Weak-Interaction Decays ofηMeson

A general discussion of the possible weak leptonic and nonleptonic decays of $\eta$ is presented, with estimates for the rates. The leptonic modes $\eta \to \pi e\left(\mu \right)\nu$ exhibit the unique feature of involving only second-class currents. If these currents are as "strong" as the first-class ones, a value of ∼2×${10}^{-9\mathrm{}}$ is expected for the ratio of the rates of these decay modes to those of the main electromagnetic decays of $\eta$. The only interesting nonleptonic decay, $\eta \to 2\pi$, is a $\mathrm{CP}$-violating process.