# A Predictive Mirror Twin Higgs with Small $\mathbf{Z}_2$ Breaking

## TL;DR

This paper proposes a modified mirror twin Higgs model with a small Z2 symmetry breaking, introducing a massive twin photon that decays before Big Bang nucleosynthesis, making the model cosmologically viable and experimentally testable.

## Contribution

It introduces a small Z2 breaking in the mirror twin Higgs model with a massive twin photon, resolving cosmological issues and providing testable predictions.

## Key findings

- Twin photon mass around 20 MeV allows viable cosmology.
- Kinetic mixing enables entropy transfer to the Standard Model.
- Future experiments can discover or exclude the twin photon.

## 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 $\mathbf{Z}_2$ 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 $\sim 20$ 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.