Unitarity bounds in the Higgs model including triplet fields with custodial symmetry

TL;DR

This paper derives upper bounds on Higgs boson masses in the Georgi-Machacek model using perturbative unitarity, showing that scalar masses are constrained, with the lightest scalar limited to about 270 GeV.

Contribution

It provides the first comprehensive unitarity bounds on Higgs masses in the Georgi-Machacek model considering all elastic scattering channels.

Findings

Scalar masses are bounded from above, some more strictly than in the Standard Model.

The upper bound for the lightest scalar boson is approximately 270 GeV.

The model remains compatible with electroweak precision data due to custodial symmetry.

Abstract

We study bounds on Higgs boson masses from perturbative unitarity in the Georgi-Machacek model, whose Higgs sector is composed of a scalar isospin doublet, a real and a complex isospin triplet fields. This model can be compatible with the electroweak precision data without fine tuning because of the imposed global SU(2)_R symmetry in the Higgs potential, by which the electroweak rho parameter is unity at the tree level. All possible two-body elastic-scattering channels are taken into account to evaluate the S-wave amplitude matrix, and then the condition of perturbative unitarity is imposed on the eigenvalues to obtain constraint on the Higgs parameters. Masses of all scalar bosons turn out to be bounded from above, some of which receive more strict upper bounds as compared to that in the standard model (712 GeV). In particular, the upper bound of the lightest scalar boson, whatever it would be, is about 270 GeV.