The curious case of high-energy deuterons in Galactic cosmic rays

TL;DR

The paper discusses unexpected high-energy deuteron levels in cosmic rays, suggesting a new source in supernova remnants with low metal abundance, challenging existing models and implications for cosmic ray composition analysis.

Contribution

It proposes that high-energy deuterons originate from hadronic interactions in old supernova remnants with low metal content, breaking the link between deuteron and B/C ratios.

Findings

Deuteron-to-helium ratio is about 1.5 at 500-2000 GeV/nucleon.

Standard models cannot explain the high deuteron levels observed.

Low-metallicity SNRs are needed to reconcile observations with theory.

Abstract

A new analysis of cosmic ray (CR) data collected by the SOKOL experiment in space found that the deuteron-to-helium ratio at energies between 500 and 2000 GeV/nucleon takes the value d/He ~ 1.5. As we will show, this result cannot be explained by standard models of secondary CR production in the interstellar medium and points to the existence of a high-energy source of CR deuterons. To account for the deuteron excess in CRs, we argue that the only viable solution is hadronic interaction processes of accelerated particles inside old supernova remnants (SNRs). From this mechanism, however, the B/C ratio is also expected to increase at energy above ~50 of GeV/nucleon, in conflict with new precision data just released by the AMS-02 experiment. Hence, if this phenomenon is a real physical effect, hadronic production of CR deuterons must occur in SNRs characterized by low metal abundance. In such a scenario, the sources accelerating C-N-O nuclei are not the same as those accelerating helium or protons, so that the connection between d/He ratio and B/C ratio is broken, and the latter cannot be used to place constraints on the production of light isotopes or antiparticles.