A bridge to new physics: proposing new — and reviving old — explanations of aμ

Abstract

The 4.2 σ tension in the combined measurement of the anomalous magnetic moment of the muon, aμ, and the Standard Model prediction strongly suggests the existence of beyond the Standard Model physics. Following the Standard Model Effective Field Theory approach, we study a particular topology, the bridge diagram, which gives a chirally enhanced contribution to aμ. We classify all possible 2- and 3-field SM extensions that can generate this contribution and present the full aμ result for them. Within our approach, we find that several 2-field fermion-scalar extensions which had been previously discarded in the literature — when only the Yukawa-suppressed contribution was considered — can actually be viable models to explain the observed anomaly. Furthermore, the 3-field extensions which generate the bridge diagram represent a new class of models to account for aμ. We explore a particular 3-field extension which, beyond explaining aμ, can also account for the neutral B-meson anomalies and the Cabibbo angle anomaly. We present the full one-loop matching for this model and a one-loop phenomenological study.