Custodial Naturalness

TL;DR

Custodial Naturalness introduces a symmetry-based framework where the Higgs boson emerges as a pseudo-Nambu-Goldstone boson, explaining the electroweak scale's stability and predicting new particles and phenomena testable at future experiments and cosmological observations.

Contribution

It presents a novel symmetry-based mechanism combining classical scale invariance and custodial symmetry, with minimal models and extensions that address the hierarchy problem and predict new testable particles.

Findings

Predicts a new heavy Z' gauge boson.

Forecasts a dilaton-like scalar detectable at colliders.

Suggests gravitational wave signals from a supercooled phase transition.

Abstract

Custodial Naturalness is a new symmetry-based idea to explain the large separation between the electroweak (EW) scale and ultraviolet completions of the Standard Model (SM). Classical scale invariance is combined with an enhanced scalar-sector custodial symmetry and both are spontaneously broken by dimensional transmutation at a new intermediate scale. The SM-like Higgs boson is an elementary pseudo-Nambu-Goldstone-Boson (pNGB) of the extended custodial symmetry, which naturally explains the suppression of the EW scale without a little hierarchy problem. We explain details of the general mechanism, its minimal realization and simplest extensions which populate Higgs-, gauge-, and neutrino portals and introduce candidates for particle Dark Matter (DM). We show the stability of the mechanism under inclusion of new sources of explicit custodial symmetry violation, as well as under variations of boundary conditions at the high scale. Custodial Naturalness is experimentally testable - including a specific correlation between the Higgs and top quark masses, as well as by the prediction of a new heavy $Z'$ gauge boson and a new dilaton-like scalar which are well-motivated targets for future colliders and Higgs factories. The cosmological evolution features a strongly supercooled phase transition implying that consequences of Custodial Naturalness may also be tested by gravitational wave observatories.