Lepton Flavor Violation in the Standard Model Extended by Heavy Singlet Dirac Neutrinos

TL;DR

This paper investigates lepton flavor violation in an extended Standard Model with heavy singlet Dirac neutrinos, providing a formalism for calculating branching ratios, analyzing decoupling effects, and proposing experimental search strategies.

Contribution

It introduces a comprehensive formalism for evaluating LFV processes in models with heavy neutrinos and improves previous results with detailed amplitude and branching ratio analysis.

Findings

Heavy neutrinos decouple at high masses when bounds are applied

Current experimental bounds limit certain LFV processes

B-meson LFV processes are promising for future searches

Abstract

Low energy neutrinoless lepton flavor violating (LFV) processes are studied in an extension of the Standard Model (SM) by heavy SU(2)xU(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 applied model the formalism not treated in the applied model the formalism for evaluating the branching ratios (BRs) is presented. The processes previously studied in the model are carefully examined. Some of the previous results are improved. Special attention is paid to the structure of the amplitudes and BRs as well as to the relations between BRs of different LFV processes. Numerical analysis of the BRs is done. The decoupling of heavy neutrinos is discussed and it is explicitely shown that the very heavy neutrinos decouple when the upper bound procedure is applied. The upper limits of the BRs are compared with the current experimental upper bounds and the processes interesting for the search for LFV are proposed. The LFV decays are shown to be unsuitable for finding upper bounds on "diagonal" LFV parameters. The B-meson LFV processes are suggested for the search of LFV in future B-factories.