Meson

In particle physics, a meson is a type of hadronic subatomic particle composed of an equal number of quarks and antiquarks, usually one of each, bound together by the strong interaction. Because mesons are composed of quark subparticles, they have a meaningful physical size, a diameter of roughly one femtometre (10−15 m), which is about 0.6 times the size of a proton or neutron. All mesons are uns... (Wikipedia)