Why is the ninth month called September?
Any of three unstable mesons, one having charge +1 and a mass of 966 electron masses, and two being electrically neutral, with a mass 974 electron masses. Their half-life is approximately 10-8 seconds, and they decay through the weak force. Their decay patterns suggest that CP invariance may be violated. Also called K-meson, K particle. See Table at subatomic particle.