# Implications of BBN Bounds for Cosmic Ray Upscattered Dark Matter

## TL;DR

This paper examines how Big Bang Nucleosynthesis constraints limit the parameter space of light dark matter that can be accelerated by cosmic rays and detected terrestrially, focusing on mass ranges and interaction cross sections.

## Contribution

It demonstrates that for low-mass dark matter (<10 MeV), BBN constraints exclude scenarios with large cross sections, while for higher masses, current or future experiments can test the models.

## Key findings

- Low-mass (<10 MeV) dark matter is incompatible with BBN constraints due to energy density contributions.
- Dark matter in the 10-100 MeV range faces experimental constraints or is within reach of upcoming searches.
- BBN bounds significantly restrict the parameter space for cosmic-ray upscattered dark matter detection.

## Abstract

We consider the Big Bang Nucleosynthesis (BBN) bounds on light dark matter whose cross section off nucleons is sufficiently large to enable acceleration by scattering off of cosmic rays in the local galaxy. Such accelerated DM could then deposit energy in terrestrial detectors. Since this signal involves DM of mass ~ keV - 100 MeV and requires large cross sections > 10^-31 cm^2 in a relativistic kinematic regime, we find that the DM population in this scenario is generically equilibrated with Standard Model particles in the early universe. For sufficiently low DM masses < 10 MeV, corresponding to the bulk of the favored region of many cosmic-ray upscattering studies, this equilibrated DM population adds an additional component to the relativistic energy density around T ~ few MeV and thereby spoils the successful predictions of BBN. In the remaining ~ 10-100 MeV mass range, the large couplings required in this scenario are either currently excluded or within reach of current or future accelerator-based searches.