Interplay between Vector-like Lepton and Seesaw Mechanism:Oblique Corrections

TL;DR

This paper investigates how mixing between right-handed neutrinos and vector-like leptons affects electroweak precision observables, especially the T parameter, providing constraints on new physics models involving TeV-scale particles.

Contribution

It analytically computes oblique parameters considering Majorana neutrino loops and constrains VLL-RHN mixing using experimental data, highlighting implications for TeV-scale new physics.

Findings

VLL-RHN mixing can produce sizable T parameter contributions.

Constraints from PDG-2021 and CDF-II data limit the mixing and mass scale.

TeV-scale VLL and RHN with large mixing are compatible with electroweak precision tests.

Abstract

The non-vanishing neutrino mass strongly hints the existence of right-handed neutrinos (RHNs), singlets of the standard model (SM). However, they are highly decoupled from the SM and difficult to probe. In this work, we consider the Majorana RHNs from the type-I seesaw mechanism may well mix with the heavy neutral lepton dwelling in certain vector-like lepton (VLL), thus acquiring a sizable electroweak charge. Such a simple scenario yields many interesting consequences, and the imprint on oblique corrections, well expected from the mass splitting between components of VLL by virtue of VLL-RHN mixing, is our focus here. We analytically calculate the Peskin-Takeuchi parameters S, T and U with full details, carefully treating the Majorana loop to obtain the self consistent expressions free of divergence. Then, we constrain on the VLL-RHN system which only gives a sizable $T$ parameter using the PDG-2021 data and CDF-II data, separately, by imposing $T\lesssim{\cal O}(0.1)$. It is found that for the RHN and VLL below the TeV scale, with a properly large mixing, stands in the frontier of the electroweak precision test such as W-boson mass.