Leptonic Dipole Transitions: A New Signature for Physics Beyond the Standard Model

Abstract

<p>In this work, we consider the addition of a single neutral massive vector boson to the Standard Model (SM). This boson, which we refer to as N⁰, induces dipolar transitions between electrons and muons. We obtain bounds on the strength of its coupling and its mass: from the scattering process e+e- to mu+mu-; from its contribution to muonium-antimuonium oscillations; and from its possible contribution to the rare muon decay mu- to e+e-e-. In particular, we examine the two cases where the mediator is both heavy and light compared with the scattering energies for, and place constraints on the relevant parameters based on their contributions to the cross section and the forward-backward asymmetry. For muonium-antimuonium oscillations, we consider only the case where the mediator is heavy compared to all other scales, reducing its effect to an effective contact interaction. Finally, we consider an SU(2) invariant theory from which the N⁰ interaction emerges, and find that flavour diagonal interactions also emerge, giving a tree-level path for the decay mu- to e+e-e-. We find that the heavy N⁰ is not strongly constrained by this contribution, while the light N⁰ is very strongly constrained by it. Very generally, we find that the heavy N⁰ is much less constrained than other lepton flavour violating processes, while the constraints on the light N0 vary in strength between processes.</p>