The decay $\eta \to 3\pi$: study of the Dalitz plot and extraction of the quark mass ratio $Q$

Gilberto Colangelo, Stefan Lanz, Heinrich Leutwyler +1

The $\eta \to 3\pi$ amplitude is sensitive to the quark mass difference $m_u-m_d$ and offers a unique way to determine the quark mass ratio $Q^2\equiv (m_s^2-m_{ud}^2)/(m_d^2-m_u^2)$ f…