Effective approach to lepton observables: The seesaw case

Rupert Coy 1, 2, * Michele Frigerio 1, 2
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 $M\sim 10^3$ 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 $\mu\to e$ and $\tau\to e,\mu$ transitions suppress flavour violation in $Z$ and Higgs decays, as well as electric dipole moments, far beyond the experimental reach. The bounds from the universality of $G_F$ and the invisible $Z$ width are more stringent than present and future limits on $\tau\to e,\mu$ transitions. We also present a general spurion analysis, to compare the seesaw with different models, thus assessing the discriminating potential of the effective approach.
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Submitted on : Monday, December 10, 2018 - 11:27:24 AM
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Rupert Coy, Michele Frigerio. Effective approach to lepton observables: The seesaw case. Physical Review D, American Physical Society, 2019, 99 (9), pp.095040. ⟨10.1103/PhysRevD.99.095040⟩. ⟨hal-01949587⟩



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