Showing papers by "International School for Advanced Studies published in 2007"

Hippocampal remapping and grid realignment in entorhinal cortex

Abstract: A fundamental property of many associative memory networks is the ability to decorrelate overlapping input patterns before information is stored. In the hippocampus, this neuronal pattern separation is expressed as the tendency of ensembles of place cells to undergo extensive 'remapping' in response to changes in the sensory or motivational inputs to the hippocampus. Remapping is expressed under some conditions as a change of firing rates in the presence of a stable place code ('rate remapping'), and under other conditions as a complete reorganization of the hippocampal place code in which both place and rate of firing take statistically independent values ('global remapping'). Here we show that the nature of hippocampal remapping can be predicted by ensemble dynamics in place-selective grid cells in the medial entorhinal cortex, one synapse upstream of the hippocampus. Whereas rate remapping is associated with stable grid fields, global remapping is always accompanied by a coordinate shift in the firing vertices of the grid cells. Grid fields of co-localized medial entorhinal cortex cells move and rotate in concert during this realignment. In contrast to the multiple environment-specific representations coded by place cells in the hippocampus, local ensembles of grid cells thus maintain a constant spatial phase structure, allowing position to be represented and updated by the same translation mechanism in all environments encountered by the animal.

A bias-exchange approach to protein folding

Abstract: By suitably extending a recent approach [Bussi, G.; et al. J. Am. Chem. Soc. 2006, 128, 13435] we introduce a powerful methodology that allows the parallel reconstruction of the free energy of a system in a virtually unlimited number of variables. Multiple metadynamics simulations of the same system at the same temperature are performed, biasing each replica with a time-dependent potential constructed in a different set of collective variables. Exchanges between the bias potentials in the different variables are periodically allowed according to a replica exchange scheme. Due to the efficaciously multidimensional nature of the bias the method allows exploring complex free energy landscapes with high efficiency. The usefulness of the method is demonstrated by performing an atomistic simulation in explicit solvent of the folding of a Triptophane cage miniprotein. It is shown that the folding free energy landscape can be fully characterized starting from an extended conformation with use of only 40 ns of sim...

Theory of neutrinos: a white paper

Abstract: This paper is a review of the present status of neutrino mass physics, which grew out of an APS sponsored study of neutrinos in 2004. After a discussion of the present knowledge of neutrino masses and mixing and some popular ways to probe the new physics implied by recent data, it summarizes what can be learned about neutrino interactions as well as the nature of new physics beyond the Standard Model from the various proposed neutrino experiments. The intriguing possibility that neutrino mass physics may be at the heart of our understanding of a long standing puzzle of cosmology, i.e. the origin of matter?antimatter asymmetry is also discussed.

Alleviation of the Fermion-sign problem by optimization of many-body wave functions

Abstract: We present a simple, robust, and highly efficient method for optimizing all parameters of many-body wave functions in quantum Monte Carlo calculations, applicable to continuum systems and lattice models. Based on a strong zero-variance principle, diagonalization of the Hamiltonian matrix in the space spanned by the wave function and its derivatives determines the optimal parameters. It systematically reduces the fixed-node error, as demonstrated by the calculation of the binding energy of the small but challenging C(2) molecule to the experimental accuracy of 0.02 eV.

The universal rotation curve of spiral galaxies II: the dark matter distribution out to the virial radius

Abstract: In the current ACDM cosmological scenario, N-body simulations provide us with a universal mass profile, and consequently a universal equilibrium circular velocity of the virialized objects, as galaxies. In this paper we obtain, by combining kinematical data of their inner regions with global observational properties, the universal rotation curve of disc galaxies and the corresponding mass distribution out to their virial radius. This curve extends the results of Paper I, concerning the inner luminous regions of Sb-Im spirals, out to the edge of the galaxy haloes.

Standing waves of some coupled nonlinear Schrödinger equations

Abstract: We deal with a class of systems of nonlinear Schrodinger equations, proving the existence of bound and ground states provided the coupling parameter is small and large, respectively.

