Fully integrated transport approach to heavy ion reactions with an intermediate hydrodynamic stage
We present a coupled Boltzmann and hydrodynamics approach to relativistic heavy ion reactions. This hybrid approach is based on the Ultra-relativistic Quantum Molecular Dynamics (UrQMD) transport approach with an intermediate hydrodynamical evolution for the hot and dense stage of the collision. Event-by-event fluctuations are directly taken into account via the nonequilibrium initial conditions generated by the initial collisions and string fragmentations in the microscopic UrQMD model. After a (3+1)-dimensional ideal hydrodynamic evolution, the hydrodynamical fields are mapped to hadrons via the Cooper-Frye equation and the subsequent hadronic cascade calculation within UrQMD proceeds to incorporate the important final state effects for a realistic freeze-out. This implementation allows us to compare pure microscopic transport calculations with hydrodynamic calculations using exactly the same initial conditions and freeze-out procedure. The effects of the change in the underlying dynamics--ideal fluid dynamics versus nonequilibrium transport theory--is explored. The freeze-out and initial state parameter dependencies are investigated for different observables. The time evolution of the baryon density and particle yields are also discussed. We find that the final pion and proton multiplicities are lower in the hybrid model calculation owing to the isentropic hydrodynamic expansion whereas the yields for strange particles are enhanced owing to the localmore » equilibrium in the hydrodynamic evolution. The results of the different calculations for the mean transverse mass excitation function, rapidity, and transverse mass spectra for different particle species at three different beam energies are discussed in the context of the available data.« less

title: Centrality and system size dependence of (multi-strange) hyperons at 40A and 158A GeV: A comparison between a binary collision model and a Boltzmann + hydrodynamic hybrid model
abstract: We present results on the centrality and system size dependence of (multi-strange) hyperons in $\mathrm{Pb}+\mathrm{Pb}$ collisions at $40A$ and $158A$ GeV from the Ultra-relativistic Quantum Molecular Dynamics (UrQMD-v2.3) model and a coupled Boltzmann $+$ hydrodynamics calculation. The second approach is realized in a hybrid fashion based on UrQMD, with an intermediate hydrodynamical evolution for the hot and dense stage of the collision. This implementation allows a comparison of microscopic transport calculations with hydrodynamic simulations to explore the transition from a system that is not fully equilibrated, such as $\mathrm{C}+\mathrm{C}$ or $\mathrm{Si}+\mathrm{Si}$ collisions, to a supposedly fully equilibrated system, such as that created in $\mathrm{Pb}+\mathrm{Pb}$ reactions. The results of our calculations are compared to measurements of the (anti-)hyperon yields at midrapidity ($|y|\ensuremath{\leqslant}0.5$) and total multiplicities performed by the NA49 and NA57 Collaborations at $40A$ and $158A$ GeV. Furthermore, we compare our predictions to the centrality dependence of $\ensuremath{\Lambda},\overline{\ensuremath{\Lambda}}$, and ${\ensuremath{\Xi}}^{\ensuremath{-}}$ rapidity spectra and total multiplicities at $40A$ and $158A$ GeV, where possible.

title: What the collective flow excitation function can tell about the quark–gluon plasma
abstract: Recent STAR data from the RHIC beam energy scan (BES) show that the slope of the net-proton directed flow v 1 at midrapidity changes sign twice within the collision energy range 7.7–39 GeV. To investigate this phenomenon, we study the collision energy dependence of v 1 utilizing a hybrid model which combines Boltzmann transport and hydrodynamic approach. Calculations with dynamically evolved initial and final state show no qualitative difference between an equation of state with a cross-over and one with a first-order phase transition, in contrast to earlier pure fluid predictions. Our analysis of the elliptic flow v 2 shows that pre-equilibrium transport dynamics are partially compensating for the diminishing contribution to the elliptic flow from the hydrodynamical phase at lower energies, which leads to a qualitative agreement with STAR BES results in midcentral collisions. No compensation from transport is found for integrated v 3 , which decreases from ≈0.02 at s N N = 27 GeV to ≈0.005 at s N N = 7.7 GeV in midcentral collisions.

title: Initial-state fluctuations at the RHIC and the LHC in event-by-event ideal hydrodynamics
abstract: Triangular flow is a new observable in heavy ion collisions that provides interesting information about the structure of the initial state that enters (viscous) hydrodynamic simulations. The longitudinal correlations of the initial state triangularity is investigated to evaluate the potential to distinguish different energy deposition scenarios. We find that radiation processes in a parton cascade introduce similar longitudinal structures as the string fragmentation processes in a hadron cascade. The weak centrality dependence of triangular flow is reproduced within an event-by-event hybrid approach and confirms its sensitivity to initial state fluctuations. The transverse momentum-dependent triangular flow of identified particles at LHC energies is predicted and shows a mass splitting similar to the one observed for elliptic flow.

title: Triangular flow in relativstic heavy ion collisions in an event-by-event hybrid approach
abstract: Triangular flow has been shown to be an interesting new observable to gain insights about the properties of hot and dense strongly interacting matter as it is produced in heavy ion collisions at RHIC and LHC. The potential of triangular flow for constraining the initial state granularity is explored by performing an explicit calculation of the triangularity and the final state anisotropic flow for initial states that exhibit different amounts of fluctuations. We present triangular flow results for Au+Au collisions at the highest RHIC energy calculated in a hybrid approach that includes a non-equilibrium initial evolution and an ideal hydrodynamic expansion with a hadronic afterburner in 3+1 dimensions. Triangular flow results for Pb+Pb collisions at LHC energies employing the same parameters that work at RHIC are compared to ALICE data. In addition, by comparing the hybrid approach calculation with a pure transport approach, the influence of viscosity is studied.

title: Initial State Fluctuations and Final State Correlations in Relativistic Heavy-Ion Collisions
abstract: We review the phenomenology and theory of bulk observables in ultra- relativistic heavy-ion collisions, focussing on recent developments involving event-by- event uctuations in the initial stages of a heavy ion collision, and how they manifest in observed correlations. We rst dene the relevant observables and show how each measurement is related to underlying theoretical quantities. Then we review the prevailing picture of the various stages of a collision, including the state-of-the-art modeling of the initial stages of a collision and subsequent hydrodynamic evolution, as well as hadronic scattering and freeze-out in the later stages. We then discuss the recent results that have shaped our current understanding and identify the challenges that remain. Finally, we point out open issues and the potential for progress in the eld.

title: Density of states in the K excitation spectra of solid and liquid lithium
abstract: Photoelectric yield measurements at the Li K edge and in an extended energy range 54-80 eV are presented. The sample temperature was varied in the range 79-480K (the liquid state). The edge consists of a steeply rising part due to the onset of transitions and a more slowly rising part which is interpreted as a rising density of states at the Fermi energy. With rising temperature the edge shifts to higher energy and broadens. Phonon broadening appears to contribute also at 79K to the width while the edge shape is consistent with a threshold exponent alpha 1 approximately=0. The structures in the range 54-80 eV are washed out on melting. These structures are well described by APW calculations by Bross (1973) of the optical density including plasmon damping.

title: LOST POWER INFLUENCE ON THE SYNCHROTRON ACCELERATING GAP. (IN GERMAN)

title: Damping antennas for spurious RF-modes in electron-positron storage rings
abstract: The maximum beam currents in DORIS were limited mainly by instabilities induced by the rf-cavities. To overcome this difficulty, the higher cavity modes must be damped. Special damping antennas were developed for this purpose.