already from 30 AGeV on !
Data for h-
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Marcus Bleicher, TBS Berkeley 2005
Elliptic flow (I)
From Xianglei Zhu
•Elliptic flow from string/hadron model is too small
• However, half of v2 is generated in the hadronic stage
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Marcus Bleicher, TBS Berkeley 2005
Elliptic flow (II)
From Xianglei Zhu
•Qualitatively non-flow contributions are reproduced
•Large difference between real v2 and 2-particle cumulants
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Marcus Bleicher, TBS Berkeley 2005
Elliptic flow (III)
From Xianglei Zhu
•Hadron/String dynamics predicts correct mass ordering
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Marcus Bleicher, TBS Berkeley 2005
Elliptic flow (IV)
From Xianglei Zhu
• Scaling with nq is present in transport calculations
• Scaling is not a unique QGP signal!
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Marcus Bleicher, TBS Berkeley 2005
Summary: or What I learned
Transport models produce to few pressure in the early stage above 30 GeV
However, at RHIC up to 50% of v2 are from hadronic stage
mass ordering is correct
non-flow correlations are correct