Effective approach to lepton observables: The seesaw case

Abstract

In the absence of direct evidence of new physics, any ultraviolet theory can be reduced to its specific set of low-energy effective operators. As a case study, we derive the effective field theory for the seesaw extension of the Standard Model, with sterile neutrinos of mass M>mW. We systematically compute all Wilson coefficients generated at one loop. Hence, it becomes straightforward to (i) identify the seesaw parameters compatible with the smallness of neutrino masses, (ii) compute precision lepton observables, which may be sensitive to scales as large as M∼103 TeV, and (iii) establish sharp correlations among those observables. We find that the flavor-conserving Wilson coefficients set an upper bound on the flavor-violating ones. The low-energy limits on μ→e and τ→e,μ transitions suppress flavor violation in Z and Higgs decays, as well as electric dipole moments, far beyond the experimental reach. The precision measurements of GF, mW, and Z partial decay widths set more stringent bounds than present and future limits on τ→e,μ transitions. We also present a general spurion analysis, to compare the seesaw with different models, thus assessing the discriminating potential of the effective approach.