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R. Aloisio

Publications and source records attributed to R. Aloisio.

At least 73 records · Page 4Linked to original sources

Super Heavy Dark Matter in light of BICEP2, Planck and Ultra High Energy Cosmic Rays Observations

The announcement by BICEP2 of the detection of B-mode polarization consistent with primordial gravitational waves with a tensor-to-scalar ratio, $r=0.2^{+0.07}_{-0.05}$, challenged predictions from most inflationary models of a lower value for $r$. More recent results by Planck on polarized dust emission show that the observed tensor modes signal is compatible with pure foreground emission. A more significant constraint on $r$ was then obtained by a joint analysis of Planck, BICEP2 and Keck Array data showing an upper limit to the tensor to scalar ratio $r\le 0.12$, excluding the case $r=0$ with low statistical significance. Forthcoming measurements by BICEP3, the Keck Array, and other CMB polarization experiments, open the possibility for making the fundamental measurement of $r$. Here we discuss how $r$ sets the scale for models where the dark matter is created at the inflationary epoch, the generically called super-heavy dark matter models. We also consider the constraints on such scenarios given by recent data from ultrahigh energy cosmic ray observatories which set the limit on super-heavy dark matter particles lifetime. We discuss how super-heavy dark matter can be discovered by a precise measurement of $r$ combined with future observations of ultra high energy cosmic rays.

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Propagation of Ultra High Energy Cosmic Rays and the Production of Cosmogenic Neutrinos

We present an updated version of the {\it SimProp} Monte Carlo code to study the propagation of ultra high energy cosmic rays in astrophysical backgrounds computing the cosmogenic neutrino fluxes expected on earth. The study of secondary neutrinos provides a powerful tool to constrain the source models of these extremely energetic particles. We will show how the newly detected IceCube neutrino events at PeV energies together with the the latest experimental results of the Pierre Auger Observatory and Telescope Array experiment are almost at the level of excluding several hypothesis on the astrophysical sources of ultra high energy cosmic rays. Results presented here can be also used to evaluate the discovery capabilities of future high energy cosmic rays and neutrino detectors.

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SimProp v2r2: a Monte Carlo simulation to compute cosmogenic neutrino fluxes

We present an updated version of the SimProp Monte Carlo code: a simulation scheme to study the propagation of ultra-high-energy cosmic rays through diffuse extragalactic background radiation. The new version of the code presents two important updates: (i) it treats in a full stochastic approach all interaction channels involving ultra-high-energy cosmic rays and (ii) it takes into account the production of secondary (cosmogenic) neutrinos. This new version of SimProp was tested against different simulations code, in particular the production of secondary neutrinos was compared with the fluxes expected in the scenario of Engel, Seckel and Stanev.

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Ultra high energy cosmic rays: implications of Auger data for source spectra and chemical composition

We use a kinetic-equation approach to describe the propagation of ultra high energy cosmic ray protons and nuclei and calculate the expected spectra and mass composition at the Earth for different assumptions on the source injection spectra and chemical abundances. When compared with the spectrum, the elongation rate $X_{max}(E)$ and dispersion $σ(X_{max})$ as observed with the Pierre Auger Observatory, several important consequences can be drawn: a) the injection spectra of nuclei must be very hard, $\sim E^{-γ}$ with $γ\sim 1-1.6$; b) the maximum energy of nuclei of charge $Z$ in the sources must be $\sim 5Z\times 10^{18}$ eV, thereby not requiring acceleration to extremely high energies; c) the fit to the Auger spectrum can be obtained only at the price of adding an {\it ad hoc} light extragalactic component with a steep injection spectrum ($\sim E^{-2.7}$). In this sense, at the ankle ($E_{A}\approx 5\times 10^{18}$ eV) all the components are of extragalactic origin, thereby suggesting that the transition from Galactic to extragalactic cosmic rays occurs below the ankle. Interestingly, the additional light extragalactic component postulated above compares well, in terms of spectrum and normalization, with the one recently measured by KASCADE-Grande.

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Self-generated magnetic turbulence and the propagation of galactic cosmic rays

Cosmic rays propagating in the Galaxy may excite a streaming instability when their motion is super-alfvenic, thereby producing the conditions for their own diffusion. We present the results of a self-consistent solution of the transport equation where diffusion occurs because of the self-generated turbulence together with a preexisting turbulence injected, for instance, by supernova explosions and cascading to smaller scales. All chemicals are included in our calculations, so that we are able to show the secondary to primary ratios in addition to the spectra of the individual elements. All predictions appear to be in good agreement with observations.