Nonlinear Analysis and Semilinear Elliptic Problems

Abstract: Preface 1. Preliminaries Part I. Topological Methods: 2. A primer on bifurcation theory 3. Topological degree, I 4. Topological degree, II: global properties Part II. Variational Methods, I: 5. Critical points: extrema 6. Constrained critical points 7. Deformations and the Palais-Smale condition 8. Saddle points and min-max methods Part III. Variational Methods, II: 9. Lusternik-Schnirelman theory 10. Critical points of even functionals on symmetric manifolds 11. Further results on Elliptic Dirichlet problems 12. Morse theory Part IV. Appendices: Appendix 1. Qualitative results Appendix 2. The concentration compactness principle Appendix 3. Bifurcation for problems on Rn Appendix 4. Vortex rings in an ideal fluid Appendix 5. Perturbation methods Appendix 6. Some problems arising in differential geometry Bibliography Index.

Chemical enrichment of galaxy clusters from hydrodynamical simulations

Abstract: We present cosmological hydrodynamical simulations of galaxy clusters aimed at studying the process of metal enrichment of the intra-cluster medium (ICM). These simulations have been performed by implementing a detailed model of chemical evolution in the TREE-PM+SPM GADGET-2 code. This model allows us to follow the metal release from Type II supernovae (SNII), Type Ia supernovae (SNIa) and asymptotic giant branch (AGB) stars by properly accounting for the lifetimes of stars of different mass, as well as to change the stellar initial mass function (IMF), the lifetime function and the stellar yields. As such, our implementation of chemical evolution represents a powerful instrument to follow the cosmic history of metal production. The simulations presented here have been performed with the twofold aim of checking numerical effects, as well as the impact of changing the model of chemical evolution and the efficiency of stellar feedback. In general, we find that the distribution of metals produced by SNII is more clumpy than for the product of low-mass stars, as a consequence of the different time-scales over which they are released. Using a standard Salpeter IMF produces a radial profile of iron abundance which is in fairly good agreement with observations available out to ≃0.6R 500 . This result holds almost independent of the numerical scheme adopted to distribute metals around star-forming regions. The mean age of enrichment of the ICM corresponds to redshift z ∼ 0.5, which progressively increases outside the virial region. Increasing resolution, we improve the description of a diffuse high-redshift enrichment of the inter-galactic medium (IGM). This turns into a progressively more efficient enrichment of the cluster outskirts, while having a smaller impact at R ≤ 0.5R 500 . As for the effect of the model of chemical evolution, we find that changing the IMF has the strongest impact. Using an IMF, which is top-heavier than the Salpeter one, provides a larger iron abundance, possibly in excess of the observed level, also significantly increasing the [O/Fe] relative abundance. Our simulations always show an excess of low-redshift star formation and, therefore, of the abundance of oxygen in central cluster regions, at variance with observations. This problem is not significantly ameliorated by increasing the efficiency of the stellar feedback.

On the dynamics of the spontaneous activity in neuronal networks.

Abstract: Most neuronal networks, even in the absence of external stimuli, produce spontaneous bursts of spikes separated by periods of reduced activity. The origin and functional role of these neuronal events are still unclear. The present work shows that the spontaneous activity of two very different networks, intact leech ganglia and dissociated cultures of rat hippocampal neurons, share several features. Indeed, in both networks: i) the inter-spike intervals distribution of the spontaneous firing of single neurons is either regular or periodic or bursting, with the fraction of bursting neurons depending on the network activity; ii) bursts of spontaneous spikes have the same broad distributions of size and duration; iii) the degree of correlated activity increases with the bin width, and the power spectrum of the network firing rate has a 1/f behavior at low frequencies, indicating the existence of long-range temporal correlations; iv) the activity of excitatory synaptic pathways mediated by NMDA receptors is necessary for the onset of the long-range correlations and for the presence of large bursts; v) blockage of inhibitory synaptic pathways mediated by GABAA receptors causes instead an increase in the correlation among neurons and leads to a burst distribution composed only of very small and very large bursts. These results suggest that the spontaneous electrical activity in neuronal networks with different architectures and functions can have very similar properties and common dynamics.

