arXiv subjects

D. Grasso

Publications and source records attributed to D. Grasso.

At least 37 records · Page 2Linked to original sources

arXiv2014

Numerical comparison between a Gyrofluid and Gyrokinetic model investigating collisionless magnetic reconnection

The first detailed comparison between gyrokinetic and gyrofluid simulations of collisionless magnetic reconnection has been carried out. Both the linear and nonlinear evolution of the collisionless tearing mode have been analyzed. In the linear regime, we have found a good agreement between the two approaches over the whole spectrum of linearly unstable wave numbers, both in the drift kinetic limit and for finite ion temperature. Nonlinearly, focusing on the small-$Δ'$ regime, with $Δ'$ indicating the standard tearing stability parameter, we have compared relevant observables such as the evolution and saturation of the island width, as well as the island oscillation frequency in the saturated phase.The results are basically the same, with small discrepancies only in the value of the saturated island width for moderately high values of $Δ'$. Therefore, in the regimes investigated here, the gyrofluid approach can describe the collisionless reconnection process as well as the more complete gyrokinetic model.

physics.plasm-ph↗

arXiv2014

The deceiving delta: on the equilibrium dependent dynamics of nonlinear magnetic islands

The linear stability parameter delta is commonly used as a figure of merit for the nonlinear dynamics of the tearing mode. It is shown, through state of the art numerical simulations, that factors other than delta can play a very important role in determining the evolution of nonlinear magnetic islands. In particular, two different equilibria are analysed and it is shown that, once perturbed, they have a qualitatively and quantitatively different response despite the fact that they are characterised by the same delta. The different behaviour can still be associated with linear properties of the equilibrium. It is also studied how the nonlinear and saturation phase are affected by an increasing delta in the two equilibria. As the instability drive is increased, the systems move from a dynamics characterised by a "universal" generalised Rutherford equation to a Y-point configuration and then to a plasmoid unstable Y-point. Finally, for even larger delta the second harmonic overcomes the fundamental,leading to an interesting double island structure.

physics.plasm-ph↗

arXiv2013

Galactic electron and positron properties from cosmic ray and radio observations

We perform a consistent modeling of cosmic ray electrons, positrons and of the radio emission of the Galaxy. For the time we reproduce all relevant data sets between 1 GeV and 1 TeV including the recent AMS-02 positron fraction results. We show that below few GeV cosmic ray and radio data require that electron primary spectrum to be drastically suppressed and the propagated spectrum be dominated by secondary particles. Above 10 GeV an electron + positron extra-component with a hard spectrum is required. The positron spectrum measured below few GeV is consistently reproduced only within low reacceleration models. We also constrain the scale-height of the cosmic-ray distribution showing that a thin halo ($z_t \lsim 2 \kpc$) is excluded.

astro-ph.HE↗

arXiv2012

Les Houches 2011: Physics at TeV Colliders New Physics Working Group Report

We present the activities of the "New Physics" working group for the "Physics at TeV Colliders" workshop (Les Houches, France, 30 May-17 June, 2011). Our report includes new agreements on formats for interfaces between computational tools, new tool developments, important signatures for searches at the LHC, recommendations for presentation of LHC search results, as well as additional phenomenological studies.

hep-ph↗

arXiv2011

Gyrofluid simulations of collisionless reconnection in the presence of diamagnetic effects

The effects of the ion Larmor radius on magnetic reconnection are investigated by means of numerical simulations, with a Hamiltonian gyrofluid model. In the linear regime, it is found that ion diamagnetic effects decrease the growth rate of the dominant mode. Increasing ion temperature tends to make the magnetic islands propagate in the ion diamagnetic drift direction. In the nonlinear regime, diamagnetic effects reduce the final width of the island. Unlike the electron density, the guiding center density does not tend to distribute along separatrices and at high ion temperature, the electrostatic potential exhibits the superposition of a small scale structure, related to the electron density, and a large scale structure, related to the ion guiding-center density.

physics.plasm-ph↗

arXiv2009

A Combined Interpretation of CR Nuclei and Antiproton High Energy Measurements

In the last months several ballon and satellite experiments improved significantly our knowledge of cosmic rays (CR) spectra at high energy. In particular CREAM allowed to measure B/C, C/O and N/O up to 1 TeV and PAMELA the anti-p/p ratio up to 100 GeV with unprecedented accuracy. These measurements offer a valuable probe of CR propagation properties. We performed a statistical analysis to test the compatibility of these results, as well as other most significant experimental data, with the predictions of a new numerical CR diffusion package (DRAGON). We found that above 1 GeV all data are consistent with a diffusion scenario in a well defined range of values of the diffusion coefficient energy power index and normalization.

astro-ph.HE↗

arXiv2009

On possible interpretations of the high energy electron-positron spectrum measured by the Fermi Large Area Telescope

