Lepton flavor violation in the standard model extended by heavy singlet Dirac neutrinos

Abstract

Low-energy neutrinoless lepton flavor-violating (LFV) processes are studied in an extension of the standard model (SM) by heavy $\mathrm{SU}(2)\ifmmode\times\else\texttimes\fi{}\mathrm{U}(1)$ singlet Dirac neutrinos. An upper-bound procedure is elaborated for the evaluation of amplitudes. A comment on the extraction of heavy-neutrino mixings from astrophysical observations is given. For processes not treated in the model applied, the formalism for evaluating the branching ratios (BR's) is presented. The processes previously studied in the model are examined and some results are improved. The structure of the amplitudes and BR's as well as the relations between BR's of different LFV processes are examined. The decoupling of heavy neutrinos is discussed and it is explicitly shown that very heavy neutrinos decouple when the upper-bound procedure is applied. The LFV decays are shown to be unsuitable for finding upper bounds on ``diagonal'' LFV parameters. Comparing the theoretical BR's with curent experimental upper bounds, a few processes interesting for the search for LFV are proposed. Particularly, B-meson LFV processes are suggested for the search of LFV in future B factories.