# How can pure leptonic interactions be mediated by the weak force?

$$\mu^+ \rightarrow e^+ + \nu_e + \bar \nu_\mu$$

Consider the above decay, it is given in my lecture notes as a weak force interaction. I thought the weak force was only involved when quarks change flavours. Here we have no quarks. Why are pure leptonic decays a weak force interaction?

• beta decay is probably the first process where the weak force is mentioned in school. The two vertices of the W boson do both - interact with quarks and with leptons, see e.g. en.wikipedia.org/wiki/Beta_decay#/media/… – Vangi May 24 at 14:35