On the Positron Fraction and Cosmic-Ray Propagation Models

TL;DR

This paper analyzes cosmic-ray propagation models to explain the positron fraction observed by experiments, proposing a model where positrons are mainly produced in the interstellar medium and boron in source regions, aligning with various observational constraints.

Contribution

It introduces a cosmic-ray propagation model that accounts for positron and boron production in different regions, explaining observed positron fractions and their energy dependence.

Findings

Positron fraction expected to reach ~0.6 at several TeV

Positron energy dependence linked to boron-to-carbon ratio

Model consistent with cosmic-ray anisotropy bounds

Abstract

The positron fraction observed by PAMELA and other experiments up to ~ 100 GeV is analyzed in terms of models of cosmic-ray propagation. It is shown that generically we expect the positron fraction to reach ~ 0.6 at energies of several TeV, and its energy dependence bears an intimate but subtle connection with that of the boron to carbon ratio in cosmic rays. The observed positron fraction can be fitted in a model that assumes a significant fraction of the boron below ~ 10 GeV is generated through spallation of cosmic-ray nuclei in a cocoonlike region surrounding the sources, and the positrons of energy higher than a few GeV are almost exclusively generated through cosmic-ray interactions in the general interstellar medium. Such a model is consistent with the bounds on cosmic-ray anisotropies and other observations.