118375168
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.
119030398
UrQMD v2.3: Changes and Comparisons
The new version of the Ultra-relativistic Quantum Molecular Dynamics model (UrQMD-2.3) is presented. The Ultra-relativistic Quantum Molecular Dynamics model (UrQMD) is a microscopic many body approach to pp, pA and AA interactions at relativistic energies. The major updates and changes are explained and a comparison to the previous version (UrQMD-1.3p1) in the context of the available data is performed. The plots and numerical data tables for hadron (i.e. $\pi$, K, p, $\bar{p}$, $\Lambda$, $\bar{\Lambda}$, $\Xi$, $\bar{\Xi}$, $\Omega$, $\bar{\Omega}$) multiplicities, and mean transverse mass momenta, $ -m_0$ excitation functions, transverse mass spectra and rapidity distributions in pp and central Au+Au/Pb+Pb reactions from $E_{\rm lab}=2A $GeV to $\sqrt{s}_{\rm NN}=200$ GeV are provided in this paper. The source code of UrQMD-2.3 is available at this http URL
120726937
Initial-state fluctuations at the RHIC and the LHC in event-by-event ideal hydrodynamics
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.
119118846
Anisotropic flow in transport + hydrodynamics hybrid approaches
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.
118584528
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.
118510480
Classification of initial state granularity via 2D Fourier expansion
A new method for quantifying fluctuations in the initial state of heavy ion collisions is presented. The initial state energy distribution is decomposed with a set of orthogonal basis functions which include both angular and radial variation. The resulting two-dimensional Fourier coefficients provide additional information about the nature of the initial state fluctuations compared to a purely angular decomposition. We apply this method to ensembles of initial states generated by both Glauber and color glass condensate Monte-Carlo codes. In addition initial state configurations with varying amounts of fluctuations generated by a dynamic transport approach are analyzed to test the sensitivity of the procedure. The results allow for a full characterization of the initial state structures that is useful to discriminate the different initial state models currently in use. Communicated by Steffen Bass
121485180
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.
181677184
LOST POWER INFLUENCE ON THE SYNCHROTRON ACCELERATING GAP. (IN GERMAN)
97292148
Density of states in the K excitation spectra of solid and liquid lithium
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.