Electroweak Precision Constraints on Vector-like Fermions

TL;DR

This paper investigates how adding vector-like fermions to the Standard Model affects electroweak precision measurements, providing constraints on their masses and mixing, and exploring implications for the Higgs boson mass.

Contribution

It introduces a formalism for analyzing electroweak corrections from vector-like fermions and derives constraints on their masses and mixing parameters.

Findings

The extension constrains the neutral fermion mass differences.

Electroweak parameters $ ilde{S}$, $W$, $Y$ are weakly constrained by data.

The model allows a heavier Higgs boson consistent with precision tests.

Abstract

We calculate the oblique electroweak corrections and confront with the experiments in an extension of the Standard Model. The new fields added are a vector-like weak doublet and a singlet fermion. After electroweak symmetry breaking there is a mixing between the components of the new fields, but no mixing allowed with the standard fermions. Four electroweak parameters, $\hat{S}$, $\hat{T}$, W, Y are presented in the formalism of Barbieri et al., these are the generalization of the Peskin-Takeuchi S, T, U's. The vector-like extension is slightly constrained, $\hat{T}$ requires the new neutral fermion masses not to be very far from each other, allowing higher mass difference for higher masses and smaller mixing. $\hat{S}, W, Y$ gives practically no constraints on the masses. This extension can give a positive contribution to $\hat{T} $, allowing a heavy Higgs boson in electroweak precision tests of the Standard Model.