marcus bleicher, tbs berkeley 2005 what have we learned from transport models? marcus bleicher...

Marcus Bleicher, TBS Berkeley 2005

What have we learned from transport models?

Marcus Bleicher

Institut für Theoretische Physik

Goethe Universität Frankfurt

Germany


In collaboration with

• Elena Bratkovskaya

• Sascha Vogel

• Xianglei Zhu

• Stephane Haussler

• Hannah Petersen

• Diana Schumacher


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


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


Included Particles


Resonance cross sections


• 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


Basic checks (I)


Basic Checks (II)

Unfortunately the data has poor quality

One has to rely on the extrapolation

This leads to ~10% systematic uncertainty


Baryon Stopping

Energy deposition is OK

Anything special here?


Particle Production

Extrapolation from pp to AA is OK


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


What can be studied:

• Kinetic observables: longitudinal pressure (Landau or Bjorken?)transverse pressure (radial flow & elliptic flow)

• Chemical observables: Strangeness enhancementFluctuationsResonances


• 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


AA Excitation functions

• 4 and mid-y abundancies: OK

• Energy dependence: OK

• Hadron-string models work well


Check for strangeness enhancement compared to pp

•Strangeness enhancement is strongest at low energies

•Apparent Lambda enhancement from stopping

•Disappearance of canonical suppression


Excitation functions: ratios

• ‘Horn’ in the ratio not reproduced

• well reproduced

• relative strange baryon enhancement reproduced

/K

/K

/


Transverse Pressure:Proton-Proton

• PP works well

• pQCD needed at RHIC

• PYTHIA included in

UrQMD 2.x and HSD


Proton-Nucleus

• pA is well under control

• CC and SiSi are also under control

• What about AA?


Transverse mass spectra

• Standard UrQMD and HSD underestimate the data

• Additional resonances of 2-3 GeV mass may improve the description


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?


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-


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


Elliptic flow (II)

From Xianglei Zhu

•Qualitatively non-flow contributions are reproduced

•Large difference between real v2 and 2-particle cumulants


Elliptic flow (III)

From Xianglei Zhu

•Hadron/String dynamics predicts correct mass ordering


Elliptic flow (IV)

From Xianglei Zhu

• Scaling with nq is present in transport calculations

• Scaling is not a unique QGP signal!


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

marcus bleicher, tbs berkeley 2005 what have we learned from transport models? marcus bleicher...

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