Indications for a high-rigidity break in the cosmic-ray diffusion coefficient

Abstract

The repeated deflections of cosmic rays (CR) onto magnetic irregularities turn their propagation into a diffusive process. This increases their permanence time in the Galaxy, increasing their interaction probability with the interstellar (IS) medium. Also, due to their energy, their collision byproducts contain species which are otherwise rare or absent in the IS medium, such as fragile elements like Li-Be-B or antiparticles, like antiprotons (p). These so-called "secondary" species have long been used to set constraints on propagation parameters in the generalized diffusion-loss equations linking the CR injection to the observable fluxes at the Earth. Using cosmic-ray boron to carbon ratio (B/C) data recently released by the AMS-02 experiment, we find tantalizing indications (decisive evidence, in Bayesian terms) in favor of a diffusive origin for the broken power-law spectra found in protons (p) and helium nuclei (He). The result is robust with respect to currently estimated uncertainties in the cross sections, and in the presence of a small component of primary boron, expected because of spallation at the acceleration site. Reduced errors at high energy as well as further cosmic ray nuclei data (as absolute spectra of C, N, O, Li, Be) may definitively confirm this scenario. This proceeding is based on [1].