Planck-scale modified dispersion relations and Finsler geometry

Abstract: A common feature of all quantum gravity (QG) phenomenology approaches is to consider a modification of the mass-shell condition of the relativistic particle to take into account quantum gravitational effects. The framework for such approaches is therefore usually set up in the cotangent bundle (phase space). However it was recently proposed that this phenomenology could be associated with an energy dependent geometry that has been coined ``rainbow metric''. We show here that the latter actually corresponds to a Finsler geometry, the natural generalization of Riemannian geometry. We provide in this way a new and rigorous framework to study the geometrical structure possibly arising in the semiclassical regime of QG. We further investigate the symmetries in this new context and discuss their role in alternative scenarios like Lorentz violation in emergent spacetimes or deformed special relativity-like models.

REM observations of GRB 060418 and GRB 060607A: the onset of the afterglow and the initial fireball Lorentz factor determination

Abstract: Context. Gamma-ray burst (GRB) emission is believed to originate in highly relativistic fireballs. Aims. Currently, only lower limits were securely set to the initia l fireball Lorentz factor 0. We aim to provide a direct measure of 0. Methods. The early-time afterglow light curve carries information about 0, which determines the time of the afterglow peak. We have obtained early observations of the near-infrared afte rglows of GRB 060418 and GRB 060607A with the REM robotic telescope. Results. For both events, the afterglow peak could be clearly singled out, allowing a firm determination of the fireball Lorentz of 0∼ 400, fully confirming the highly relativistic nature of GRB fi reballs. The deceleration radius was inferred to be Rdec≈ 10 17 cm. This is much larger than the internal shocks radius (believed to power the prompt emission), thus providing further evidence for a different origin of the prompt and afterglow stages of the GRB.

Population III stars: hidden or disappeared?

Abstract: A Pop III/Pop II transition from massive to normal stars is predicted to occur whenthe metallicity of the star forming gas crosses the critical range Z cr = 10 −5±1 Z ⊙ . Toinvestigate the cosmic implications of such process we use numerical simulations whichfollow the evolution, metal enrichment and energy deposition of both Pop III and PopII stars. We find that: (i) due to inefficient heavy element transport by outflows andslow ”genetic” transmission during hierarchical growth, large fluctuations around theaverage metallicity arise; as a result Pop III star formation continues down to z = 2.5,but at a low peak rate of 10 −5 M ⊙ yr 1 Mpc −3 occurring at z ≈6 (about 10 −4 of thePop II one); (ii) Pop III star formation proceeds in a ”inside-out” mode in whichformation sites are progressively confined at the periphery of collapsed structures,where the low gas density and correspondingly long free-fall timescales result in a veryinefficient astration. These conclusions strongly encourage deep searches for pristinestar formation sites at moderate (2 < z < 5) redshifts where metal free stars are likelyto be hidden.Key words: galaxies: formation - cosmology: theory - cosmology: observations -intergalactic medium

Astrophysical and cosmological information from large-scale submillimetre surveys of extragalactic sources

Abstract: We present a quantitative analysis of the astrophysical and cosmological information that can be extracted from the many important wide-area, shallow surveys that will be carried out in the next few years. Our calculations combine the predictions of the physical model by Granato et al. for the formation and evolution of spheroidal galaxies with up-to-date phenomenological models for the evolution of starburst and normal late-type galaxies and of radio sources. We compute the expected number counts and the redshift distributions of these source populations separately and then focus on protospheroidal galaxies. For the latter objects, we predict the counts and redshift distributions of strongly lensed sources at 250, 350, 500 and 850 μm, the angular correlation function of sources detected in the surveys considered, and the angular power spectra due to clustering of sources below the detection limit in Herschel and Planck surveys. An optimal survey for selecting strongly lensed protospheroidal galaxies is described, and it is shown how they can be easily distinguished from the other source populations. We also discuss the detectability of the imprints of the one-halo and two-halo regimes on angular correlation functions and clustering power spectra, as well as the constraints on cosmological parameters that can be obtained from the determinations of these quantities. The novel data relevant to derive the first submillimetre estimates of the local luminosity functions of starburst and late-type galaxies, and the constraints on the properties of rare source populations, such as blazars, are also briefly described.

Shifts in coding properties and maintenance of information transmission during adaptation in barrel cortex.

