Constraints on dark axion portal: missing energy and fermion EDMs

TL;DR

This paper investigates a dark axion portal model linking the Standard Model photon, dark photon, and ALP, analyzing experimental signatures, sensitivities, and constraints from missing energy searches and fermion EDM measurements.

Contribution

It introduces new constraints on dark photon and ALP couplings from fixed-target experiments and EDM bounds, exploring novel production mechanisms and their experimental implications.

Findings

Projected sensitivity curves for NA64e and LDMX experiments.

New constraints on dark photon and ALP parameter space from EDM measurements.

Analysis of production mechanisms for invisible dark sector states.

Abstract

We study a model in which a new interactions between the Standard Model (SM) photon and both the dark photon ($\gamma_D)$ and an ALP ($a$) are described by the dark axion portal operator. The implications of this dark axion portal scenario for electron fixed-target experiments are presented. In particular, we investigate the missing energy signatures associated with production of dark photons and their subsequent invisible decays into stable dark sector fermions, $\gamma_D \to \chi \bar{\chi}$. We discuss the discovery potential for such a scenario and derive projected sensitivity curves for the NA64$e$ and LDMX experiments. Furthermore, novel constraints of the NA64$e$ for $9.37\times 10^{11}$ electrons on target are derived by considering two production mechanisms for invisible states: (i)~the bremsstrahlung-like emission of an $a\gamma_D$ pair, $e N \to e N \gamma^* (\to a \gamma_D)$, and (ii)~the exclusive vector meson photoproduction, $\gamma^* N \to N V$, followed by the invisible decays of vector mesons, $V \to a \gamma_D$. Additionally, the constraints on the parameter space of $CP$-violating, fermion-specific ALP and dark photon couplings are established. These constraints are derived from current experimental bounds on the electric dipole moments (EDMs) of SM fermions, incorporating loop-induced contributions to the EDMs of the electron, muon, and neutron.