Suppression of lepton number violation mediated by ΔI = 0 mass fermions

Abstract

We add a lepton doublet with ΔI = 0 mass M to the minimal Weinberg-Salam model. We then calculate the transition rate for μ → eγ for all M as a function of an arbitrary mixing parameter δ of order a normal lepton mass. For δ ⪅ 0.4 GeV, M can be as low as 20 GeV. We show that heavy gDI = 0 fermions decouple throught their highly but naturally suppressed mixing angles with normal fermions and not through diagrammatic cancellations. Models with heavy ΔI = 0 fermions evade the commonly used conditions for natural suppression of rare processes in gauge theories.