Does the kaon participate in the weak interaction?

Question

I'm looking at a physics textbook for A-level and in the book it states that kaons are only affected by the strong force and the electromagnetic force. Isn't this incorrect? Aren't kaons affected by the weak force as well?

Answer 1

You are correct. All of the quarks and leptons of the Standard Model, and their composites, participate in the strong, electromagnetic, and weak interactions. The leptons, which lack color charge, don't participate in the strong interaction; the neutrinos, which lack electric charge, don't participate in the electromagnetic interaction either. But a particle which doesn't participate in the weak interaction isn't a member of the Standard Model.

The kaon, a hadron with one unit of strangeness, decays via the charged weak current into zero-strangeness pions and/or leptons.

Note that some people will try to tell you that the weak interaction isn't properly a "force." They're wrong, but the approximation is useful, because the interaction is, to use the unfortunately punny technical term, much weaker or feebler than electromagnetism or the strong interaction.

Answer 2

This text book is really a shame. As you had detected correctly, the kaons feel also the weak force, e.g. in the decay mode $K^+ \to \mu^+ \nu_\mu$ being mediated by the exchange of a virtual $W^+$ boson. But this is not the only error in this table. Also the pions interact weakly, in particular the dominant decay mode of the charged pion, $\pi^+ \to \mu^+ \nu_\mu$, is a weak decay. It is also not correct that the neutral pion does not feel the electromagnetic interaction (as suggested in the table), as it decays electromagnetically via $\pi^0 \to \gamma \gamma$. Also the neutron can interact with the electromagnetic field via its magnetic moment.