The Fermi-LAT experiment recently reported high precision measurements of the spectrum of cosmic-ray electrons-plus-positrons (CRE) between 20 GeV and 1 TeV. The spectrum shows no prominent spectral features, and is significantly harder than that inferred from several previous experiments. Here we discuss several interpretations of the Fermi results based either on a single large scale Galactic CRE component or by invoking additional electron-positron primary sources, e.g. nearby pulsars or particle Dark Matter annihilation. We show that while the reported Fermi-LAT data alone can be interpreted in terms of a single component scenario, when combined with other complementary experimental results, specifically the CRE spectrum measured by H.E.S.S. and especially the positron fraction reported by PAMELA between 1 and 100 GeV, that class of models fails to provide a consistent interpretation. Rather, we find that several combinations of parameters, involving both the pulsar and dark matter scenarios, allow a consistent description of those results. We also briefly discuss the possibility of discriminating between the pulsar and dark matter interpretations by looking for a possible anisotropy in the CRE flux.

astro-ph.HE↗

arXiv2008

Hamiltonian formulation and analysis of a collisionless fluid reconnection model

The Hamiltonian formulation of a plasma four-field fluid model that describes collisionless reconnection is presented. The formulation is noncanonical with a corresponding Lie-Poisson bracket. The bracket is used to obtain new independent families of invariants, so-called Casimir invariants, three of which are directly related to Lagrangian invariants of the system. The Casimirs are used to obtain a variational principle for equilibrium equations that generalize the Grad-Shafranov equation to include flow. Dipole and homogeneous equilibria are constructed. The linear dynamics of the latter is treated in detail in a Hamiltonian context: canonically conjugate variables are obtained; the dispersion relation is analyzed and exact thresholds for spectral stability are obtained; the canonical transformation to normal form is described; an unambiguous definition of negative energy modes is given; and thresholds sufficient for energy-Casimir stability are obtained. The Hamiltonian formulation also is used to obtain an expression for the collisionless conductivity and it is further used to describe the linear growth and nonlinear saturation of the collisionless tearing mode.

physics.plasm-ph↗

arXiv2007

Science with the new generation high energy gamma- ray experiments

This Conference is the fifth of a series of Workshops on High Energy Gamma- ray Experiments, following the Conferences held in Perugia 2003, Bari 2004, Cividale del Friuli 2005, Elba Island 2006. This year the focus was on the use of gamma-ray to study the Dark Matter component of the Universe, the origin and propagation of Cosmic Rays, Extra Large Spatial Dimensions and Tests of Lorentz Invariance.

astro-ph↗

arXiv2007

Gamma-ray and neutrino diffuse emissions of the Galaxy above the TeV

In this contribution we will discuss recent results concerning the intensity and the angular distribution of the gamma-ray and neutrino emissions as should be originated from the hadronic scattering of cosmic rays (CR) with the interstellar medium (ISM). We assumed that CR sources are supernova remnants (SNR) and estimated the spatial distribution of primary nuclei by solving numerically the diffusion equation. For the ISM, we considered recent models for the 3D spatial distributions of molecular hydrogen. Respect to previous results, we find the secondary gamma-ray and neutrino emissions to be more peaked along the galactic equator and in the galactic centre which improves significantly the perspectives of a positive detection. We compare our predictions with the experimental limits/observations by MILAGRO and TIBET (for the gamma-rays) and by AMANDA-II (for the neutrinos) and discuss the detection perspectives for a km3 neutrino telescope to be built in the North hemisphere.

astro-ph↗

arXiv2005

The SU(N) Matrix Model at Two Loops

Multi-loop calculations of the effective action for the matrix model are important for carrying out tests of the conjectured relationship of the matrix model to the low energy description of M-theory. In particular, comparison with N-graviton scattering amplitudes in eleven-dimensional supergravity requires the calculation of the effective action for the matrix model with gauge group SU(N). A framework for carrying out such calculations at two loops is established in this paper. The two-loop effective action is explicitly computed for a background corresponding to the scattering of a single D0-brane from a stack of N-1 D0-branes, and the results are shown to agree with known results in the case N=2.

hep-th↗

arXiv2004

Diffusion of Ultra High Energy Protons in Galaxy Clusters and Secondary X and Gamma Ray Emissions

