Universality

The decay the $\pi$ meson

$$\pi^{\pm}\rightarrow\mu^{\pm} + \stackrel{(-)}{\nu}_{\mu}$$ (65)

indicates the existence of $\nu_{\mu}$ different from the electronic neutrino. Nevertheless, that reaction may be described by a Lagrangian similar to the one introduced above. If the neutron disintegration is characterized by the weak coupling constant $G_{\beta}$, the decay of the muon

$$\mu^{-}\rightarrow e^{-} + \bar{\nu}_{e} + \nu_{\mu}$$ (66)

by the constant $G_{\mu}$ and the muonic capture

$$\mu^{-} + p \rightarrow n + \nu_{\mu}$$ (67)

by $G_{\mu n}$, the experimental evidence indicates

$$G_{\beta} \simeq G_{\mu} \simeq G_{\mu n} \equiv G_{F}$$ (68)

This is the phenomenological expression of the universality of the weak interactions.