Abstract
The twin Higgs mechanism is a solution to the little hierarchy problem in which the top partner is neutral under the Standard Model (SM) gauge group. The simplest mirror twin Higgs (MTH) model — where a Z2 symmetry copies each SM particle — has too many relativistic degrees of freedom to be consistent with cosmological observations. We demonstrate that MTH models can have an observationally viable cosmology if the twin mass spectrum leads to twin neutrino decoupling before the SM and twin QCD phase transitions. Our solution requires the twin photon to have a mass of ∼ 20 MeV and kinetically mix with the SM photon to mediate entropy transfer from the twin sector to the SM. This twin photon can be robustly discovered or excluded by future experiments. Additionally, the residual twin degrees of freedom present in the early Universe in this scenario would be detectable by future observations of the cosmic microwave background.
Highlights
While the twin Higgs mechanism is theoretically appealing, it is difficult to reconcile with cosmological observations
We demonstrate that mirror twin Higgs (MTH) models can have an observationally viable cosmology if the twin mass spectrum leads to twin neutrino decoupling before the Standard Model (SM) and twin QCD phase transitions
The fundamental Z2 symmetry predicts that the entropy of light twin particles is eventually transferred into twin photons and twin neutrinos, which behave as extra radiation components
Summary
The MTH model consists of a twin sector that is related to the SM by a Z2 symmetry at a scale above the SM electroweak scale. Approximate SU(4) global symmetry in the full Higgs sector is spontaneously broken when the twin Higgs doublet acquires a VEV f. Tuning in Twin Higgs models is always greater than ≈ 20%. In a supersymmetric UV completion of the twin Higgs model with an SU(4) symmetric potential from an F term, fine-tuning of a few percent is already required [15], and f /v 10 does not introduce additional fine-tuning. We assume that the gauge coupling constants preserve the Z2 symmetry up to the quantum correction from Z2-breaking fermion masses, which raises the twin QCD scale.
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