Constraining models with a large scalar multiplet

TL;DR

This paper demonstrates that most models extending the Higgs sector with a large scalar multiplet are excluded by cosmological and direct detection constraints, except for a specific T=2 case with a decaying lightest component.

Contribution

It shows that all U(1)-symmetric models with large scalar multiplets are excluded, except for a special T=2 case with a decay mechanism.

Findings

Most large scalar multiplet models are excluded by relic density and direct detection constraints.

The T=2 multiplet is an exception due to its decaying neutral component.

The T=2 case involves a Planck-suppressed decay channel.

Abstract

Models in which the Higgs sector is extended by a single electroweak scalar multiplet X can possess an accidental global U(1) symmetry at the renormalizable level if X has isospin T greater or equal to 2. We show that all such U(1)-symmetric models are excluded by the interplay of the cosmological relic density of the lightest (neutral) component of X and its direct detection cross section via Z exchange. The sole exception is the T=2 multiplet, whose lightest member decays on a few-day to few-year timescale via a Planck-suppressed dimension-5 operator.