International School for Advanced Studies

About: International School for Advanced Studies is a education organization based out in Trieste, Friuli-Venezia Giulia, Italy. It is known for research contribution in the topics: Galaxy & Dark matter. The organization has 3751 authors who have published 13433 publications receiving 588454 citations. The organization is also known as: SISSA & Scuola Internazionale Superiore di Studi Avanzati.

Ab initio calculation of properties of carbon in the amorphous and liquid states

Abstract: We have carried out a detailed investigation of liquid carbon (l-C) at T\ensuremath{\simeq}5000 K, and quenched amorphous carbon (a-C) at room temperature, using a first-principles molecular-dynamics method. In this paper we report calculations performed for a fixed density corresponding to low pressures. From our results, which agree well with the limited experimental information available, we analyze the short-range order, particularly the fractions of sp, ${\mathit{sp}}^{2}$, and ${\mathit{sp}}^{3}$ sites, and the electronic properties of the two systems. In addition, we discuss new features of a-C and l-C, predicted by our calculation, and present an analysis of some models proposed in the literature to account for their electronic and bonding properties, as well as of recent controversial experiments.

Common and Differential Neural Mechanisms Supporting Imitation of Meaningful and Meaningless Actions

Abstract: Neuropsychological studies indicate that, after brain damage, the ability to imitate meaningful or meaningless actions can be selectively impaired. However, the neural bases supporting the imitation of these two types of action are still poorly understood. Using PET, we investigated in 10 healthy individuals the neural mechanisms of imitating novel, meaningless actions and familiar, meaningful actions. Data were analyzed using SPM99. During imitation, a significant positive correlation (p < .05, corrected) of regional cerebral blood flow with the amount of meaningful actions was observed in the left inferior temporal gyrus only. In contrast, a significant positive correlation (p < .05, corrected) with the amount of meaningless movements was observed in the right parieto-occipital junction. The direct categorical comparison of imitating meaningful (100%) relative to meaningless (100%) actions showed differential increases in neural activity (p < .001, uncorrected) in the left inferior temporal gyrus, the left parahippocampal gyrus, and the left angular gyrus. The reverse categorical comparison of imitating meaningless (100%) relative to meaningful (100%) actions revealed differential increases in neural activity (p < .001, uncorrected) in the superior parietal cortex bilaterally, in the right parieto-occipital junction, in the right occipital–temporal junction (MT, V5), and in the left superior temporal gyrus. Increased neural activity common to imitation of meaningless and meaningful actions compared to action observation was observed in a network of areas known to be involved in imitation of actions including the primary sensorimotor cortex, the supplementary motor area, and the ventral premotor cortex. These results are compatible with the two-route model of action imitation which suggests that there are at least two mechanisms involved in imitation of actions: a direct mechanism transforming a novel action into a motor output, and a semantic mechanism, on the basis of stored memories, that allows reproductions of known actions. Our results indicate that, in addition to shared neural processes, the direct and the semantic mechanisms that underlie action imitation also draw upon differential neural mechanisms. The direct mechanism underlying imitation of meaningless actions differentially involves visuospatial transformation processes as evidenced by activation of areas belonging to the dorsal stream. In contrast, imitation of meaningful actions differentially involves semantic processing as evidenced by activation of areas belonging to the ventral stream.

Hamiltonian replica exchange in GROMACS: a flexible implementation

Abstract: A simple and general implementation of Hamiltonian replica exchange for the popular molecular dynamics software GROMACS is presented. In this implementation, arbitrarily different Hamiltonians can be used for the different replicas without incurring in any significant performance penalty. The implementation was validated on a simple toy model – alanine dipeptide in water – and applied to study the rearrangement of an RNA tetraloop, where it was used to compare recently proposed force-field corrections.

Impact of dark matter decays and annihilations on reionization

Abstract: One of the possible methods to distinguish among various dark matter (DM) candidates is to study the effects of DM decays. We consider four different DM candidates [light dark matter (LDM), gravitinos, neutralinos and sterile neutrinos], for each of them deriving the decaying/annihilation rate, the influence on reionization, matter temperature and cosmic microwave background (CMB) spectra. We find that LDM particles (1–10 MeV) and sterile neutrinos (2–8 keV) can be sources of partial early reionization (z ~< 100). However, their integrated contribution to Thomson optical depth is small (~< 0.01) with respect to the 3-yr WMAP results (τe = 0.09 ± 0.03). Finally, they can significantly affect the behaviour of matter temperature. On the contrary, effects of heavy DM candidates (gravitinos and neutralinos) on reionization and heating are minimal. All the considered DM particles have completely negligible effects on the CMB spectra.

Accurate relic densities with neutralino, chargino and sfermion coannihilations in mSUGRA

Abstract: Neutralinos arise as natural dark matter candidates in many supersymmetric extensions of the Standard Model. We present a novel calculation of the neutralino relic abundance in which we include all so-called coannihilation processes between neutralinos, charginos and sfermions, and, at the same time, we apply the state of the art technique to trace the freeze-out of a species in the early Universe. As a first application, we discuss here results valid in the mSUGRA framework; we describe general trends as well as performing a detailed study of the neutralino relic densities in the mSUGRA parameter space. The emerging picture is fair agreement with previous analyses in the same framework, however we have the power to discuss it in much more detail than previously done. For example, we find that the cosmological bound on the neutralino mass is pushed up to ~565 GeV in the stau coannihilation region and to ~1500 GeV in the chargino coannihilation region.