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 8Be 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 8Be 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 8Be particularly predictive. The underlying idea of this minimal “prototype model” can be readily incorporated into other protophobic U(1) extensions of the Standard Model.
Highlights
Denoted 8Be∗ and 8Be∗, into the fundamental state (8Be0)
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 8Be 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
10−2 (g − 2)μ all the most intriguing since instead of following a naıve scaling proportional to powers of the light lepton masses, the comparison of ∆ae,μ suggests the presence of a New Physics which violates lepton flavour universality
Summary
Fμν and Fμν correspond to the field strengths of the (kinetically mixed) hypercharge boson Band the U(1)B−L boson B ; k denotes the kinetic mixing parameter. The gauge kinetic terms can be cast in matrix form as. This transformation is obtained by a Cholesky decomposition, allowing the resulting triangular matrices to be absorbed into a redefinition of the gauge fields. The resulting mass matrix, in which the neutral bosons mix amongst themselves, can be diagonalised, leading to the following relations between physical and interaction gauge boson states. In the limit of small ε (corresponding to small kinetic mixing, cf eq (2.7)), one finds the following approximate expressions for the mixing angle and the masses of the Z and.
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