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Propagation of galactic cosmic rays in the presence of self-generated turbulence

Cosmic rays propagating in the Galaxy may excite a streaming instability when their motion is super-alfvenic, thereby producing the conditions for their own diffusion. In this paper we present the results of a self-consistent solution of the transport equation where diffusion occurs because of the self-generated turbulence together with a pre-existing turbulence injected, for instance, by supernova explosions and cascading to smaller scales. All chemicals are included in our calculations, so that we are able to show the secondary to primary ratios in addition to the spectra of the individual elements. All predictions appear to be in good agreement with observations. The fact that data are explained with no need for artificial breaks in the injection spectrum and/or in the diffusion coefficient as functions of momentum can be interpreted as a strong indication that the phenomenon proposed here is in fact being observed in Nature. We also show that there is very good agreement between the calculated proton spectrum and the cosmic ray spectrum inferred from observations of the gamma ray emission from clouds in the Gould's belt.

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Propagation of UHECRs in cosmological backgrounds: some results from SimProp

Ultra-High-Energy Cosmic Ray (UHECR) nuclei propagating in cosmological radiation backgrounds produce secondary particles detectable at Earth. SimProp is a one dimensional code for extragalactic propagation of UHECR nuclei, inspired by the kinetic approach of Aloisio et al. As in this approach, only a subset of nuclei and nuclear channels are used as representative. We discuss the validation of the code and present applications to UHECR experimental results. In particular we present the expected fluxes of neutrinos produced in some astrophysical scenario.

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Ultra High Energy Cosmic Rays: A Short Review

We will review the main physical aspects of Ultra High Energy Cosmic Rays. We will discuss in particular their propagation through astrophysical backgrounds, focusing on the latest experimental observations of HiRes, Telescope Array and Auger. We will also review the issue of the transition between galactic and extra-galactic cosmic rays.

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Transition from galactic to extragalactic cosmic rays

The study of the transition between galactic and extragalactic cosmic rays can shed more light on the end of the Galactic cosmic rays spectrum and the beginning of the extragalactic one. Three models of transition are discussed: ankle, dip and mixed composition models. All these models describe the transition as an intersection of a steep galactic component with a flat extragalactic one. Severe bounds on these models are provided by the Standard Model of Galactic Cosmic Rays according to which the maximum acceleration energy for Iron nuclei is of the order of $E_{\rm Fe}^{\rm max} \approx 1\times 10^{17}$ eV. In the ankle model the transition is assumed at the ankle, a flat feature in the all particle spectrum which observationally starts at energy $E_a \sim (3 - 4)\times 10^{18}$ eV. This model needs a new high energy galactic component with maximum energy about two orders of magnitude above that of the Standard Model. The origin of such component is discussed. As observations are concerned there are two signatures of the transition: change of energy spectra and mass composition. In all models a heavy galactic component is changed at the transition to a lighter or proton component.

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Computational Schemes for the Propagation of Ultra High Energy Cosmic Rays

We discuss the problem of ultra high energy particles propagation in astrophysical backgrounds. We present two different computational schemes based on both kinetic and Monte Carlo approaches. The kinetic approach is an analytical computation scheme based on the hypothesis of continuos energy losses while the Monte Carlo scheme takes into account also the stochastic nature of particle interactions. These schemes, that give quite reliable results, enable the computation of fluxes keeping track of the different primary and secondary components, providing a fast and useful workbench to study Ultra High Energy Cosmic Rays.

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SimProp: a Simulation Code for Ultra High Energy Cosmic Ray Propagation

A new Monte Carlo simulation code for the propagation of Ultra High Energy Cosmic Rays is presented. The results of this simulation scheme are tested by comparison with results of another Monte Carlo computation as well as with the results obtained by directly solving the kinetic equation for the propagation of Ultra High Energy Cosmic Rays. A short comparison with the latest flux published by the Pierre Auger collaboration is also presented.

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Analytic calculations of the spectra of ultra high energy cosmic ray nuclei. II. The general case of background radiation

We discuss the problem of ultra high energy nuclei propagation in extragalactic background radiations. The present paper is the continuation of the accompanying paper I where we have presented three new analytic methods to calculate the fluxes and spectra of ultra high energy cosmic ray nuclei, both primary and secondary, and secondary protons. The computation scheme in this paper is based on the analytic solution of coupled kinetic equations, which takes into account the continuous energy losses due to the expansion of the universe and pair-production, together with photo-disintegration of nuclei. This method includes in the most natural way the production of secondary nuclei in the process of photo-disintegration of the primary nuclei during their propagation through extragalactic background radiations. In paper I, in order to present the suggested analytical schemes of calculations, we have considered only the case of the cosmic microwave background radiation, in the present paper we generalize this computation to all relevant background radiations, including infra-red and visible/ultra-violet radiations, collectively referred to as extragalactic background light. The analytic solutions allow transparent physical interpretation of the obtained spectra. Extragalactic background light plays an important role at intermediate energies of ultra high energy cosmic ray nuclei. The most noticeable effect of the extragalactic background light is the low-energy tail in the spectrum of secondary nuclei.