Abstract: Neuronal responses to ongoing stimulation in many systems change over time, or “adapt.” Despite the ubiquity of adaptation, its effects on the stimulus information carried by neurons are often unknown. Here we examine how adaptation affects sensory coding in barrel cortex. We used spike-triggered covariance analysis of single-neuron responses to continuous, rapidly varying vibrissa motion stimuli, recorded in anesthetized rats. Changes in stimulus statistics induced spike rate adaptation over hundreds of milliseconds. Vibrissa motion encoding changed with adaptation as follows. In every neuron that showed rate adaptation, the input–output tuning function scaled with the changes in stimulus distribution, allowing the neurons to maintain the quantity of information conveyed about stimulus features. A single neuron that did not show rate adaptation also lacked input–output rescaling and did not maintain information across changes in stimulus statistics. Therefore, in barrel cortex, rate adaptation occurs on a slow timescale relative to the features driving spikes and is associated with gain rescaling matched to the stimulus distribution. Our results suggest that adaptation enhances tactile representations in primary somatosensory cortex, where they could directly influence perceptual decisions.

Metastable vacua and D-branes at the conifold

Abstract: We consider quiver gauge theories arising on D-branes at simple Calabi-Yau singularities (quotients of the conifold). These theories have metastable supersymmetry breaking vacua. The field theoretic mechanism is basically the one exhibited by the examples of Intriligator, Seiberg and Shih in SUSY QCD. In a dual description, the SUSY breaking is captured by the presence of anti-branes. In comparison to our earlier related work, the main improvements of the present construction are that we can reach the free magnetic range of the SUSY QCD theory where the existence of the metastable vacua is on firm footing, and we can see explicitly how the small masses for the quark flavors (necessary to the existence of the SUSY breaking vacua) are dynamically stabilized. One crucial mass term is generated by a stringy instanton. Finally, our models naturally incorporate R-symmetry breaking in the non-supersymmetric vacuum, in a way similar to the examples of Kitano, Ooguri and Ookouchi.

Gauge/gravity duality and meta-stable dynamical supersymmetry breaking

Abstract: We engineer a class of quiver gauge theories with several interesting features by studying D-branes at a simple Calabi-Yau singularity. At weak 't Hooft coupling we argue using field theory techniques that these theories admit both supersymmetric vacua and meta-stable non-supersymmetric vacua, though the arguments indicating the existence of the supersymmetry breaking states are not decisive. At strong 't Hooft coupling we find simple candidate gravity dual descriptions for both sets of vacua.

Ultraviolet properties of f(R)-Gravity

Abstract: We discuss the existence and properties of a nontrivial fixed point in f(R)-gravity, where f is a polynomial of order up to six. Within this seven-parameter class of theories, the fixed point has three ultraviolet-attractive and four ultraviolet-repulsive directions; this brings further support to the hypothesis that gravity is nonperturbatively renormalizabile.

NGC 3741: the dark halo profile from the most extended rotation curve

Abstract: We present new H i observations of the nearby dwarf galaxy NGC 3741. This galaxy has an extremely extended H i disc, which allows us to trace the rotation curve out to unprecedented distances in terms of the optical disc: we reach 42 B-band exponential scalelengths or about 7 kpc. The H i disc is strongly warped, but the warp is very symmetric. The distribution and kinematics are accurately derived by building model data cubes, which closely reproduce the observations. In order to account for the observed features in the data cube, radial motions of the order of 5–13 km s−1 are needed. They are consistent with an inner bar of several hundreds of pc and accretion of material in the outer regions. The observed rotation curve was decomposed into its stellar, gaseous and dark components. The Burkert dark halo (with a central constant density core) provides very good fits. The dark halo density distribution predicted by the Λ cold dark matter (CDM) theory fails to fit the data, unless NGC 3741 is a 2.5σ exception to the predicted relation between concentration parameter and virial mass and at the same time a high value of the virial mass (though poorly constrained) of 1011 M⊙. Noticeably, modified Newtonian dynamics (MOND) seems to be consistent with the observed rotation curve. Scaling up the contribution of the gaseous disc also gives a good fit.

Structure of a CH3S monolayer on Au(111) solved by the interplay between molecular dynamics calculations and diffraction measurements.

