Cosmic-ray deuteron excess from a primary component

The recent AMS-02 measurements of cosmic-ray (CR) deuteron flux suggest the presence of primary deuterons in quantities far exceeding predictions from big bang nucleosynthesis. This poses a significant challenge to modern astrophysics, as no known processes can account for such large amounts of deuterons without violating existing constraints [R. I. Epstein et al., Nature (London) 263, 198 (1976)]. In contrast, it has recently been proposed that the AMS-02 measurements can be explained by a purely secondary origin when contributions from heavier nuclei are considered. In this study, we recalculate the secondary deuteron flux using production cross sections updated with the latest data. We find that the deuteron production cross sections are overestimated in the commonly used calculation tools for CR propagation, and a primary deuteron component is still necessary. We then propose a novel process for generating primary deuterons at CR sources through a fusion mechanism, which is naturally unique to deuterons. This model could explain the observed deuteron excess while maintaining consistency with other CR measurements.