Frankfurt Institute for Advanced Studies

About: Frankfurt Institute for Advanced Studies is a facility organization based out in Frankfurt am Main, Germany. It is known for research contribution in the topics: Baryon & Quark–gluon plasma. The organization has 798 authors who have published 2733 publications receiving 82799 citations. The organization is also known as: FIAS.

Second order dissipative fluid dynamics from kinetic theory

Abstract: We derive the equations of second order dissipative fluid dynamics from the relativistic Boltzmann equation following the method of W. Israel and J. M. Stewart. We present a frame independent calculation of all first- and second-order terms and their coefficients using a linearised collision integral. Therefore, we restore all terms that were previously neglected in the original papers of W. Israel and J. M. Stewart.

A model comparison of resonance lifetime modifications, a soft equation of state and non-Gaussian effects on π–π correlations at FAIR/AGS energies

Abstract: Hanbury-Brown–Twiss (HBT) correlations of π−–π− pairs at FAIR/AGS energies are investigated by using the UrQMD transport model and the CRAB analyzing program. Three different possible sources (treatment of resonance lifetimes, a soft equation of state and non-Gaussian effects) to understand the HBT RO/RS puzzle are investigated. First, we find that different treatments of the resonance decay time cannot resolve the HBT time-related puzzle, however it can modify the HBT radii at low transverse momenta to some extent to explain the data slightly. Second, with a soft equation of state with momentum dependence, the measured transverse momentum-dependent HBT radii and the RO/RS ratio can be described fairly well. Third, non-Gaussian effects are visible in the calculated correlation function. Using the Edgeworth expansion, one finds that the non-Gaussian effect is strongest in the longitudinal direction and weakest in the sideward direction.

An Energy-Efficient Multi-GPU Supercomputer

Abstract: During recent years, heterogeneous HPC systems, which combine commodity processors with GPUs have proven to deliver superior energy efficiency. In this paper an international collaboration of research groups from Germany and Saudi Arabia presents SANAM, the prototype of a general-purpose 10 PFLOPS supercomputer based on off-the-shelf components. Leveraging an advanced multi-GPU architecture, supported by particular software optimizations that aim at energy efficiency, the system ranked second in the Green500 list of November 2012 with a power efficiency of 2351 MFLOPS/W.

On the use of tracer particles in simulations of binary neutron stars

Abstract: In grid-based codes that provide the combined solution of the Einstein equations and of relativistic hydrodynamics, the history of the fluid is not simple to track, especially when compared with particle-based codes. The use of tracers, namely massless particles that are advected with the flow, represents a simple and effective way to solve this problem. Yet, the use of tracers in numerical relativity is far from being settled and several issues, such as the impact of different placements in time and space of the tracers, or the relation between the placement and the description of the underlying fluid, have not yet been addressed. In this paper we present the first detailed discussion of the use tracers in numerical-relativity simulations focussing on both unbound material -- such as the one leading to r-process nucleosynthesis in binary mergers of neutron stars -- and on bound material -- such as the one in the core of the object produced from the merger of two neutron stars. In particular, when interested in unbound matter, we have evaluated four different placement schemes that could be used to initially distribute the tracers and how well their predictions match those obtained when using information from the actual fluid flow. Countering our naive expectations, we found that the most effective method does not rely on the rest-mass density distribution nor on the fluid that is unbound, but simply distributes tracers uniformly in rest-mass density. This prescription leads to the closest matching with the information obtained from the hydrodynamical solution. When considering bound matter, we demonstrate that tracers can provide insight into the fine details of the fluid motion as they can be used to track the evolution of fluid elements or to calculate the variation of quantities that are conserved along streamlines of adiabatic flows.

Learning independent causes in natural images explains the spacevariant oblique effect

Abstract: The efficient coding hypothesis posits that sensory processing increases independence between neural responses to natural stimuli by removing their statistical redundancy reflective of the structure present in the natural environment. While there is consensus on the role of the statistical structure of the physical environment in shaping the natural input to the sensory system, it is not well understood how the sensory apparatus itself and its active use during behavior determine the statistics of the input. To explore this issue, a virtual human agent is simulated navigating through a wooded environment under full control of its gaze allocation during walking. Independent causes for the images obtained during navigation are learned with algorithms that have been shown to extract computationally useful representations similar to those encountered in the primary visual cortex of the mammalian brain. The distributions of properties of the learned simple cell like units are in good agreement with a wealth of data on the visual system including the oblique effect, the meridional effect, properties of neurons in the macaque visual cortex, and functional Magnetic Resonance Imaging (fMRI) data on orientation selectivity in humans and monkeys. Finally, this analysis sheds new light on the discussion on orientation anisotropies based on carpented environments. Thus, when learning computational representations it is not sufficient to consider only the regularities of the environment but also the regularities imposed by the sensory apparatus and its use during behavior need to be taken into account.