Resonance Dynamics in Heavy Ion Collisions

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 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

3. Motivation 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 Thanks to Christina Markert, STAR Collaboration

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

5. Motivation Particle yields Particle spectra A+A Hot and dense medium time Motivation 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

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“) Au+Au 40% to 80% sNN = 200 GeV ρ0 f0K0S ωK*0 STAR Preliminary 1.2 pT 1.4 GeV/c |y|  0.5 counts/(10 MeV/c2) How does the experiment (reconstruct) the resonance? • Invariant mass reconstruction of decay products Statistical error only

7. Quick reminder on resonances π- ρ0 π+ ρ0 ρ0 ρ0 π- π+ π- ρ0 + + + π+ - - - 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. r0 p+p- , r0  e+e-) dileptonic decay channel hadronic 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… r0(770) p+p-B.R. ~1  = 1.3 fm D++(1232)  p p+B.R. ~1  = 1.6 fm f 0(980)  p+p-B.R. ~ 2/3  = 2.6 fm K*0±(892) pK B.R. ~ 2/3  = 4 fm S (1385)  LpB.R. ~ 0.88  = 5.5 fm L (1520) p KB.R. ~ 0.45  = 12.6 fm F (1020) K+K-B.R. ~ 0.49  = 44 fm

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

10. Model UrQMD • 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. Model Fochler, Vogel et al, Physical Review C, in print (arxiv.org/abs/nucl-th/0601062) Model UrQMD • 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 • Generates full space-time dynamics of hadrons and strings Bleicher et al., J.Phys.G25:1859-1896,1999

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 Note: L.h.s. would be visible in a dilepton analysis (multiplied with the corresponding branching ratio). All decayed particles Reconstructable particles

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

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

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

17. Re-feeding of resonances Tch freeze-out Tkin freeze-out Rough estimate of the re-feeding probability • 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

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

19. Decay time analysis Decay time analysis r mesons are emitted earlier than D baryons Peak emission times: D ~ 22 fm/c r ~ 15 fm/c

20. Cross sections and collision rates Cross sections and collision rates K* and L* show rescattering S* shows regeneration Regeneration/Rescattering cross section: s(L+p) > s (K+p) > s (S+p) ? S* K* L* Production channel for measured resonances: S+p  L(1520) K+p  K* N+K L(1520) L+p  S(1385) preliminary L+X Y K+X Y S+X Y

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

22. Mean center of mass energy pp -> D pp -> r Kp -> K* Mean center of mass energy (in linear scale) pp Kp pp Kp

23. Summary 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 r meson • The probability to regenerate a r or K* meson is lower than the chance to regenerate a D baryon • The cross section for L* production is lower than for S*production Thank you!

24. Backup slides Backup slides

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