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Inelastic dark matter revives the dark photon interpretation of the muon g-2 anomaly

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 semi-visible 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 semi-visible events we discuss may have been vetoed by experiments searching for invisible dark photon decays. A re-analysis 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.