Leptonic scalar portal: Origin of muon g−2 anomaly and dark matter?

Abstract

We present a model explaining both the $4.2\ensuremath{\sigma}$ muon $g\ensuremath{-}2$ anomaly and the relic density of dark matter (DM) in which DM interacts with the Standard Model (SM) via a scalar portal boson $\ensuremath{\varphi}$ carrying both dark and SM leptonic numbers, and mediating a nondiagonal interaction between the electron and muon that allows $e\ensuremath{\leftrightarrow}\ensuremath{\mu}$ transitions. The $\ensuremath{\varphi}$ could be produced in high-energy electron scattering off a target nuclei in the reaction $eZ\ensuremath{\rightarrow}\ensuremath{\mu}Z\ensuremath{\varphi}$ followed by the prompt invisible decay $\ensuremath{\varphi}\ensuremath{\rightarrow}\mathrm{DM}$ particles and searched for in events with large missing energy accompanied by a single outgoing muon in the final state. Interestingly, several events with a similar signature have been observed in a data sample of $\ensuremath{\simeq}3\ifmmode\times\else\texttimes\fi{}{10}^{11}$ electrons on target collected during 2016-2018 for the search for light dark matter in the NA64 experiment at the CERN SPS [D. Banerjee et al. (NA64 Collaboration), Phys. Rev. Lett. 123, 121801 (2019).]. Attributing so far these events to background allows us to set first constraints on the $\ensuremath{\varphi}$ mass and couplings while leaving at the same time decisively probing the origin of these events and a large fraction of the remaining parameter space to a near exiting future with the upgraded NA64 detector or other planned experiments.