Lepton flavor violation and seesaw symmetries
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Lepton flavor violation and seesaw symmetries D. Aristizabal Sierra
Published online: 5 February 2013 © Springer Science+Business Media Dordrecht 2013
Abstract When the standard model is extended with right-handed neutrinos the symmetries of the resulting Lagrangian are enlarged with a new global U(1) R Abelian factor. In the context of minimal seesaw models we analyze the implications of a slightly broken U(1) R symmetry on charged lepton flavor violating decays. We find, depending on the R-charge assignments, models where charged lepton flavor violating rates can be within measurable ranges. In particular, we show that in the resulting models due to the structure of the light neutrino mass matrix muon flavor violating decays are entirely determined by neutrino data (up to a normalization factor) and can be sizable in a wide right-handed neutrino mass range. Keywords Neutrino mass and mixings · Right handed neutrinos · Decays of leptons
1 Introduction Apart from demonstrating that neutrinos are massive and have non-vanishing mixing angles among the different generations [1, 2], neutrino oscillation experiments have also proved that lepton flavor is not conserved in the neutral lepton sector. Once the standard model is extended to account for neutrino masses—unavoidably—lepton flavor violation (LFV) also takes place in the charged lepton sector. This, however, not necessarily implies that these effects are sizable, so whether these processes
The 5th International Symposium on Symmetries in Subatomic Physics (SSP 2012), Groningen, The Netherlands, 18–22 June 2012. D. Aristizabal Sierra (B) IFPA, Department AGO, Universite de Liege, Bat B5 Sart Tilman, 4000 Liege 1, Belgium e-mail: [email protected]
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D. Aristizabal Sierra
can or not have measurable rates depends to a large extent on the details of the corresponding model. Despite this fact, from a general point of view, charged lepton flavor violating processes are expected to have large decay branching fractions as long as the LFV mediators have O(TeV) masses and their couplings to the standard model leptons are about 10−2 . Majorana neutrino masses can be generated in a model independent way by adding the dimension-five effective operator O5 ∼ LLH H to the standard model Lagrangian [3]. And in turn the different concrete realizations of this operator constitute a model for neutrino masses.1 Among the tree-level realizations the type-I seesaw is certainly the most popular one [14–18]. In this model, light neutrino masses are generated via the exchange of electroweak fermionic singlets (right-handed (RH) neutrinos for brevity). Consistency with neutrino data then requires either heavy RH neutrino masses (O(M N ) ∼ GUT ) or tiny Yukawa couplings (O ∼ 10−6 ), thus implying negligibly small charged lepton flavor violating effects. In addition to the standard model gauge symmetry the seesaw Lagrangian features a global Abelian U(1) R symmetry, typically related with phase rotations of the standard model lepton SU(2) singlets, and thus broken by the charge
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