Probing Gamma Rays from MeV-Scale Dark Matter with Effective Field Theory
Experiments have recently placed stringent constraints on the most natural incarnations of weakly interacting massive particles (WIMPs), which motivates exploring alternative dark matter (DM) candidates. One interesting possibility is that DM has mass at the MeV scale. Indirect detection of gamma rays produced in its self-annihilations is a particularly timely subject since proposed telescopes such as e-ASTROGAM are poised to revolutionize our understanding of the MeV gamma ray spectrum.
This thesis presents a comprehensive analysis of how existing and future gamma ray detectors can probe three realistic simplified models of MeV DM. After employing chiral perturbation theory to write down effective Lagrangians for the DM's interactions with mesons, the relevant strongly interacting degrees of freedom at the MeV scale, the gamma-ray spectra for DM annihilation are computed in detail. The final state radiation from annihilation into leptons and charged pions is calculated exactly, and contributions to the charged pion decay spectrum are accounted for at each step of the decay chain. The electron and positron spectra are also determined in these models and used to derive accurate cosmic microwave background (CMB) constraints. A common concern is that CMB constraints rule out the possibility of detecting photons from MeV DM. This is found not to be the case over most of the parameter space of the simplified models, with e-ASTROGAM providing stronger constraints even when the DM annihilates in an s-wave. For p-wave DM, e-ASTROGAM will place bounds approaching the thermal relic cross section.