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R. Aloisio

Publications and source records attributed to R. Aloisio.

At least 91 records · Page 5Linked to original sources
arXiv2008

Secondary protons from ultra high energy cosmic ray nuclei: is the Greisen-Zatsepin-Kuzmin cutoff unavoidable?

We discuss the production of ultra high energy secondary protons by cosmic ray primary nuclei propagating in the intergalactic space through Cosmic Microwave Background (CMB) and Infrared (IR) radiations. Under the assumption that only primary nuclei with a fixed atomic mass number $A_0$ are accelerated, the spectrum of secondary protons is calculated. It is found that for all $A_0$ the diffuse flux of secondary protons starts to dominate over that of primary nuclei at energy $E \sim (1 - 2)\times 10^{19}$ eV, and thus the standard Greisen-Zatsepin -Kuzmin (GZK) cutoff is produced.

astro-ph↗
arXiv2008

Super Heavy Dark Matter and UHECR Anisotropy at Low Energy

Super Heavy quasi-stable particles are naturally produced in the early universe and could represent a substantial fraction of the Dark Matter: the so-called Super Heavy Dark Matter (SHDM). The decay of SHDM represents also a possible source of Ultra High Energy Cosmic Rays (UHECR), with a reliably calculated spectrum of the particles produced in the decay $(\propto E^{-1.9})$. The SHDM model for the production of UHECR can explain quantitatively only the excess of UHE events observed by AGASA. In the case of an observed spectrum not showing the AGASA excess the SHDM model can provide only a {\it subdominant} contribution to the UHECR flux. We discuss here the basic features of SHDM for the production of a {\it subdominant} UHECR flux, we refer our study to the possible signatures of the model at the Auger observatory discussing in particular the expected chemical composition and anisotropy.

astro-ph↗
arXiv2008

Detection of GRB signals with Fluorescence Detectors

Gamma Ray Bursts are being searched in many ground based experiments detecting the high energy component (GeV $÷$ TeV energy range) of the photon bursts. In this paper, Fluorescence Detectors are considered as possible candidate devices for these searches. It is shown that the GRB photons induce fluorescence emission of UV photons on a wide range of their spectrum. The induced fluorescence flux is dominated by GRB photons from 0.1 to about 100 MeV and, once the extinction through the atmosphere is taken into account, it is distributed over a wide angular region. This flux can be detected through a monitor of the diffuse photon flux, provided that its maximum value exceeds a threshold value, that is primarily determined by the sky brightness above the detector. The feasibility of this search and the expected rates are discussed on the basis of the current GRB observations and the existing fluorescence detectors.

astro-ph↗
arXiv2007

Signatures of the transition from galactic to extragalactic cosmic rays

We discuss the signatures of the transition from galactic to extragalactic cosmic rays in different scenarios, giving most attention to the dip scenario. The dip is a feature in the diffuse spectrum of ultra-high energy (UHE) protons in the energy range $1\times 10^{18} - 4\times 10^{19}$ eV, which is caused by electron-positron pair production on the cosmic microwave background (CMB) radiation. The dip scenario provides a simple physical description of the transition from galactic to extragalactic cosmic rays. Here we summarize the signatures of the pair production dip model for the transition, most notably the spectrum, the anisotropy and the chemical composition. The main focus of our work is however on the description of the features that arise in the elongation rate and in the distribution of the depths of shower maximum $X_{\rm max}$ in the dip scenario. We find that the curve for $X_{\max}(E)$ shows a sharp increase with energy, which reflects a sharp transition from an iron dominated flux at low energies to a proton dominated flux at $E\sim 10^{18}$ eV. We also discuss in detail the shape of the $X_{\max}$ distributions for cosmic rays of given energy and demonstrate that this represents a powerful tool to discriminate between the dip scenario and other possible models of the transition.

astro-ph↗
arXiv2007

Ultra High Energy Cosmic Rays Diffusion in an Expanding Universe

We study the solution of the diffusion equation for Ultra-High Energy Cosmic Rays in the general case of an expanding universe, comparing it with the well known Syrovatsky solution obtained in the more restrictive case of a static universe. The formal comparison of the two solutions with all parameters being fixed identically reveals an appreciable discrepancy. This discrepancy is less important if in both models a different set of best-fit parameters is used.

astro-ph↗
arXiv2006

A dip in the UHECR spectrum and the transition from galactic to extragalactic cosmic rays

