The Unnatural Composite Higgs

TL;DR

This paper explores a class of composite Higgs models with a high symmetry breaking scale, addressing hierarchy, unification, and dark matter, and predicts potential collider signatures of new resonances and metastable particles.

Contribution

It introduces a framework for composite Higgs models with a large symmetry breaking scale, providing bounds and phenomenological implications for future experiments.

Findings

Symmetry breaking scale f < 100-1000 TeV, accessible at future colliders.

Dark matter as a pseudo Nambu-Goldstone boson in the SU(7)/SU(6) x U(1) coset.

Metastable colour-triplet pseudo Nambu-Goldstone boson decays with displaced vertices.

Abstract

Composite Higgs models can trivially satisfy precision-electroweak and flavour constraints by simply having a large spontaneous symmetry breaking scale, f > 10 TeV. This produces a 'split' spectrum, where the strong sector resonances have masses greater than 10 TeV and are separated from the pseudo Nambu-Goldstone bosons, which remain near the electroweak scale. Even though a tuning of order 10^{-4} is required to obtain the observed Higgs boson mass, the big hierarchy problem remains mostly solved. Intriguingly, models with a fully-composite right-handed top quark also exhibit improved gauge coupling unification. By restricting ourselves to models which preserve these features we find that the symmetry breaking scale cannot be arbitrarily raised, leading to an upper bound f < 100-1000 TeV. This implies that the resonances may be accessible at future colliders, or indirectly via rare-decay experiments. Dark matter is identified with a pseudo Nambu-Goldstone boson, and we show that the smallest coset space containing a stable, scalar singlet and an unbroken SU(5) symmetry is SU(7) / SU(6) x U(1). The colour-triplet pseudo Nambu-Goldstone boson also contained in this coset space is metastable due to a residual symmetry. It can decay via a displaced vertex when produced at colliders, leading to a distinctive signal of unnaturalness.