arXiv subjects

Nicola Tomassetti

Publications and source records attributed to Nicola Tomassetti.

39 records · Page 3Linked to original sources

arXiv2015

Examination of uncertainties in nuclear data for cosmic ray physics with the AMS experiment

High-energy Li-Be-B nuclei in cosmic rays are being measured with unprecedent accuracy by the AMS experiment. These data bring valuable information to the cosmic ray propagation physics. In particular, combined measurements of B/C and Be/B ratios may allow to break the parameter degeneracy between the cosmic-ray diffusion coefficient and the size of the propagation region, which is crucial for dark matter searches. The parameter determination relies in the calculation of the Be and B production from collisions of heavier nuclei with the gas. Using the available cross-section data, I present for the first time an evaluation of the nuclear uncertainties and their impact in constraining the propagation models. I found that the AMS experiment can provide tight constraints on the transport parameters allowing to resolutely break the degeneracy, while nuclear uncertainties in the models are found to be a major limiting factor. Once these uncertainties are accounted, the degeneracy remains poorly resolved. In particular, the Be/B ratio at ~1 - 10 GeV/n is found not to bring valuable information for the parameter extraction. On the other hand, precise Be/B data at higher energy may be useful to test the nuclear physics inputs of the models.

astro-ph.HE↗

arXiv2015

Cosmic-ray protons, nuclei, electrons, and antiparticles under a two-halo scenario of diffusive propagation

We report calculations of cosmic-ray proton, nuclei, antiproton, electron and positron energy spectra within a "two-halo model" of diffusive transport. The two halos represent a simple, physically consistent generalization of the standard diffusion models, which assume a unique type of diffusion for cosmic rays in the whole Galactic halo. We believe instead that cosmic rays may experience a smaller energy dependence of diffusion when they are in proximity of the Galactic disk. Our scenario is supported by recent observations of cosmic-ray protons, nuclei, anisotropy, and gamma-rays. We predict remarkably hard antiparticle spectra at high energy. In particular, at E>10 GeV, the antiproton/proton ratio is expected to flatten, while the positron fraction is found to increase with energy. We discuss the implications for cosmic-ray physics and dark matter searches via antimatter.

astro-ph.HE↗

arXiv2015

Origin of the spectral upturn in the cosmic-ray C/Fe and O/Fe ratios

The observed spectrum of Galactic cosmic rays has several exciting features such as the rise in the positron fraction above ~10 GeV of energy and the spectral hardening of protons and helium at ~300 GeV/nucleon of energy. The ATIC-2 experiment has recently reported an unexpected spectral upturn in the elemental ratios involving iron, such as the C/Fe or O/Fe ratios, at energy above 50 GeV per nucleon. It is recognized that the observed positron excess can be explained by pion production processes during diffusive shock acceleration of cosmic-ray hadrons in nearby sources. Recently, it was suggested that a scenario with nearby source dominating the GeV-TeV spectrum may be connected with the change of slope observed in protons and nuclei, which would be interpreted as a flux transition between the local component and the large-scale distribution of Galactic sources. Here I show that, under a two-component scenario with nearby source, the shape of the spectral transition is expected to be slightly different for heavy nuclei, such as iron, because their propagation range is spatially limited by inelastic collisions with the interstellar matter. This enables a prediction for the primary/primary ratios between light and heavy nuclei. From this effect, a spectral upturn is predicted in the C/Fe and O/Fe ratios in good accordance with the ATIC-2 data.

astro-ph.HE↗