The dip is a feature in the diffuse spectrum of ultra-high energy (UHE) protons caused by electron-positron pair production on the cosmic microwave background (CMB) radiation. For a power-law generation spectrum $E^{-2.7}$, the calculated position and shape of the dip is confirmed with high accuracy by the spectra observed by the Akeno-AGASA, HiRes, Yakutsk and Fly's Eye detectors. When the particle energies, measured in these detectors, are calibrated by the dip, their fluxes agree with a remarkable accuracy. The predicted shape of the dip is quite robust. The dip is only modified strongly when the fraction of nuclei heavier than protons is high at injection, which imposes some restrictions on the mechanisms of acceleration operating in UHECR sources. The existence of the dip, confirmed by observations, implies that the transition from galactic to extragalactic cosmic rays occurs at $E \lsim 1\times 10^{18}$ eV. We show that at energies lower than a characteristic value $E_{\rm cr}\approx 1\times 10^{18}$ eV, the spectrum of extragalactic cosmic rays flattens in all cases of interest, and it provides a natural transition to a steeper galactic cosmic ray spectrum. This transition occurs at some energy below $E_{\rm cr}$, corresponding to the position of the so-called second knee. We discuss extensively the constraints on this model imposed by current knowledge of acceleration processes and sources of UHECR and compare it with the traditional model of transition at the ankle.

astro-ph↗
arXiv2006

Modified Special Relativity on a fluctuating spacetime

It was recently proposed that deformations of the relativistic symmetry, as those considered in Deformed Special Relativity (DSR), can be seen as the outcome of a measurement theory in the presence of non-negligible (albeit small) quantum gravitational fluctuations [1,2]. In this paper we explicitly consider the case of a spacetime described by a flat metric endowed with stochastic fluctuations and, for a free particle, we show that DSR-like nonlinear relations between the spaces of the measured and classical momenta, can result from the average of the stochastic fluctuations over a scale set be the de Broglie wavelength of the particle. As illustrative examples we consider explicitly the averaging procedure for some simple stochastic processes and discuss the physical implications of our results.

gr-qc↗
arXiv2006

On the status of superheavy dark matter

Superheavy particles are a natural candidate for the dark matter in the universe and our galaxy, because they are produced generically during inflation in cosmologically interesting amounts. The most attractive model for the origin of superheavy dark matter (SHDM) is gravitational production at the end of inflation. The observed cosmological density of dark matter determines the mass of the SHDM particle as $m_X=$(a few)$\:\times 10^{13}$ GeV, promoting it to a natural candidate for the source of the observed ultra-high energy cosmic rays (UHECR). After a review of the theoretical aspects of SHDM, we up-date its predictions for UHECR observations: no GZK cutoff, flat energy spectrum with $dN/dE\approx 1/E^{1.9}$, photon dominance and galactic anisotropy. We analyze the existing data and conclude that SDHM as explanation for the observed UHECRs is at present disfavored but not yet excluded. We calculate the anisotropy relevant for future Auger observations that should be the conclusive test for this model. Finally, we emphasize that negative results of searches for SHDM in UHECR do not disfavor SHDM as a dark matter candidate. Therefore, UHECRs produced by SHDM decays and with the signatures as described should be searched for in the future as subdominant effect.

astro-ph↗
arXiv2005

Anti-GZK effect in UHECR spectrum

In this paper we discuss the anti-GZK effect that arises in the framework of the diffusive propagation of Ultra High Energy (UHE) protons. This effect consists in a jump-like increase of the maximum distance from which UHE protons can reach the observer. The position of the jump is independent of the Intergalactic Magnetic Field (IMF) strength and depends only on the energy losses of protons, namely on the transition energy from adiabatic and pair-production energy losses. The Ultra High Energy Cosmic Rays (UHECR) spectrum presents a low-energy steepening approximately at this energy, which is very close to the position of the observed second knee. The dip, seen in the universal spectrum as a signature of the proton interaction with the Cosmic Microwave Background (CMB) radiation, is also present in the case of diffusive propagation in magnetic fields.

astro-ph↗
arXiv2005

Anti-GZK effect in Ultra High Energy Cosmic Rays diffusive propagation

We discuss the antiGZK effect in the diffusive propagation of ultra high energy protons in intergalactic magnetic fields, which consists in a jump-like increase of the maximum distance from which ultra high energy protons can reach an observer. The position of this jump, $E_j \approx 2\times 10^{18}$ eV, is determined exclusively by energy losses (transition from adiabatic to pair-production energy losses) and it is independent of the diffusion parameters. The diffuse spectrum presents a low-energy steepening approximately at this energy, which is very close to the position of the second knee observed in the cosmic ray spectrum. The dip, seen in the universal spectrum as a signature of the interaction with the cosmic microwave background radiation, is also present in the case of diffusive propagation in magnetic fields.

astro-ph↗
arXiv2005

Phenomenological Quantum Gravity: the birth of a new frontier?

