Amplifying Non-Resonant Production of Dark Sector Particles in Scattering Dominance Regime

TL;DR

This paper demonstrates that in the scattering dominance regime, nonresonant production of dark photons can be significantly enhanced, leading to improved experimental constraints on their mixing parameters, unlike sterile neutrinos.

Contribution

It introduces the concept of collisional enhancement of dark photon production in the scattering dominance regime and applies it to improve laboratory constraints.

Findings

Enhanced dark photon production by a factor of λ_abs/λ_sca in the scattering dominance regime

Significant improvement in TEXONO experiment sensitivity to dark photon mixing parameter ε

No similar enhancement observed for sterile neutrino production

Abstract

We investigate the enhancement of dark sector particle production within the scattering dominant regime. These particles typically exhibit a slight mixing with Standard Model particles through various portals, allowing for their generation through in-medium oscillation from Standard Model particle sources. Our analysis reveals that in the scattering dominance regime, with a significantly smaller scattering mean free path $\lambda_{\rm sca}$ compared to the absorption mean free path $\lambda_{\rm abs}$, the nonresonant production of sterile states can experience an enhancement by a factor of $\lambda_{\rm abs}/\lambda_{\rm sca}$. This phenomenon is demonstrated within the context of kinetic mixing dark photon production at a reactor, precisely satisfying this condition. By incorporating this collisional enhancement, we find that the current sensitivity to the mixing parameter $\epsilon$ for dark photons in the TEXONO experiment can be significantly improved across a range spanning from tens of eV to MeV. This advancement establishes the most stringent laboratory constraint within this mass spectrum for the dark photon. Sterile neutrino production, however, does not exhibit such enhancement, either due to the failure to meet the scattering dominance criterion or the neutrino damping in resonant production.