Anomalies in ${}^8$Be nuclear transitions and $(g-2{)}_{e,\mu }$: towards a minimal combined explanation

Abstract

Motivated by a simultaneous explanation of the apparent discrepancies in the light charged lepton anomalous magnetic dipole moments, and the anomalous internal pair creation in ${}^8$Be nuclear transitions, we explore a simple New Physics model, based on an extension of the Standard Model gauge group by a $U(1{)}_{B-L}$. The model further includes heavy vector-like fermion fields, as well as an extra scalar responsible for the low-scale breaking of $U(1{)}_{B-L}$, which gives rise to a light $Z^{\prime }$ boson. The new fields and currents allow to explain the anomalous internal pair creation in ${}^8$Be while being consistent with various experimental constraints. Interestingly, we find that the contributions of the $Z^{\prime }$ and the new $U(1{)}_{B-L}$-breaking scalar can also successfully account for both $(g-2{)}_{e,\mu }$ anomalies; the strong phenomenological constraints on the model's parameter space ultimately render the combined explanation of $(g-2{)}_e$ and the anomalous internal pair creation in ${}^8$Be particularly predictive. The underlying idea of this minimal "prototype model" can be readily incorporated into other protophobic $U(1)$ extensions of the Standard Model.

32 pages, 6 figures

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