Lepton Flavor Violation and the Origin of the Seesaw Mechanism

The right–handed neutrino mass matrix that is central to the understanding of small neutrino masses via the seesaw mechanism can arise either (i) from renormalizable operators or (ii) from nonrenormalizable or superrenormalizable operators, depending on the symmetries and the Higgs content of the theory beyond the Standard Model. In this paper, we study lepton flavor violating (LFV) effects in the first class of seesaw models wherein the νR Majorana masses arise from renormalizable Yukawa couplings involving a B − L = 2 Higgs field. We present detailed predictions for τ → μ + γ and μ → e + γ branching ratios in these models taking the current neutrino oscillation data into account. Focusing on minimal supergravity models, we find that for a large range of MSSM parameters suggested by the relic abundance of neutralino dark matter and that is consistent with Higgs boson mass and other constraints, these radiative decays are in the range accessible to planned experiments. We compare these predictions with lepton flavor violation in the second class of models arising entirely from the Dirac Yukawa couplings. We study the dependence of the ratio r ≡ B(μ → e + γ)/B(τ → μ + γ) on the MSSM parameters and show that measurement of r can provide crucial insight into the origin of the seesaw mechanism.