In this work we simulate the propagation of Ultra High Energy (UHE) protons in the magnetised intergalactic medium of Galaxy Clusters (GCs). Differently from previous works on the subject, we trace proton trajectories in configurations of the Intra Cluster Magnetic Field (ICMF) which have been extracted from a constrained Magnetic-SPH simulation of the local universe. Such an approach allows us to take into account the effects of several features of the ICMFs, e.g. irregular geometrical structure and field fluctuations due to merger shocks,which cannot be investigated analitically or with usual numerical simulations. Furthermore, we are able to simulate a set of clusters which have properties quite similar to those of GCs observed in the nearby universe. We estimate the time that UHE protons take to get out of the clusters and found that in the energy range $5\times 10^{18} \simleq E \simleq 3 \times 10^{19} \eV$ proton propagation takes place in the Bohm scattering diffusion regime passing smoothly to a small pitch angle diffusion regime at larger energies. We apply our results to estimate the secondary gamma and Hard X Ray (HXR) emissions produced by UHE protons in a rich GC. We show that the main emission channel is due to the synchrotron HXR radiation of secondary electrons originated by proton photo-pair production scattering onto the CMB. This process may give rise to a detectable signal if a relatively powerful AGN, or a dead quasar, accelerating protons at UHEs is harboured by a rich GC in the local universe.

astro-ph↗

arXiv2003

Constrained Simulations of the Magnetic Field in the Local Supercluster and the Propagation of UHECR

Magnetic fields (MF) in the Local Supercluster (LSC) of galaxies may have profound consequences for the propagation of Ultra High Energy Cosmic Rays (UHECR). Faraday rotations measurements provide some informations about MF in compact clusters. However, very few is known about less dense regions and about the global structure of MF in the LSC. In order to get a better knowledge of these fields we are performing constrained magnetohydrodynamical simulations of the LSC magnetic field. We will present the results of our simulation and discuss their implications for the angular distribution of expected UHECR deflections.

astro-ph↗

arXiv2001

Generation of Cosmic Magnetic Fields and Gravitational Waves at Neutrino Decoupling

We show that an inhomogeneous cosmological lepton number may have produced turbulence in the primordial plasma when neutrinos entered (almost) free-streaming regime. This effect may be responsible for the origin of cosmic magnetic fields and give rise to a detectable background of gravitational waves. An existence of inhomogeneous lepton asymmetry could be naturally generated by active-sterile neutrino oscillations or by some versions of Affleck and Dine baryogenesis scenario.

astro-ph↗

arXiv1999

The Sphaleron in a Magnetic Field and Electroweak Baryogenesis

The presence of a primordial magnetic field in the early universe affects the dynamic of the electroweak phase transition enhancing its strength. This effect may enlarge the window for electroweak baryogenesis in the minimal supersymmetric extension of the standard model or even resurrect the electroweak baryogenesis scenario in the standard model. We compute the sphaleron energy in the background of the magnetic field and show that, due to the sphaleron dipole moment, the barrier between topologically inequivalent vacua is lowered. Therefore, the preservation of the baryon asymmetry calls for a much stronger phase transition than required in the absence of a magnetic field. We show that this effect overwhelms the gain in the phase transition strength, and conclude that magnetic fields do not help electroweak baryogenesis.

hep-ph↗

arXiv1998

Pulsar Velocities without Neutrino Mass

We show that pulsar velocities may arise from anisotropic neutrino emission induced by resonant conversions of massless neutrinos in the presence of a strong magnetic field. The main ingredient is a small violation of weak universality and neither neutrino masses nor magnetic moments are required.

astro-ph↗

arXiv1996

Revisiting Nucleosynthesis Constraints on Primordial Magnetic Fields

In view of several conflicting results, we reanalyze the effects of magnetic fields on the primordial nucleosynthesis. In the case the magnetic field is homogeneous over a horizon volume, we show that the main effects of the magnetic field are given by the contribution of its energy density to the Universe expansion rate and the effect of the field on the electrons quantum statistics. Although, in order to get an upper limit on the field strength, the weight of the former effect is numerically larger, the latter cannot be neglected. Including both effects in the PN code we get the upper limit $B \le 1\times 10^{11}$ Gauss at the temperature $T = 10^9~^oK$. We generalize the considerations to cases when instead the magnetic is inhomogeneous on the horizon length. We show that in these cases only the effect of the magnetic field on the electrons statistics is relevant. If the coherence length of the magnetic field at the end of the PN is in the range $10 \ll L_0 \ll 10^{11}$ cm our upper limit is $B \le 1\times 10^{12}$ Gauss.

astro-ph↗

arXiv1994

Limits on possible magnetic fields at nucleosynthesis time

In this paper we discuss limits on magnetic fields that could have been present at nucleosynthesis time. We considered several effects that could be relevant modifing light elements relic abundances. They include: changes in reaction rates, mass shifts due to strong and electromagnetic interactions, variation of the expansion rate of the Universe due to both the magnetic field energy density and the increasing of the electrons density in overcritical magnetic fields. We find that the latter is the main effect. It was not taken into account in previous calculations. The allowed field intensity at the end of nucleosynthesis ($T =1\times 10^9\;^o{\rm K}$) is $B \le 3\times 10^{10}$ Gauss.

astro-ph↗