Implications of BBN bounds for cosmic ray upscattered dark matter
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 $\ensuremath{\sim}\mathrm{keV}\ensuremath{-}\mathrm{GeV}$ and requires large cross sections $\ensuremath{\gtrsim}{10}^{\ensuremath{-}31}\text{ }\text{ }{\mathrm{cm}}^{2}$ 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 $\ensuremath{\lesssim}10\text{ }\text{ }\mathrm{MeV}$, corresponding to much of the favored region of many cosmic-ray upscattering studies, this equilibrated DM population adds an additional component to the relativistic energy density around $T\ensuremath{\sim}\mathrm{few}$ MeV and thereby spoils the successful predictions of BBN. In the remaining $\ensuremath{\sim}10\text{ }\text{ }\mathrm{MeV}--\mathrm{GeV}$ mass range, the large couplings required in this scenario are either currently excluded or within reach of current or future accelerator-based searches.