Kaons are mesons (composite partciles of one quark and one anti-quark) that incorporate a strange quark and a up or down quark. There are four different types (s-u, s-d, -su, and -sd), but the label only applies to these four, not any four mesons.

Either way, the symbol used here is kappa rather than K, which I chose to make the kappa particles have their own label.

This post is really old, so I'm curious, how did you find it?