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Analytic calculations of the spectra of ultra-high energy cosmic ray nuclei. I. The case of CMB radiation

We present a systematic study of different methods for the analytic calculation of ultra-high energy nuclei diffuse spectra. Nuclei propagating in the intergalactic space are photo-disintegrated and decrease their Lorentz factor due to the interaction with cosmic microwave background and extragalactic background light. We calculate the evolution trajectories in the backward time, that describe how atomic mass number $A$ and Lorentz factor $Γ$ change with redshift $z$. Three methods of spectra calculations are investigated and compared: {\it (i)} trajectory method, {\it(ii)} kinetic equation combined with trajectory calculations and {\it (iii)} coupled kinetic equations. We believe that these three methods exhaust at least the principal possibilities for any analytic solution of the problem. In the most straightforward method {\it(i)} only trajectory calculations are used to connect the observed nuclei flux with the production rate of primary (accelerated) nuclei $A_0$. In the second method {\it (ii)} the flux (space density) of primary nuclei, and secondary nuclei and protons are calculated with the help of kinetic equation and trajectories are used only to determine the generation rates of these nuclei. The third method {\it (iii)} consists in solving the complete set of coupled kinetic equations, written starting with primary nuclei $A_0$, then for $A_0-1$ etc down to the $A$ of interest. The solution of the preceding equation gives the generation rate for the one which follows. An important element of the calculations for all methods is the systematic use of Lorentz factor instead of energy. We consider here the interaction of nuclei only with the cosmic microwave background, this case is particularly suitable for understanding the physical results.

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Ultra High Energy Cosmic Rays: Anisotropies and Spectrum

The recent results of the Pierre Auger Observatory on the possible correlation of Ultra High Energy Cosmic Rays events and several nearby discrete sources could be the starting point of a new era with charged particles astronomy. In this paper we introduce a simple model to determine the effects of any local distribution of sources on the expected flux. We consider two populations of sources: faraway sources uniformly distributed and local point sources. We study the effects on the expected flux of the local distribution of sources, referring also to the set of astrophysical objects whose correlation with the Auger events is experimentally claimed.

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Disappointing model for ultrahigh-energy cosmic rays

Data of Pierre Auger Observatory show a proton-dominated chemical composition of ultrahigh-energy cosmic rays spectrum at (1 - 3) EeV and a steadily heavier composition with energy increasing. In order to explain this feature we assume that (1 - 3) EeV protons are extragalactic and derive their maximum acceleration energy, E_p^{max} \simeq 4 EeV, compatible with both the spectrum and the composition. We also assume the rigidity-dependent acceleration mechanism of heavier nuclei, E_A^{max} = Z x E_p^{max}. The proposed model has rather disappointing consequences: i) no pion photo-production on CMB photons in extragalactic space and hence ii) no high-energy cosmogenic neutrino fluxes; iii) no GZK-cutoff in the spectrum; iv) no correlation with nearby sources due to nuclei deflection in the galactic magnetic fields up to highest energies.

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Ultra High Energy Cosmic Rays: The disappointing model

We develop a model for explaining the data of Pierre Auger Observatory (Auger) for Ultra High Energy Cosmic Rays (UHECR), in particular, the mass composition being steadily heavier with increasing energy from 3 EeV to 35 EeV. The model is based on the proton-dominated composition in the energy range (1 - 3) EeV observed in both Auger and HiRes experiments. Assuming extragalactic origin of this component, we argue that it must disappear at higher energies due to a low maximum energy of acceleration, E_p^{\max} \sim (4 - 10) EeV. Under an assumption of rigidity acceleration mechanism, the maximum acceleration energy for a nucleus with the charge number Z is ZE_p^{\max}, and the highest energy in the spectrum, reached by Iron, does not exceed (100 - 200) EeV. The growth of atomic weight with energy, observed in Auger, is provided by the rigidity mechanism of acceleration, since at each energy E=ZE_p^{\max} the contribution of nuclei with Z' < Z vanishes. The described model has disappointing consequences for future observations in UHECR: Since average energies per nucleon for all nuclei are less than (2 - 4) EeV, (i) pion photo-production on CMB photons in extragalactic space is absent; (ii) GZK cutoff in the spectrum does not exist; (iii) cosmogenic neutrinos produced on CMBR are absent; (iv) fluxes of cosmogenic neutrinos produced on infrared - optical background radiation are too low for registration by existing detectors and projects. Due to nuclei deflection in galactic magnetic fields, the correlation with nearby sources is absent even at highest energies.

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Superluminal problem in diffusion of relativistic particles and its phenomenological solution

We discuss the superluminal problem in the diffusion of ultra high energy protons with energy losses taken into account. The phenomenological solution of this problem is found with help of the generalized Jüttner propagator, originally proposed for relativization of the Maxwellian gas distribution. It is demonstrated that the generalized Jüttner propagator gives the correct expressions in the limits of diffusive and rectilinear propagation of the charged particles in the magnetic fields, together with the intermediate regime, in all cases without superluminal velocities. This solution, very general for the diffusion, is considered for two particular cases: diffusion inside the stationary objects, like e.g. galaxies, clusters of galaxies etc, and for expanding universe. The comparison with the previously obtained solutions for propagation of UHE protons in magnetic fields is performed.

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