HADRON

in physics, any of the subatomic particles that are built from quarks and thus react through the agency of the strong nuclear force. The hadrons embrace mesons (e.g., pions and kaons), baryons (e.g., protons, neutrons, and sigma particles), and their many resonances. All observed subatomic particles except bosons (e.g., photons, W particles, and Z particles) and leptons (e.g., electrons, muons, and neutrinos) are hadrons. Except for protons and for neutrons that are bound in nuclei, all hadrons have short lives and are produced in the high-energy collisions of subatomic particles. The other three basic forces of nature also affect hadron behaviour: all are subject to gravitation; charged hadrons obey electromagnetic laws; and some hadrons break up by way of the weak nuclear force (as in radioactive decay), while others decay via the strong and the electromagnetic forces.