# Spontaneous Twin Symmetry Breaking

## TL;DR

This paper explores a twin Higgs model with spontaneously broken $Z_2$ symmetry, linking Higgs metastability to the symmetry breaking scale, and discusses how introducing twin vector-like leptons can reduce fine-tuning.

## Contribution

It introduces a twin Higgs scenario with spontaneous $Z_2$ breaking and shows how twin vector-like leptons can lower the required symmetry breaking scale, reducing tuning.

## Key findings

- The $SU(4)$ breaking scale $f$ is linked to Higgs metastability.
- Minimal model with exact twin symmetry predicts $f o 10^{10}$ GeV, implying unnaturalness.
- Adding twin vector-like leptons reduces the tuning and lowers $f$ to around 2.7 TeV.

## Abstract

We consider a twin Higgs scenario where the $Z_2$ twin symmetry is broken spontaneously, not explicitly. This scenario provides an interesting interpretation of the Higgs metastability in the standard model; the $SU(4)$ breaking scale $f$ is determined by the scale where the Higgs self quartic coupling flips its sign. However, for the misalignment of nonzero vacuum expectation values of the twin Higgs fields, parameter tuning of ${\cal O}(m_h^2/f^2)$ is required like explicit twin symmetry breaking scenarios. For the minimal model with the exact twin symmetry, $f$ is ${\cal O}$($10^{10}$ GeV), so the model is very unnatural. We point out that the tuning can be significantly reduced ($f\gtrsim 2.7$ TeV) if there are twin vector-like leptons with large Yukawa coupling to twin Higgs fields.