# Breaking Mirror Twin Hypercharge

## TL;DR

This paper proposes a simple extension to the Mirror Twin Higgs model that introduces a new hypercharge scalar, which is a pseudo-Nambu-Goldstone boson, and explores its implications for twin sector fermion masses and experimental signatures.

## Contribution

It addresses the nature of twin hypercharge and the origin of $ ext{Z}_2$ symmetry breaking within a minimal model with spontaneous symmetry breaking.

## Key findings

- A new hypercharge scalar appears as a pseudo-Nambu-Goldstone boson.
- Couplings between the hypercharge scalar and matter can generate twin fermion masses.
- Experimental signatures include flavor violation, neutrino masses, and collider signals.

## Abstract

The Twin Higgs scenario stabilizes the Higgs mass through an approximate global symmetry and has remained natural in the face of increasingly stringent LHC bounds on colored top partners. Two basic structural questions in this framework concern the nature of the twin hypercharge gauge symmetry and the origin of the $\mathbb{Z}_2$ symmetry breaking needed to achieve the correct vacuum alignment. Both questions are addressed in a simple extension of the Mirror Twin Higgs model with an exact $\mathbb{Z}_2$ symmetry and a scalar field that spontaneously breaks both twin hypercharge and $\mathbb{Z}_2$. Due to the $\mathbb{Z}_2$ symmetry and an approximate $U(2)$ symmetry in the potential, a new hypercharge scalar appears in the visible sector and, like the Higgs, is a pseudo-Nambu-Goldstone boson with a weak-scale mass. Couplings between the hypercharge scalar and matter provide a new dynamical source of twin sector fermion masses. Depending on the nature and size of these couplings, a variety of experimental signatures may arise, including quark and lepton flavor violation, neutrino masses and mixings as well as direct collider probes of the hypercharged scalar. These signals are correlated with the twin matter spectrum, which can differ dramatically from the visible one, including dynamical realizations of fraternal-like scenarios.