Abstract: We have investigated the controversy surrounding the $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{}$ structure of self-assembled monolayers of methylthiolate on Au(111) by first principles molecular dynamics simulations, energy and angle resolved photoelectron diffraction, and grazing incidence x-ray diffraction. Our simulations find a dynamic equilibrium between bridge site adsorption and a novel structure where 2 ${\mathrm{CH}}_{3}\mathrm{S}$ radicals are bound to an Au adatom that has been lifted from the gold substrate. As a result, the interface is characterized by a large atomic roughness with both adatoms and vacancies. This result is confirmed by extensive photoelectron and grazing incidence x-ray diffraction measurements.

Connecting low energy leptonic CP-violation to leptogenesis

Abstract: It was commonly thought that the observation of low energy leptonic $CP$-violating phases would not automatically imply the existence of a baryon asymmetry in the leptogenesis scenario. This conclusion does not generically hold when the issue of flavor is relevant and properly taken into account in leptogenesis. We illustrate this point with various examples studying the correlation between the baryon asymmetry and the $CP$-violating asymmetry in neutrino oscillations and the effective Majorana mass in neutrinoless double beta decay.

Neuronal Activity in Rat Barrel Cortex Underlying Texture Discrimination

Abstract: Rats and mice palpate objects with their whiskers to generate tactile sensations. This form of active sensing endows the animals with the capacity for fast and accurate texture discrimination. The present work is aimed at understanding the nature of the underlying cortical signals. We recorded neuronal activity from barrel cortex while rats used their whiskers to discriminate between rough and smooth textures. On whisker contact with either texture, firing rate increased by a factor of two to ten. Average firing rate was significantly higher for rough than for smooth textures, and we therefore propose firing rate as the fundamental coding mechanism. The rat, however, cannot take an average across trials, but must make an immediate decision using the signals generated on each trial. To estimate single-trial signals, we calculated the mutual information between stimulus and firing rate in the time window leading to the rat's observed choice. Activity during the last 75 ms before choice transmitted the most informative signal; in this window, neuronal clusters carried, on average, 0.03 bits of information about the stimulus on trials in which the rat's behavioral response was correct. To understand how cortical activity guides behavior, we examined responses in incorrect trials and found that, in contrast to correct trials, neuronal firing rate was higher for smooth than for rough textures. Analysis of high-speed films suggested that the inappropriate signal on incorrect trials was due, at least in part, to nonoptimal whisker contact. In conclusion, these data suggest that barrel cortex firing rate on each trial leads directly to the animal's judgment of texture.

Weak binding between two aromatic rings: feeling the van der Waals attraction by quantum Monte Carlo methods.

Abstract: We report a systematic study of the weak chemical bond between two benzene molecules. We first show that it is possible to obtain a very good description of the C(2) dimer and the benzene molecule by using pseudopotentials for the chemically inert 1s electrons and a resonating valence bond wave function as a variational ansatz, expanded on a relatively small Gaussian basis set. We employ an improved version of the stochastic reconfiguration technique to optimize the many-body wave function, which is the starting point for highly accurate simulations based on the lattice regularized diffusion Monte Carlo method. This projection technique provides a rigorous variational upper bound for the total energy, even in the presence of pseudopotentials, and substantially improves the accuracy of the trial wave function, which already yields a large fraction of the dynamical and nondynamical electron correlation. We show that the energy dispersion of two benzene molecules in the parallel displaced geometry is significantly deeper than the face-to-face configuration. However, contrary to previous studies based on post-Hartree-Fock methods, the binding energy remains weak ( approximately 2 kcal/mol) also in this geometry, and its value is in agreement with the most accurate and recent experimental findings [H. Krause et al., Chem. Phys. Lett. 184, 411 (1991)].

Triggering dynamics of the high-pressure benzene amorphization.

Abstract: Success in designing and tailoring solid-state reactions depends on the knowledge of the mechanisms regulating the reactivity at the microscopic level In spite of several attempts to rationalize the reactivity of crystals1,2,3,4,5, the question of the existence of a critical distance for a reaction to occur remains unsolved In this framework, the role of lattice phonons, which continuously tune the relative distance and orientation of the molecules, is still not fully understood Here, we show that at the onset of the transformation of crystalline benzene to an amorphous hydrogenated carbon the intermolecular C–C distance is always the same (about 26 A) once collective motions are taken into account, and it is independent of the pressure and temperature conditions This conclusion is supported by first-principles molecular-dynamics simulations This is a clear demonstration of the role of lattice phonons in driving the reactivity in the crystalline phase by fine-tuning of the nearest-neighbour distances The knowledge of the critical C–C distance can be crucial in planning solid-state reactions at moderate pressure

