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.