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.

31 pages, 7 figures and 4 tables

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• The Ellipse of Muon Dipole Moments
• Minimal model for the $W$-boson mass, $(g-2{)}_{\mu }$, $h\to {\mu }^{+}{\mu }^{-}$ and quark-mixing-matrix unitarity
• Predictions for Muon Electric and Magnetic Dipole Moments from $h\to {\mu }^{+}{\mu }^{-}$ in Two-Higgs-Doublet Models with New Leptons
• The Muon Magnetic Moment and Physics Beyond the Standard Model
• The Low-Scale Seesaw Solution to the $M_W$ and $(g-2{)}_{\mu }$ Anomalies
• Effective Field Theory of Chirally-Enhanced Muon Mass and Dipole Operators
• From the EFT to the UV: the complete SMEFT one-loop dictionary
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• From Higgs physics to lepton flavour violation: current bounds and future prospects for vector-like lepton models