γ rays from in-flight positron annihilation as a probe of new physics

The γ ray emission originating from in-flight annihilation (IA) of positrons is a powerful observable for constraining high-energy positron production from exotic sources. By comparing diffuse γ ray observations of INTEGRAL, COMPTEL and EGRET to theoretical predictions, we set the most stringent constraints on electrophilic feebly interacting particles (FIPs), thereby proving IA as a valuable probe of new physics. In particular, we extensively discuss the case of MeV-scale sterile neutrinos, where IA sets the most stringent constraints, excluding |U_μ4|^2≳ 10^-13 and |U_τ4|^2≳ 2× 10^-13 for sterile neutrinos mixed with μ and τ neutrinos respectively. These constraints improve existing limits by more than an order of magnitude. We briefly discuss the application of these results to a host of exotic positron sources such as dark photons, axion-like particles, primordial black holes (PBHs) and sub-GeV dark matter (DM).