pion

The term pion refers to any one of three mesons containing up and down quarks and their antiparticles. The positively charged is composed of an up quark and an anti-down quark. Its antiparticle is the negatively charged , which is composed of an anti-up quark and a down quark. The neutral pion, , is made of down, anti-down, up, and anti-up quarks. Pions are the lightest mesons , and play a role in strong interactions in the nuclei of atoms.