Effects of heavy Majorana neutrinos on lepton flavor violating processes

TL;DR

This paper investigates how heavy Majorana neutrinos influence lepton flavor violating processes, providing detailed calculations of one-loop contributions and updating constraints on neutrino mixings.

Contribution

It introduces a 5-parameter model with heavy neutrinos, analyzes their effects on flavor violation, and compares Majorana versus Dirac cases with updated experimental bounds.

Findings

Heavy Majorana neutrinos induce significant flavor violating processes.

Analytic expressions for one-loop contributions are provided.

Updated bounds on heavy-light neutrino mixings are derived.

Abstract

The observation of lepton flavor violating processes at colliders could be a clear signal of a non-minimal neutrino sector. We define a 5-parameter model with a pair of TeV fermion singlets and arbitrary mixings with the three active neutrino flavors. Then we analyze several flavor violating transitions ($\ell\to \ell^{\prime}\gamma,\,\ell^{\prime}\ell^{\prime\prime}\bar{\ell}^{\prime\prime\prime}$ or $\mu-e$ conversions in nuclei) and $Z\to\bar{\ell}\,\ell^{\prime}$ decays induced by the presence of heavy neutrinos. In particular, we calculate all the one-loop contributions to these processes and present their analytic expressions. We focus on the genuine effects of the heavy Majorana masses, comparing the results in that case with the ones obtained when the two heavy neutrinos define a Dirac field. Finally, we use our results to update the bounds on the heavy-light mixings in the neutrino sector.