Revisiting the dark photon explanation of the muon anomalous magnetic moment

Abstract

A massive U(1)′ gauge boson known as a "dark photon" or A′, has long been proposed as a potential explanation for the discrepancy observed between the experimental measurement and theoretical determination of the anomalous magnetic moment of the muon (gμ-2) anomaly. Recently, experimental results have excluded this possibility for a dark photon exhibiting exclusively visible or invisible decays. In this work, we revisit this idea and consider a model where A′ couples inelastically to dark matter and an excited dark sector state, leading to a more exotic decay topology we refer to as a semivisible decay. We show that for large mass splittings between the dark sector states this decay mode is enhanced, weakening the previous invisibly decaying dark photon bounds. As a consequence, A′ resolves the gμ-2 anomaly in a region of parameter space the thermal dark matter component of the Universe is readily explained. Interestingly, it is possible that the semivisible events we discuss may have been vetoed by experiments searching for invisible dark photon decays. A reanalysis of the data and future searches may be crucial in uncovering this exotic decay mode or closing the window on the dark photon explanation of the gμ-2 anomaly.