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Charged-lepton decays from soft flavour violation

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 Added by Walter Grimus
 Publication date 2016
  fields
and research's language is English




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We consider a two-Higgs-doublet extension of the Standard Model, with three right-handed neutrino singlets and the seesaw mechanism, wherein all the Yukawa-coupling matrices are lepton flavour-diagonal and lepton flavour violation is soft, originating solely in the non-flavour-diagonal Majorana mass matrix of the right-handed neutrinos. We consider the limit $m_R to infty$ of this model, where $m_R$ is the seesaw scale. We demonstrate that there is a region in parameter space where the branching ratios of all five charged-lepton decays $ell_1^- to ell_2^- ell_3^+ ell_3^-$ are close to their experimental upper bounds, while the radiative decays $ell_1^- to ell_2^- gamma$ are invisible because their branching ratios are suppressed by $m_R^{-4}$. We also consider the anomalous magnetic moment of the muon and show that in our model the contributions from the extra scalars, both charged and neutral, can remove the discrepancy between its experimental and theoretical values.



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We consider the one-loop radiative corrections to the light-neutrino mass matrix and their consequences for the predicted branching ratios of the five lepton-flavour-violating decays $ell_1^- to ell_2^- ell_3^+ ell_3^-$ in a two-Higgs-doublet model furnished with the type-I seesaw mechanism and soft lepton-flavour violation. We find that the radiative corrections are very significant; they may alter the predicted branching ratios by several orders of magnitude and, in particular, they may help explain why $mbox{BR}(mu^- to e^- e^+ e^-)$ is strongly suppressed relative to the branching ratios of the decays of the $tau^-$. We conclude that, in any serious numerical assessment of the predictions of this model, it is absolutely necessary to take into account the one-loop radiative corrections to the light-neutrino mass matrix.
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