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 > m W . 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 ∼ 10 3 TeV; and (iii) establish sharp correlations among those observables. We find that the flavour-conserving Wilson coefficients set an upper bound on the flavour-violating ones. The low-energy limits on µ → e and τ → e, µ transitions suppress flavour violation in Z and Higgs decays, as well as electric dipole moments, far beyond the experimental reach. The precision measurements of G F , m W , 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.

Majoron at two loops

“…As already noted in Ref. [17], the non-Majoron limits are completely dominated by flavor-conserving observables such as Z → e + e − and other electroweak precision data. LFV such as τ → eγ and τ → eee reside far in the already excluded region and even future improvements, e.g.…”

“…with complex z and real ξ α without loss of generality [17]. 1 This product structure of M D implies…”

“…7, the latter adopted from Ref. [17]. We set M R /TeV = 1 1 as well as f = 1 TeV and stress again that the Majoron limits can be suppressed arbitrarily by increasing f .…”

“…R = f = 1 TeV, (M D M D † ) eτ = [(M D M D † ) ee (M D M D † ) ττ ] TeV, (M D M D † ) eμ = [(M D M D † ) ee (M D M D † ) μμ ] 1/Limits on (MDM † D ) αβ /v 2 fromflavor-conserving non-Majoron (gray region) and flavor-violating observables (colored lines), dashed lines denoting future reach, adopted from Ref [17]…”

Constraining a general U(1)′ inverse seesaw model from vacuum stability, dark matter, and collider

“…In addition, it has been shown in reference [52] that the µ → e conversion in nuclei can give the strongest bound out of all the flavor violating observables in the case of type-I seesaw models. The bound on the branching ratio for the µ → e conversion in Gold (Au) nucleus reads as [53],…”