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

TL;DR

This paper proposes a scalar portal model that explains the muon g-2 anomaly and dark matter relic density, predicts detectable signals in electron scattering experiments, and constrains model parameters based on existing experimental data.

Contribution

It introduces a leptonic scalar portal model linking muon g-2 and dark matter, and provides experimental constraints from NA64 data.

Findings

Constraints on the scalar portal mass and couplings.

Potential for future detection with upgraded experiments.

Explanation of observed missing energy events in NA64.

Abstract

We present a model explaining both the 4.2 $\sigma$ muon $g-2$ anomaly and the relic density of dark matter (DM) in which DM interacts with the Standard Model (SM) via a scalar portal boson $\varphi$ carrying both dark and SM leptonic numbers, and mediating a nondiagonal interaction between the electron and muon that allows $e \leftrightarrow \mu$ transitions. The $\varphi$ could be produced in high-energy electron scattering off a target nuclei in the reaction $e Z \to \mu Z\varphi$ followed by the prompt invisible decay $\varphi~\to$~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 $\simeq 3\times 10^{11}$ electrons on target collected during 2016-2018 for the search for light dark matter in the NA64 experiment at the CERN SPS [PRL {\bf 123}, 121801 (2019)]. Attributing so far these events to background allows us to set first constraints on the $\varphi$ 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.