Naturalness in the Dark at the LHC

TL;DR

This paper investigates the Twin Higgs mechanism as a dark, natural solution to the little hierarchy problem, introducing a minimal Fraternal Twin Higgs model in which twin particles are SM-neutral and a confining twin color sector generates rich Hidden Valley phenomenology. The authors derive the one-loop effective potential, discuss matching to the SM EFT, and show that electroweak tuning can be mild (roughly 2 v^2/f^2), with f ≈ 3 v yielding ~20% tuning. They then analyze the twin color dynamics, confinement, and the spectrum of twin hadrons (glueballs and bottomonium), establishing production and decay channels via Higgs portals, including potentially visible and long-lived decays with displaced vertices at the LHC. The collider implications are explored in depth, highlighting displaced Higgs decays to twin hadrons as a key signature, alongside heavy twin-Higgs decays and precision Higgs measurements, with practical search strategies outlined. Overall, the work demonstrates a concrete, testable hidden-sector naturalness framework that motivates a broad program of LHC and future-lepton collider searches for exotic Higgs decays and long-lived particles.

Abstract

We revisit the Twin Higgs scenario as a "dark" solution to the little hierarchy problem, identify the structure of a minimal model and its viable parameter space, and analyze its collider implications. In this model, dark naturalness generally leads to Hidden Valley phenomenology. The twin particles, including the top partner, are all Standard-Model-neutral, but naturalness favors the existence of twin strong interactions -- an asymptotically-free force that confines not far above the Standard Model QCD scale -- and a Higgs portal interaction. We show that, taken together, these typically give rise to exotic decays of the Higgs to twin hadrons. Across a substantial portion of the parameter space, certain twin hadrons have visible and often displaced decays, providing a potentially striking LHC signature. We briefly discuss appropriate experimental search strategies.

Figures (10)

• Figure 1: Example of a Twin Higgs collider event. The SM-like Higgs decays through a loop of the twin tops into a pair of twin gluons, which subsequently hadronize to produce various twin glueballs. While some glueballs are stable at the collider scale, ${G_{0+}}$ decay to Standard Model particles is sufficiently fast to give LHC-observable effects, including possible displaced vertices. The $h{\hat{g}}{\hat{g}}$ coupling, indicated by a black dot, is generated by small mixing of the Higgs and the twin Higgs.
• Figure 2: Cancellation of the top divergence in the Twin Higgs model. The effective vertex in the second diagram arises upon integrating out the heavy radial mode.
• Figure 3: The confinement scale $\hat{\Lambda}_3$ of the twin $SU(3)$ coupling given fractional variations in $\hat{g}_3$ and $\hat{y}_b$ at the cutoff $\Lambda$ for the minimal Twin Higgs (dependence on $\hat{y}_t$ is negligible). Here we take $\Lambda = 10 \hat{m}_t$ and $f = 3v$. The mild kinks are due to the $\hat{b}$ threshold.
• Figure 4: Sketch of the twin hadron spectrum in the regime where $m_0<2m_{\hat{b}}<2m_0$. In addition to the ${G_{0+}}$, of mass $m_0$, about a dozen other glueballs, with mass splittings of order $m_0$, are stable against twin strong decays. Numerous twin bottomonium states, including a tower of $0^{++}$ states ${\hat{\chi}}$, are stable against twin strong decays. The circled ${G_{0+}}$ and $G'_{0+}$ glueballs, and potentially the ${\hat{\chi}}$ quarkonia, can dominantly decay via annihilation through an $s$-channel off-shell Higgs to the SM.
• Figure 5: The parameter space of the model in terms of the masses of the lightest glueball ${G_{0+}}$ and the lightest quarkonium ${\hat{\eta}}$. In region A, only glueballs are produced; in region B, the relevant quarkonia decay to glueballs; in region C, glueballs are either not produced or decay to quarkonia, so only quarkonia appear in the final state; and in region D there are both metastable glueballs and metastable quarkonia, with the potential for mixing. Solid lines indicate kinematic boundaries.