Can annihilating Dark Matter be lighter than a few GeVs?

TL;DR

This paper investigates whether annihilating dark matter can be lighter than a few GeV by estimating gamma-ray fluxes from residual annihilations for $m_{dm}$ in the MeV–GeV range and comparing with observations. It shows that relic-density considerations fix $\langle \sigma v_r \rangle_{ann}$ to a nearly mass-independent value around $7 \times 10^{-27}$ cm$^3$ s$^{-1}$, with the cross section written as $\sigma v_r \sim a + b (v/c)^2$, which favors light DM candidates that annihilate predominantly via S-wave or P-wave terms depending on the model. Indirect detection limits from gamma rays and radio emission constrain light DM to masses above roughly 100 MeV unless the S-wave term is strongly suppressed or a light mediator alters the annihilation dynamics. Cosmological damping and decoupling considerations further require $m_{dm} \gtrsim 1$ MeV in standard histories, narrowing the viable parameter space and highlighting specific velocity-dependent or mediator-based scenarios where MeV–GeV annihilating DM could still be viable. Overall, the work underscores that low-mass annihilating DM remains possible only under particular cross-section and particle-physics configurations, with current astrophysical and cosmological data providing stringent tests.

Abstract

We estimate the gamma ray fluxes from the residual annihilations of Dark Matter particles having a mass mdm \in [MeV, O(GeV)] and compare them to observations. We find that particles lighter than O(100 MeV) are excluded unless their cross section is S-wave suppressed.