A systematic study of the sensitivity of triangular flow to the initial state fluctuations in relativistic heavy-ion collisions
Experimental data from the Relativistic Heavy-Ion Collider suggest that the quark gluon plasma behaves almost like an ideal fluid. Due to its short lifetime, many QGP properties can only be inferred indirectly through a comparison of the final state measurements with transport model calculations. Among the current phenomena of interest are the interdependences between two collective flow phenomena, elliptic and triangular flow. The former is mostly related to the initial geometry and collective expansion of the system whereas the latter is sensitive to the fluctuations of the initial state. For our investigation we use a hybrid transport model based on the ultra-relativistic quantum molecular dynamics (UrQMD) transport approach using an ideal hydrodynamic expansion for the hot and dense stage. Using UrQMD initial conditions for an Au–Au collision, particles resulting from a collision are mapped into an energy density distribution that is evolved event-by-event with a hydrodynamic calculation. By averaging these distributions over different numbers of events, we have studied how the granularity/smoothness of the distribution affects the initial eccentricity, the initial triangularity, and the resulting flow components. The average elliptic flow in non-central collisions is not sensitive to the granularity, while triangular flow is. The triangularity might thus provide a good measure of the amount of initial state fluctuations that is necessary to reproduce the experimental data.

title: Anisotropic flow in transport + hydrodynamics hybrid approaches
abstract: This contribution to the focus issue covers anisotropic flow in hybrid approaches. The historical development of hybrid approaches and their impact on the interpretation of flow measurements is reviewed. The major ingredients of a hybrid approach and the transition criteria between transport and hydrodynamics are discussed. The results for anisotropic flow in (event-by-event) hybrid approaches are presented. Some hybrid approaches rely on hadronic transport for the late stages for the reaction (so called afterburner) and others employ transport approaches for the early non-equilibrium evolution. In addition, there are ‘full’ hybrid calculations where a fluid evolution is dynamically embedded in a transport simulation. After demonstrating the success of hybrid approaches at high Relativistic Heavy Ion Collider and Large Hadron Collider energies, existing hybrid caluclations for collective flow observables at lower beam energies are discussed and remaining challenges outlined.

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: 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: 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: Temperature Dependence of the K Absorption Edge of Lithium
abstract: The shape of the K absorption edge of Li has been measured at 293, 373, 443, and ∼480 K (liquid) by photoyield methods. Unpublished transmission data on the shape of the edge at and below room temperature have been reanalyzed. There is a marked increase of the width of the edge with increasing temperature above 293 K, indicating that the lattice plays a major role in the broadening of the edge.

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)