Semi-Visible Dark Photon Phenomenology at the GeV Scale

TL;DR

This paper explores semi-visible dark photons at the GeV scale, showing they can evade previous experimental constraints and opening new avenues for detection and understanding of dark sector physics.

Contribution

It introduces generalized models of semi-visible dark photons with multiple dark fermions, revises experimental constraints, and proposes new search strategies at NA64.

Findings

BaBar and NA64 constraints can be relaxed for large kinetic mixing

Semi-visible dark photons can explain muon g-2 discrepancy

Modified missing-energy searches can probe new parameter space

Abstract

In rich dark sector models, dark photons heavier than tens of MeV can behave as semi-visible particles: their decays contain both visible and invisible final states. We present models containing multiple dark fermions which allow for such decays and inscribe them in the context of inelastic dark matter and heavy neutral leptons scenarios. Our models represent a generalization of the traditional inelastic dark matter model by means of a charge conjugation symmetry. We revisit constraints on dark photons from $e^+e^-$ colliders and fixed target experiments, including the effect of analysis vetoes on semi-visible decays, $A^\prime \to \psi_i (\psi_j \to \psi_k \ell^+\ell^-)$. We find that in some cases, the BaBar and NA64 experiments no longer exclude large kinetic mixing, $\varepsilon \sim 10^{-2}$, and, specifically, the related explanation of the discrepancy in the muon $(g-2)$. This reopens an interesting window in parameter space for dark photons with exciting discovery prospects. We point out that a modified missing-energy search at NA64 can target short-lived $A^\prime$ decays and directly probe the newly-open parameter space.