Marcus Bleicher, TBS Berkeley 2005 What have we learned from transport models? Marcus Bleicher Institut für Theoretische Physik Goethe Universität Frankfurt Germany

Marcus Bleicher, TBS Berkeley 2005 In collaboration with Elena Bratkovskaya Sascha Vogel Xianglei Zhu Stephane Haussler Hannah Petersen Diana Schumacher

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

Marcus Bleicher, TBS Berkeley 2005 Why we need transport… to avoid being fooled…

Marcus Bleicher, TBS Berkeley 2005 The tool: UrQMD v2.2 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

Marcus Bleicher, TBS Berkeley 2005 Included Particles

Marcus Bleicher, TBS Berkeley 2005 Resonance cross sections

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

Marcus Bleicher, TBS Berkeley 2005 Basic checks (I)

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

Marcus Bleicher, TBS Berkeley 2005 Baryon Stopping Energy deposition is OK Anything special here?

Marcus Bleicher, TBS Berkeley 2005 Particle Production Extrapolation from pp to AA is OK

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 160 AGeV

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 enhancement  Fluctuations  Resonances

Marcus Bleicher, TBS Berkeley st Order phase transition at high No P.T. at low Search for irregularities around Ebeam = GeV: Flow, strangeness, E-by-E Where do we expect interesting effects? Plot adapted from L. Bravina

Marcus Bleicher, TBS Berkeley 2005 AA Excitation functions 4 and mid-y abundancies: OK Energy dependence: OK Hadron-string models work well

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

Marcus Bleicher, TBS Berkeley 2005 Excitation functions: ratios ‘Horn’ in the ratio not reproduced well reproduced relative strange baryon enhancement reproduced

Marcus Bleicher, TBS Berkeley 2005 Transverse Pressure: Proton-Proton PP works well pQCD needed at RHIC PYTHIA included in UrQMD 2.x and HSD

Marcus Bleicher, TBS Berkeley 2005 Proton-Nucleus pA is well under control CC and SiSi are also under control What about AA?

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

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?

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-

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

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

Marcus Bleicher, TBS Berkeley 2005 Elliptic flow (III) From Xianglei Zhu Hadron/String dynamics predicts correct mass ordering

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!

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