Review of Neutral Naturalness

TL;DR

Neutral naturalness offers a symmetry-based solution to the electroweak hierarchy problem by introducing colorless top partners related by a discrete symmetry, thereby canceling Higgs mass divergences without visible-color partners. The review surveys concrete realizations (notably the Twin Higgs and variants), their UV completions, collider phenomenology, nonperturbative hidden-sector dynamics such as glueballs and quirks, and the rich connections to dark matter, cosmology, neutrinos, and flavor. It emphasizes the role of lattice gauge theory as a nonperturbative input to hidden-sector spectra and decays, and highlights exotic collider signatures (hidden-sector glueballs, semivisible/emerging jets, and quirks) as main probes. The work also identifies pressing open questions and promising directions, including UV completions, precise Higgs coupling tests, and novel dark-matter scenarios, underscoring the model\'s testability at current and future collider and astrophysical experiments.

Abstract

The hierarchy between the mass parameter of the Higgs boson and larger mass scales becomes ever more puzzling as experiments explore higher energies. Neutral naturalness is the umbrella term for symmetry-based explanations for these hierarchies whose quark symmetry partners are not charged under the SU(3) color gauge group of the Standard Model. Though the first manifestations of this idea predate the physics runs of the Large Hadron Collider, since the Higgs discovery this paradigm has grown and developed to include a wide variety of concrete realizations with connections to intriguing collider signals. Determining the phenomenology of such models often requires the characterization - typically relying on lattice calculations - of a new confining gauge symmetry. This presents additional motivation to further develop our understanding of nonperturbative field theory as well as to pursue specific lattice studies. The wide range of suggested hidden sectors also produces a variety of dark matter candidates, intersections with astrophysics and cosmology, and ties to neutrinos and flavor. In this review, we orient the reader within both this growing collection of specific models and the physical phenomena they produce. We also survey the often less familiar dynamics of hidden-sector glueballs and quirks. In addition to providing a guide to past efforts, we reveal interesting directions for further study.

Figures (23)

• Figure 1: Schematic contribution to Higgs mass from top quark loop. The scale $\Lambda_\text{UV}$ stands in for interactions due to high mass particles.
• Figure 2: Cancellation of quadratic sensitivity to $\Lambda_\text{UV}$ due to the top quark by its scalar partner stops $\widetilde{t}$ in SUSY.
• Figure 3: Top Row: LHC searches for top squarks at ATLAS ATLAS:2024exu and CMS CMSstopSum. Bottom Row: LHC searches for fermionic top partners at ATLAS ATLAS:2024fdw and CMS CMS:2024bni. Current LHC limits on stops extend up to about 1.3 TeV, depending on the lightest supersymmetric particle (LSP) mass, while bounds on fermionic top partners reach up to around 1.6 TeV, depending on their branching ratios.
• Figure 4: Cancellation of quadratic sensitivity to $\Lambda_\text{UV}$ in folded SUSY. The particle loops are shaded with different colors to illustrate that the SU(3) gauge groups under which the stops are charged need not be the SU(3$)_c$ of the SM quarks.
• Figure 5: Inspired by Curtin:2015fna, an illustration of the types of symmetry based solutions to the electroweak hierarchy problem, how the top partners in each might be produced at a hadron collider and some early examples of each model type. In the first row, the top partners carry SM color charge and are thus copiously produced at the LHC in direct strong interactions.