Sascha Vogel

Production and evolution path of dileptons at energies accessible to the HADES detector

Institut für Theoretische Physik, Goethe-Universität, Max-von-Laue-Str. 1, D-60438 Frankfurt am Main, Germany Institut für Kernphysik, Goethe-Universität, Max-von-Laue-Str. 1, D-60438 Frankfurt am Main, Germany Frankfurt Institute for Advanced Studies (FIAS), Ruth-Moufang-Str. 1, D-60438 Frankfurt am Main, Germany GSI Helmholtzzentrum für Schwerionenforschung GmbH, Planckstrasse 1, D-64291 Darmstadt, Germany (Dated: December 8, 2008)

System size and energy dependence of dilepton production in heavy-ion collisions at 1-2 GeV/nucleon energies

We study the dilepton production in heavy-ion collisions at energies of 1-2 AGeV as well as in proton induced pp,pn,pd and p + A reactions from 1 GeV up to 3.5 GeV where data have been taken by the HADES collaboration. For the analysis we employ three different transport models - the microscopic off-shell Hadron-String-Dynamics (HSD) transport approach, the Isospin Quantum Molecular Dynamics (IQMD) approach as well as the Ultra-relativistic Quantum Molecular Dy- namics (UrQMD) approach. We find that the HSD and IQMD models describe very reasonably the elementary pp, pn andN reactions despite of different assumptions on quantities like the excitation function of themultiplicity, where solid experimental constraints are not available. Taking these data on elementary collisions as input, the three models provide a good description of the presently available heavy ion data. In particular, we confirm the experimentally observed enhancement of the dilepton yield (normalized to the multiplicity of neutral pions N�0) in heavy-ion collisions with respect to that measured in NN = (pp + pn)/2 collisions. We identify two contributions to this enhancement: a) the pN bremsstrahlung which scales with the number of collisions and not with the number of participants, i.e. pions; b) the dilepton emission from intermediate �s which are part of the reaction cycles � ! �N;�N ! � and NN ! N�;N� ! NN. With increasing system size more generations of intermediate �s are created. If suchdecays into a pion, the pion can be reabsorbed, however, if it decays into a dilepton, the dilepton escapes from the system. Thus, experimentally one observes only one pion (from the last produced �) whereas the dilepton yield accumulates the contributions from all �s of the cycle. We show as well that the Fermi motion en- hances the production of pions and dileptons in the same way. Furthermore, employing the off-shell HSD approach, we explore the influence of in-medium effects like the modification of self-energies and spectral functions of the vector mesons due to their interactions with the hadronic environment. We find only a modest influence of the in-medium effects on the dilepton spectra in the invariant mass range where data with small error bars exist.

How sensitive are di-leptons from rho mesons to the high baryon density region?

We show that the measurement of dileptons might provide only a restricted view into the most dense stages of heavy-ion reactions. Thus, possible studies of meson and baryon properties at high baryon densities, as, e.g., done at the GSI High Acceptance DiElectron Spectrometer (HADES) and envisioned for the Facility for Antiproton and Ion Research (FAIR) compressed baryonic matter experiments, might observe weaker effects than currently expected in certain approaches. We argue that the strong absorption of resonances in the high-baryon-density region of the heavy-ion collision masks information from the early hot and dense phase due to a strong increase of the total decay width because of collisional broadening. To obtain additional information, we also compare the currently used approaches to extract dileptons from transport simulations, i.e., shining, only vector mesons from final baryon resonance decays and instant emission of dileptons and find a strong sensitivity on the method employed in particular at FAIR and the CERN Super Proton Synchrotron energies. It is shown explicitly that a restriction to \ensuremath{\rho} meson (and therefore dilepton) production only in final-state baryon resonance decays provide a strong bias toward rather low baryon densities. The results presented are obtained from ultrarelativistic quantum molecular dynamics v2.3 calculations using the standard setup.

Canonical strangeness suppression in microscopic transport models

We demonstrate the occurrence of canonical suppression associated with the conservation of a U(1) charge in current transport models. For this study a pion gas is simulated within two different transport approaches by incorporating inelastic and volume-limited collisions

Theoretical analysis of dilepton spectra in heavy ion collisions at GSI-FAIR energies

\ensuremath{\pi}\ensuremath{\pi}\ensuremath{\leftrightarrow}K\overline{K}

Elliptic flow fluctuations in heavy ion collisions and the perfect fluid hypothesis

for the production of kaon pairs. Both descriptions can dynamically account for the suppression in the yields of rare strange particles in a limited box, being in full accordance with a canonical statistical description.

Resonances as a possible observable of hot and dense nuclear matter

This paper addresses the theoretical analysis of dilepton spectra in C+C collisions at GSI-SIS energies. Theoretical predictions for the recent data of the HADES Collaboration at SIS energies are made with the help of a hadron-string transport model, the Ultra-relativistic Quantum Molecular Dynamics (UrQMD) model. A mass shift of the ρ meson due to kinematical effects is discussed.

Reconstructing ρ 0 and ω mesons from nonleptonic decays in C+C collisions at 2 GeV/nucleon in transport model calculations

We analyze the recently measured v2 fluctuation in the context of establishing the degree of fluidity of the matter produced in heavy ion collisions. We argue that flow observables within systems with a non-negligible mean free path should acquire a “dynamical” fluctuation, due to the random nature of each collision between the system’s degrees of freedom. Because of this, v2 fluctuations can be used to estimate the Knudsen number of the system produced at the relativistic heavy ion collider (RHIC). To illustrate this quantitatively, we apply the UrQMD model, with scaled cross sections, to show that collisions at RHIC have a Knudsen number at least one order of magnitude below the expected value for an interacting hadron gas. Furthermore, we argue that the Knudsen number is also bound from below by the v2 fluctuation data, because too small a Knudsen number would break the observed scaling of v2 fluctuations due to the onset of turbulent flow. We propose, therefore that v2 fluctuation measurements, together with an understanding of the turbulent regime for relativistic hydrodynamics, will provide an upper as well as a lower limit for the Knudsen number. We also argue that an energy scan of v2 fluctuations could shed light on the onset of the fluid regime.

The use of Kolmogorov-Smirnov test in event-by-event analysis

One of the most fundamental questions in the field of relativistic heavy-ion physics is how to reach and explore densities which are needed to cross the chiral and/or the deconfinement phase transition. In this analysis we investigate the information we can gather by analyzing baryonic and mesonic resonances on the hot and dense phases in such nuclear reactions. The decay products of these resonances carry information on the properties of the resonances at the spacetime point of their decay. We especially investigate the percentage of reconstructable resonances as a function of density for heavy-ion collisions in the energy range between E lab = 30 A GeV and √s = 200 A GeV, the energy domain between the future FAIR facility and the present RHIC collider.

Evidence for the a(1) meson being a difficult messenger for the restoration of chiral symmetry

We predict transverse and longitudinal momentum spectra and yields of