Stringy instantons at orbifold singularities

Abstract: We study the effects produced by D-brane instantons on the holomorphic quantities of a D-brane gauge theory at an orbifold singularity. These effects are not limited to reproducing the well known contributions of the gauge theory instantons but also generate extra terms in the superpotential or the prepotential. On these brane instantons there are some neutral fermionic zero-modes in addition to the ones expected from broken supertranslations. They are crucial in correctly reproducing effects which are dual to gauge theory instantons, but they may make some other interesting contributions vanish. We analyze how orientifold projections can remove these zero-modes and thus allow for new superpotential terms. These terms contribute to the dynamics of the effective gauge theory, for instance in the stabilization of runaway directions.

Analysis of rotation curves in the framework of Rn gravity

Abstract: We present an analysis of a devised sample of Rotation Curves (RCs), aimed atchecking the consequences of a modified f(R) gravity on galactic scales. Originallymotivated by the the dark energy mystery, this theory may serve as a possibility ofexplaining the observed non-Keplerian profiles of galactic RCs in terms of a break-down of the Einstein General Relativity. We show that in general the power-law f(R)version could fit well the observations with reasonable values for the mass modelparameters, encouraging further investigation on R n gravity from both observationaland theoretical points of view.Key words: gravitation – dark matter – galaxies: kinematics and dynamics 1 INTRODUCTIONIt is well-known that the RCs of spiral galaxies show a non-Keplerian circular velocity profile which cannot be explainedby considering a Newtonian gravitational potential gener-ated by the baryonic matter (Persic, Salucci & Stel 1996).Current possible explanation of this controversy includes,among others, the postulate of a new yet not detected stateof matter, the dark matter (Rubin 1983), a phenomeno-logical modification of the Newtonian dynamics (Milgrom1983; Brownstein & Moffat 2006; Sanders & McGaugh 2002;Bekenstein 2007), and higher order gravitational theories(originally devoted to solve the dark energy issue, see e.g.,(Carroll et al. 2004; Capozziello et al. 2004)).The recent theory proposed by Capozziello, Cardone &Troisi 2007 (hereafter CCT), modifies the usual Newtonianpotential generated by baryonic matter in such a way thatthe predicted galaxy kinematics and the observed one havea much better agreement. They consider power-law fourthorder⋆ theories of gravity obtained by replacing in the grav-ity action the Ricci scalar R with a function f(R) ∝ R

Weak binding between two aromatic rings: feeling the van der Waals attraction by quantum Monte Carlo methods

Abstract: We report a systematic study of the weak chemical bond between two benzene molecules. We first show that it is possible to obtain a very good description of the C_2 dimer and the benzene molecule, by using pseudopotentials for the chemically inert 1s electrons, and a resonating valence bond wave function as a variational ansatz, expanded on a relatively small Gaussian basis set. We employ an improved version of the stochastic reconfiguration technique to optimize the many-body wave function, which is the starting point for highly accurate simulations based on the lattice regularized diffusion Monte Carlo (LRDMC) method. This projection technique provides a rigorous variational upper bound for the total energy, even in the presence of pseudopotentials, and allows to improve systematically the accuracy of the trial wave function, which already yields a large fraction of the dynamical and non-dynamical electron correlation. We show that the energy dispersion of two benzene molecules in the parallel displaced geometry is significantly deeper than the face-to-face configuration. However, contrary to previous studies based on post Hartree-Fock methods, the binding energy remains weak (~ 2 kcal/mol) also in this geometry, and its value is in agreement with the most accurate and recent experimental findings.

Mott-insulating and glassy phases of polaritons in 1D arrays of coupled cavities.

Abstract: By means of analytical and numerical methods we analyze the phase diagram of polaritons in one-dimensional coupled cavities. We locate the phase boundary, discuss the behavior of the polariton compressibility and visibility fringes across the critical point, and find a nontrivial scaling of the phase boundary as a function of the number of atoms inside each cavity. We also predict the emergence of a polaritonic glassy phase when the number of atoms fluctuates from cavity to cavity.