Two-body lepton-flavour-violating decays in a 2HDM with soft family-lepton-number breaking

Abstract

We evaluate the decays ${\ell }_1^{\pm }\to {\ell }_2^{\pm }\gamma$, $Z\to {\ell }_1^{+}{\ell }_2^{-}$, and $h\to {\ell }_1^{+}{\ell }_2^{-}$, where ${\ell }_1$ and ${\ell }_2$ are charged leptons with different flavours and $h$ is the scalar particle with mass 125.25 GeV, in a two-Higgs-doublet model where all the Yukawa-coupling matrices conserve the lepton flavours but the Majorana mass terms of the right-handed neutrinos break the flavour lepton numbers. We find that (1) the decays ${\ell }_1^{\pm }\to {\ell }_2^{\pm }\gamma$ require large Yukawa couplings and very light right-handed neutrinos in order to be visible, (2) the decays $Z\to {\ell }_1^{+}{\ell }_2^{-}$ will be invisible in all the planned experiments, except in a very restricted range of circumstances, but (3) the decays $h\to {\ell }_1^{+}{\ell }_2^{-}$ may be detected in future experiments for rather relaxed sets of input parameters.

57 pages, 19 figures; v2: added references; v3: a general revision of the manuscript; v4 matches published version

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