Current status and new perspectives on cosmic ray deuterons

Deuterons are the most abundant secondary cosmic ray species in the Galaxy, but their study has been severely limited due to experimental challenges. In an era with new experiments and high-precision measurements in cosmic rays, having a low-uncertainty deuteron flux in a wide energy range becomes possible. The deuteron-over-helium ratio ($d$/$^4$He) is important to understand the propagation of cosmic rays in the Galaxy and in the heliosphere, complementing observations with heavier nuclei like the boron-to-carbon ratio. In this work, the most up-to-date results of the deuteron flux and the $d$/$^4$He ratio at the top of the atmosphere have been obtained using GALPROP and a 3D solar modulation model. It was found that the simulation describes the deuteron flux and $d$/$^4$He data below 1 GeV/$n$ within the uncertainties of the model. However, the model underestimates the best-published measurements available at high energy. This discrepancy suggests a differentiated approach has to be considered in the diffusion between light and heavier nuclei and, therefore, a possible break of the universality in cosmic ray propagation. In the future, AMS-02 will provide low-uncertainty results at high energies that will help to test this scenario.