For 10 points, and up or down quark and an up or down antiquark compose what lightest meson?
This is the lightest particle to be an eigenstate of the G-parity operator but not the charge conjugation operator. The decay of the phi meson into three of these particles is suppressed since the Feynman diagram can be cut in two while cutting only internal gluon lines according to the OZI rule. Since the kaon can decay into either two or three of these particles, the kaon was originally thought to be two particles: the tau and the theta. The primary decay mode for the (*) neutral version of this particle is into two photons, while charged versions will decay into a muon and a muon neutrino. The exchange of one of these particles between two nucleons was predicted by Hideki Yukawa to mediate the strong force. For 10 points, and up or down quark and an up or down antiquark compose what lightest meson?
This is the lightest particle to be an eigenstate of the G-parity operator but not the charge conjugation operator.
pion [or pi meson, accept more specific answers like pi+ or pi- or pi0]