1. Resonance Dynamics in Heavy Ion Collisions 22nd Winter Workshop on Nuclear Dynamics 17.03.2006, La Jolla, California Sascha Vogel, Marcus Bleicher UrQMD group (Mohammed Abdel-Aziz, Marcus Bleicher, Stephane Haussler, Quingfeng Li, Hannah Petersen, Diana Schumacher, Sascha Vogel, Xianglei Zhu)

2. Outline Introduction and motivation (more or less a reminder) Model Rescattering of resonances  Rapidity, transverse momentum, mass spectra Re-feeding of resonances  Average cross sections  Collision rates  Center of mass energies Summary Outline

3. Motivation We want to … learn something about freeze-out dynamics of heavy ion collisions understand why statistical models cannot describe resonance data understand quantitatively the effect of rescattering and regeneration of daughter particles in order to understand the data already measured learn something about in-medium properties of hadrons Motivation Thanks to Christina Markert, STAR Collaboration

4. Particle ratios well reproduced Resonance ratios not reproduced (Braun-Munzinger, Schweda QM 2004)  ++ /p too low K*/K too high Braun-Munzinger et al. Motivation Statistical model fitting

5. Hot and dense medium A+A Particle yieldsParticle spectra time p+p p+p interactions: No extended initial medium Chemical freeze-out (no thermalisation) Kinetic freeze-out close to the chemical freeze-out A+A interactions: Extended hot and dense phase Kinetic freeze-out separated from chemical freeze-out in medium effects Rescattering effects Regeneration effects Thanks to Christina Markert, STAR Collaboration Motivation

6. Quick reminder on resonances Resonances in a hadronic medium Since they are unstable (decaying) particles with a cross section, they can  scatter  decay Decay products (or daughter particles) can  escape the collision zone  (re-)scatter  build another resonance (“regenerate“) How does the experiment (reconstruct) the resonance?  Invariant mass reconstruction of decay products Au+Au 40% to 80% Statistical error only ρ 0 f 0 K 0 S ω K *0 1.2  p T  1.4 GeV/c |y|  0.5 STAR Preliminary counts/(10 MeV/c 2 )  s NN = 200 GeV

7. Quick reminder on resonances Resonances in a hadronic medium  Since hadronic decay products interact with the surrounding medium the experiment cannot reconstruct all resonances  The consequence is, that all spectra one observes by reconstructing hadronic decay products are altered by the hadronic medium  Interesting effect for resonances which have a hadronic and a dileptonic decay channel! (e.g.       ,    e + e - ) ρ0ρ0 ρ0ρ0 + + - - π-π- π+π+ ρ0ρ0 - + ρ0ρ0 ρ0ρ0 π-π- π+π+ π+π+ π-π- hadronic decay channel dileptonic decay channel

8. Quick reminder on resonances Resonances in a hadronic medium Differences in observables between the different decay channels depend on various factors, e.g:  system (p+p, Au+Au?)  centrality  life time of the resonance (see below)  freeze-out mechanism  life time of the medium  density of the medium  etc…  0 (770)   +  - B.R. ~1  = 1.3 fm  ++ (1232)  p  + B.R. ~1  = 1.6 fm f 0 (980)   +  - B.R. ~ 2/3  = 2.6 fm K *0± (892)   K B.R. ~ 2/3  = 4 fm  (1385)   B.R. ~ 0.88  = 5.5 fm  (1520)  p K B.R. ~ 0.45  = 12.6 fm  (1020)  K + K - B.R. ~ 0.49  = 44 fm

9. Models What kind of model do we need for our study of resonance rescattering and refeeding? initial final thermodynamical models hydrodynamical models transport models Transport model, since we need to keep track of the particles throughout the whole collision. Models

10. UrQMD Model Ultra Relativistic Quantum Molecular Dynamics Non equilibrium transport model All hadrons and resonances up to 2.2 GeV included String excitation and fragmentation pQCD hard scattering at high energies with PYTHIA Bratkovskaya, Bleicher et al., Phys.Rev.C69:054907,2004

11. UrQMD Model Bleicher et al., J.Phys.G25:1859-1896,1999 Fochler, Vogel et al, Physical Review C, in print (arxiv.org/abs/nucl-th/0601062) Generates full space-time dynamics of hadrons and strings Cross sections are fitted to available experimental data or calculated by the principle of detailed balance and the additive quark model Does dynamically account for canonical suppression

12. Rescattering Baryon resonances in central AuAu collisions at RHIC Experimental signal loss due to rescattering of decay products. All decayed particles Reconstructable particles

13. Rescattering Meson resonances in central AuAu collisions at RHIC All decayed particles Reconstructable particles Note: L.h.s. would be visible in a dilepton analysis (multiplied with the corresponding branching ratio).

14. Rescattering p T spectra Open symbols: reconstructable particles, filled symbols: all decayed Stronger suppression towards lower transverse momenta  apparent ‚heating‘ of the spectra

15. Mass spectrum of the  meson Note: C+C collisions at 2AGeV S.Vogel, M. Bleicher, Physical Review C, in print (arxiv.org/abs/nucl-th/0509105)  meson mass mass drops towards central reactions mass drops towards low p T AuAu E cm =200AGeV AuAu E cm =200AGeV

16. Motivation Some data from STAR Increase of the  /K - ratio from pp to central AuAu Decrease of the K*/K - and  */  ratio from pp to central AuAu Thanks to Christina Markert, STAR Collaboration Rescattering and Regeneration effects are to be considered! Preliminary

17. T ch freeze-out T kin freeze-out Strong decrease in kinetic freeze-out temperature from central to peripheral collisions Kinetic freeze-out as low as 80 - 90 MeV Consequences for resonance re-feeding Blast Wave Fit by Olga Barranikova, STAR Re-feeding of resonances Rough estimate of the re-feeding probability

18. baryons can re-created until end of the reaction meson re-creation is only possible near chemical freeze- out Estimate of available energy for re-feeding at different reaction stages with a simple thermal ansatz: T kin Re-feeding of resonances Rough estimate of the re-feeding probability

19.  mesons are emitted earlier than  baryons Peak emission times:  fm/c  fm/c Decay time analysis

20. Cross sections and collision rates  +X  Y  +X  Y  +X  Y Production channel for measured resonances:              * and  * show rescattering  * shows regeneration Regeneration/Rescattering cross section:       

21. Mean center of mass energy pp  Blue: Collision rate of the corresponding reactions Red: Average center of mass energy Green: Probability to form a resonance Mean center of mass energy

22.   p     pp  Mean center of mass energy (in linear scale)

23. Summary Resonances provide additional information compared to stable hadrons and HBT measurements Thermal models do not describe all resonance yields When trying to understand resonance data one has to consider both effects, rescattering and refeeding The effect of rescattering is huge and can be measured for example with the  meson The probability to regenerate a  or K * meson is lower than the chance to regenerate a  baryon The cross section for   production is lower than for   production Summary Thank you!

24. Backup slides

25. Q. Li, M.Bleicher, H. Stoecker, nucl-th/0602032; Data: STAR Correlations are well described except for most central reactions Model UrQMD - correlations