In the last years a general consensus has emerged that, contrary to intuition, quantum-gravity effects may have relevant consequences for the propagation and interaction of high energy particles. This has given birth to the field of ``Phenomenological Quantum Gravity'' We review some of the aspects of this new, very exciting frontier of Physics.

astro-ph↗
arXiv2005

A note on DSR-like approach to space-time

In this note we discuss the possibility to define a space-time with a DSR based approach. We show that the strategy of defining a non linear realization of the Lorentz symmetry with a consistent vector composition law cannot be reconciled with the extra request of an invariant length (time) scale. The latter request forces to abandon the group structure of the translations and leaves a space-time structure where points with relative distances smaller or equal to the invariant scale can not be unambiguously defined.

gr-qc↗
arXiv2004

Approaching Space Time Through Velocity in Doubly Special Relativity

We discuss the definition of velocity as dE/dp, where E,p are the energy and momentum of a particle, in Doubly Special Relativity (DSR). If this definition matches dx/dt appropriate for the space-time sector, then space-time can in principle be built consistently with the existence of an invariant length scale. We show that, within different possible velocity definitions, a space-time compatible with momentum-space DSR principles can not be derived.

gr-qc↗
arXiv2004

Quantum-Gravity phenomenology and high energy particle propagation

Quantum-gravity effects may introduce relevant consequences for the propagation and interaction of high energy cosmic rays particles. Assuming the space-time foamy structure results in an intrinsic uncertainty of energy and momentum of particles, we show how low energy (under GZK) observations can provide strong constraints on the role of the fluctuating space-time structure.

astro-ph↗
arXiv2004

Ultra High Energy Cosmic Rays Spectra in Top-Down models

In this work we present a detailed computation of the spectra of UHECR in the top-down scenario. We compare the spectra of hadrons obtained by two different methods in QCD and supersymmetric (SUSY) QCD with large primary energies $\sqrt{s}$ up to $10^{16}$ GeV. The two methods discussed are a Monte Carlo (MC) simulation and the evolution of the hadron fragmentation functions as described by the Dokshitzer-Gribov-Lipatov-Altarelli-Parisi (DGLAP) equations. The hadron spectra obtained by the two methods agree fairly well in the interesting energy range $10^{-5}M_{X}<E<0.3M_{X}$ ($M_X$ is the energy scale of the process $M_{X}\ge 10^{12}$ GeV). We have also computed the spectra of photons, neutrinos and nucleons obtaining a good agreement with other published results. The consistency of the spectra computed by different methods allows us to consider the spectral shape as a signature of the production model for UHECR, such as the decay of super heavy relic particles or topological defects.

astro-ph↗
arXiv2004

Diffusive propagation of UHECR and the propagation theorem

We present a detailed analytical study of the propagation of ultra high energy (UHE) particles in extragalactic magnetic fields. The crucial parameter which affects the diffuse spectrum is the separation between sources. In the case of a uniform distribution of sources with a separation between them much smaller than all characteristic propagation lengths, the diffuse spectrum of UHE particles has a {\em universal} form, independent of the mode of propagation. This statement has a status of theorem. The proof is obtained using the particle number conservation during propagation, and also using the kinetic equation for the propagation of UHE particles. This theorem can be also proved with the help of the diffusion equation. In particular, it is shown numerically, how the diffuse fluxes converge to this universal spectrum, when the separation between sources diminishes. We study also the analytic solution of the diffusion equation in weak and strong magnetic fields with energy losses taken into account. In the case of strong magnetic fields and for a separation between sources large enough, the GZK cutoff can practically disappear, as it has been found early in numerical simulations. In practice, however, the source luminosities required are too large for this possibility.

astro-ph↗
arXiv2004

Particle and Antiparticle sectors in DSR1 and kappa-Minkowski space-time

In this paper we explore the problem of antiparticles in DSR1 and $κ$-Minkowski space-time following three different approaches inspired by the Lorentz invariant case: a) the dispersion relation, b) the Dirac equation in space-time and c) the Dirac equation in momentum space. We find that it is possible to define a map $S_{dsr}$ which gives the antiparticle sector from the negative frequency solutions of the wave equation. In $κ$-Poincaré, the corresponding map $S_{kp}$ is the antipodal mapping, which is different from $S_{dsr}$. The difference is related to the composition law, which is crucial to define the multiparticle sector of the theory. This discussion permits to show that the energy of the antiparticle in DSR is the positive root of the dispersion relation, which is consistent with phenomenological approaches.

hep-th↗