A bridge to new physics: proposing new -- and reviving old -- explanations of $a_\mu$

Abstract

The $4.2\,\sigma$ tension in the combined measurement of the anomalous magnetic moment of the muon, $a_\mu$, 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 \emph{bridge} diagram, which gives a chirally enhanced contribution to $a_\mu$. We classify all possible 2- and 3-field SM extensions that can generate this contribution and present the full $a_\mu$ 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 $a_\mu$. We explore a particular 3-field extension which, beyond explaining $a_\mu$, 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.