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

International audienceMotivated 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′ 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′ and the new U(1)$_{B−L}$-breaking scalar can also successfully account for both (g −2)$_{e,μ}$ 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