$\gamma$ rays from in-flight positron annihilation as a probe of new physics

TL;DR

This paper uses gamma rays from in-flight positron annihilation to set new constraints on exotic particles and sources, improving previous limits significantly and exploring implications for various dark matter and new physics models.

Contribution

It introduces in-flight positron annihilation gamma rays as a novel probe for constraining feebly interacting particles and exotic sources, providing the most stringent limits to date.

Findings

Set new constraints on sterile neutrinos with mixing angles |U_{μ4}|^2 and |U_{τ4}|^2.

Improved existing limits by over an order of magnitude.

Discussed implications for dark photons, axion-like particles, PBHs, and sub-GeV dark matter.

Abstract

The $\gamma$ 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 $\gamma$ 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 $|U_{\mu4}|^{2} \gtrsim 10^{-13}$ and $|U_{\tau4}|^{2} \gtrsim 2\times 10^{-13}$ for sterile neutrinos mixed with $\mu$ and $\tau$ 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).