Constraints from the S-parameter on models with scalar triplets

TL;DR

This paper investigates how electroweak precision measurements, especially the S-parameter, constrain models with scalar triplets like the Georgi-Machacek model, limiting mass splittings unless additional fermions are introduced.

Contribution

It provides a detailed analysis of S-parameter constraints on scalar triplet models, highlighting restrictions on mass splittings without extra fermionic fields.

Findings

Large mass splittings are disallowed by S-parameter constraints.

Scalar triplet models are highly restricted by electroweak precision data.

Future Higgs factories will tighten these constraints further.

Abstract

Models with scalar triplets can have a large impact on the electroweak precision S-parameter and have to be dealt "with extreme prejudice". Such models, and in particular the Georgi-Machacek (GM) model, involve a complex triplet scalar containing doubly-charged Higgses plus a real triplet and are of recent phenomenological interests. It turns out that large mass splittings within the triplet will not be allowed by the S-parameter in the absence of extra fermionic degrees of freedom. Simply speaking, having only an extra triplet of scalars severely restricts the mass splitting within that triplet even with the present precision on the S-parameter which will further be improved with the proposed Higgs factories.