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## Dynamics of Nucleus-Nucleus Collisions at CBM energies

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**Frankfurt Institute for Advanced Studies**Dynamics of Nucleus-Nucleus Collisions at CBM energies Elena Bratkovskaya 19.09.2006 , CBM Workshop „ The Physics of High Baryon Density „, IPHC Strasbourg**Introduction**• FAIR energies are well suited to study dense and hot nuclear matter – • a phase transition to QGP , • chiral symmetry restoration, • in-medium effects Observables for CBM: • Excitation function of particle yields and ratios • Transverse mass spectra • Collective flow • Dileptons • Open and hidden charm • Fluctuations and correlations • ... Way to study: • Experimental energy scan of different observables in order to find an ‚anomalous‘ behaviour in comparison with theory Microscopical transport models provide a unique dynamical description of nonequilibrium effects in heavy-ion collisions**Basic concept of HSD**• HSD – Hadron-String-Dynamics transport approach • for each particle species i (i = N, R, Y, p, r, K, …) the phase-space density fi followsthe transport equations • with collision termsIcoll describing: • elastic and inelastic hadronic reactions: • baryon-baryon, meson-baryon, meson-meson • formation and decay of baryonic and mesonicresonances • string formation and decay • Implementation of detailed balance on the level of 1<->2 • and 2<->2 reactions (+ 2<->n multi-meson fusion reactions in HSD)**Degrees of freedom in HSD**• hadrons - baryons and mesons including excited states (resonances) • strings – excited color singlet states (qq - q) or (q – qbar) • Based on the LUND string model • &perturbative QCD via PYTHIA • leading quarks (q, qbar) & diquarks • (q-q, qbar-qbar) • NOT included in HSD 2.5 presented here : • explicit parton-parton interactions (i.e. between quarks and gluons) outside strings! • explicit phase transition from hadronic to partonic degrees of freedom • QCD EoS for partonic phase under construction: PHSD –Parton-Hadron-String-Dynamics**Dense baryonic matter – average quantities**Time evolution of the baryon density in a central cell (A+A, b=0 fm) enormous energy and baryon densities are reached (e > ecrit=1 GeV/fm3) at FAIR energies**Changes of the particle properties in the hot and dense**baryonic medium r meson spectral function • In-medium models: • chiral perturbation theory • chiral SU(3) model • coupled-channel G-matrix approach • chiral coupled-channel effective field theory predict changes of the particle properties in the hot and dense medium, e.g. broadening of the spectral function How to treat in-medium effects in transport approaches?**From on-shell to off-shell transport dynamics**W. Cassing et al., NPA 665 (2000) 377; 672 (2000) 417; 677 (2000) 445 Off-shell transport approach: Generalized transport equations on the basis of the Kadanoff-Baym equations for Greens functions || Dynamical equations of motion for ‚test-particle‘ propagation in 8-dimensional phase space (r(t), p(t), E(t)): Application to strangeness: In-medium transition rates with momentum, temperature and density dependent spectral function of antikaons from a coupled channel G-matrix approach W. Cassing, L. Tolos, E.L.B., A. Ramos., NPA 727 (2003) 59 Application to dileptons: In-medium transition rates with momentum and density dependent dynamical spectral functions of vector mesonsE.L..B., NPA 686 (2001), HSD predictions for CBM (2006)**Generalized collision integral for n<->m reactions:**Treatment of multi-particle collisions in transport approaches W. Cassing, NPA 700 (2002) 618 2<->3 Multi-meson fusion reactions important for antiproton, antilambda dynamics**AGS**NA49 BRAHMS Excitation function of particle yields Overview on the experimental meson and strange baryon abundancies from central Au+Au/Pb+Pb collisions versus s 1/2**Excitation function of particle ratios**Transport models: HSD, UrQMD, GiBUU Exp. data are not well reproduced within the hadron-string picture => evidence for nonhadronic degrees of freedom CBM probes the ‚horn‘ energy range**Transverse mass spectra**• Transport models: • HSD 2.0 (+ Cronin effect) • UrQMD 2.0 • UrQMD 2.2 (+ effective resonances with masses 2 < M < 3 GeV and isotropic decay) • GiBUU • All transport models fail to reproduce the T-slope without introducing special „tricks“which are, however, inconsistent with other observables! • 3D-fluid hydrodynamical model gives the right slope! • Is the matter a parton liquid?**Collective flow: v2 excitation function**• Proton v2 at low energy shows sensitivity to the nucleon potential. • Cascade codes fail to describe the exp. data. • AGS energies: transition from squeeze-out to in-plane elliptic flow**Collective flow: elliptic flow at 25 A GeV – predictions**for CBM • Transport models • HSD • UrQMD • GiBUU • QGSM (v. Dubna; • v. Oslo-Tuebingen) • AMPT without string melting • predict similar v2 for charged particles (except QGSM)! • AMPT with string melting shows • much stronger v2 for charged particles! Charged particles Challenge for CBM!**Elliptic flow at 25 A GeV – predictions for pions, kaons**and protons AMPT without string melting shows v2 similar to other models for all particles! Including string melting leads to a larger v2**Dileptons**• Dileptons are an ideal probe for vector meson spectroscopy in the nuclear medium and for the nuclear dynamics ! • Study of in-medium effects with dilepton experiments: • „History“ – DLS, KEK, SPS (HELIOS) • Novel experiments – HADES, NA60, CERES, PHENIX • Future – CBM • High precision NA60 data allow to • distinguish among in-medium models! • Clear evidence for a broadening • of the r spectral function! • Direct photons as a possible observable for CBM ?!**Dileptons: excitation function**• Dilepton yield increases with energy due to a higher production of mesons • r melts at practically all energies; w and f show clear peaks on an approx. exponential background in mass!**Dileptons – HSD predictions for CBM**HSD predictions In-medium modifications of e+e- and m+m- spectra are very similar!**Heavy flavor sector reflects the actual dynamics since heavy**hadrons can onlybe formed in the very early phase of heavy-ion collisions at FAIR/SPS! Open and hidden charm • Hidden charm: J/Y , Y‘ Anomalous J/Y suppression in A+A (NA38/NA50) • Comover dissociation in thetransport • approaches – HSD & UrQMD: • NA50 data are consistent with • comover absorption models**Open charm: D-mesons**• Dropping D-meson masses with increasing light quark density might give a large enhancement of the open charm yield at 25 A GeV !**Open and hidden charm – HSD predictions for CBM**Open charm Hidden charm CBM • Open charm: • without medium effects: suppression of D-meson spectra by factor ~ 10 relative to the global mT-scaling • with medium effects:restoration of the global mT-scaling for the mesons • Hidden charm: • J/Y suppression due to comover absorption at FAIR is lower than at SPS**Summary**• FAIRis anexcellent facilitytostudy the properties of the sQGP (strongly interacting ‚color liquid‘) as well as hadronic matter • Transport theory is thegeneral basisfor an understanding of nuclear dynamics on a microscopic level UrQMD: U+U, 25 A GeV