What have we learned from transport models?

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

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

3. 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

4. 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 Marcus Bleicher, TBS Berkeley 2005

5. Included Particles Marcus Bleicher, TBS Berkeley 2005

6. Resonance cross sections Marcus Bleicher, TBS Berkeley 2005

7. Reaction stages • 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 Marcus Bleicher, TBS Berkeley 2005

8. Basic checks (I) Marcus Bleicher, TBS Berkeley 2005

9. Basic Checks (II) Unfortunately the data has poor qualityOne has to rely on the extrapolation This leads to ~10% systematic uncertainty Marcus Bleicher, TBS Berkeley 2005

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

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

12. 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 Marcus Bleicher, TBS Berkeley 2005

13. 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 2005

14. Where do we expect interesting effects? • 1st Order phase transition at high • No P.T. at low • Search for irregularities around Ebeam = 10-40 GeV: • Flow, strangeness, E-by-E Plot adapted from L. Bravina Marcus Bleicher, TBS Berkeley 2005

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

16. 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

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

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

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

20. 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

21. 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

22. Data for h- 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 ! Marcus Bleicher, TBS Berkeley 2005

23. Elliptic flow (I) • Elliptic flow from string/hadron model is too small • However, half of v2 is generated in the hadronic stage From Xianglei Zhu Marcus Bleicher, TBS Berkeley 2005

24. Elliptic flow (II) • Qualitatively non-flow contributions are reproduced • Large difference between real v2 and 2-particle cumulants From Xianglei Zhu Marcus Bleicher, TBS Berkeley 2005

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

26. Elliptic flow (IV) • Scaling with nq is present in transport calculations • Scaling is not a unique QGP signal! From Xianglei Zhu Marcus Bleicher, TBS Berkeley 2005

27. Summary: orWhat 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