# Presentation: 501072

## 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...

- Why we need transport...to avoid being fooled...

- 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

- Marcus Bleicher, TBS Berkeley 2005

## Included Particles

- Marcus Bleicher, TBS Berkeley 2005

## Resonance cross sections

- Marcus Bleicher, TBS Berkeley 2005

##  Reaction stages

-  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 qualityOne 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

- Pb+Pb @ 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 2005

## Where do we expect interesting effects?

- No P.T. at low
-  Search for irregularities  around Ebeam = 10-40 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 !

- 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

- Marcus Bleicher, TBS Berkeley 2005