Low-Energy Effective Theory, Unitarity, and Non-Decoupling Behavior in a Model with Heavy Higgs-Triplet Fields

TL;DR

This paper analyzes a model with a Higgs doublet and triplet, showing how heavy triplet states affect low-energy physics, unitarity, and the non-decoupling behavior, with implications for the triplet mass scale and electroweak phenomenology.

Contribution

It constructs the low-energy effective theory for a Higgs triplet model and clarifies the non-decoupling behavior and unitarity constraints related to heavy triplet bosons.

Findings

Perturbative unitarity breaks down at a scale inversely related to the triplet VEV.

Heavy triplet bosons do not decouple at one-loop, but the model remains consistent with decoupling theorem.

Triplet-Higgs masses at GUT scale require a negligible triplet VEV for perturbative consistency.

Abstract

We discuss the properties of a model incorporating both a scalar electroweak Higgs doublet and an electroweak Higgs triplet. We construct the low-energy effective theory for the light Higgs-doublet in the limit of small (but nonzero) deviations in the rho parameter from one, a limit in which the triplet states become heavy. For small deviations in the rho parameter from one, perturbative unitarity of WW scattering breaks down at a scale inversely proportional to the renormalized vacuum expectation value of the triplet field (or, equivalently, inversely proportional to the square-root of the deviation of the rho parameter from one). This result imposes an upper limit on the mass-scale of the heavy triplet bosons in a perturbative theory; we show that this upper bound is consistent with dimensional analysis in the low-energy effective theory. Recent articles have shown that the triplet bosons do not decouple, in the sense that deviations in the rho parameter from one do not necessarily vanish at one-loop in the limit of large triplet mass. We clarify that, despite the non-decoupling behavior of the Higgs-triplet, this model does not violate the decoupling theorem since it incorporates a large dimensionful coupling. Nonetheless, we show that if the triplet-Higgs boson masses are of order the GUT scale, perturbative consistency of the theory requires the (properly renormalized) Higgs-triplet vacuum expectation value to be so small as to be irrelevant for electroweak phenomenology.