Hadron << HAD ron >> is a major group of subatomic particles. Hadrons include both protons and neutrons, which are found in an atom’s nucleus. Hadrons are affected by the strong interaction, a force holding the nucleus together.
Hadrons consist of smaller particles called quarks and antiquarks. These smaller particles occur in two combinations, which form the chief types of hadrons—mesons and baryons. A quark linked to an antiquark makes up a meson. Three quarks combined comprise a baryon. The baryon has an antimatter counterpart called an antibaryon, which consists of three antiquarks. See Antimatter.
All hadrons except protons and neutrons are extremely unstable. They disintegrate spontaneously in a few hundred millionths of a second or less. For this reason, only protons and neutrons are found in ordinary matter. However, physicists can readily produce unstable hadrons by causing a beam of high-energy particles to collide with matter. They have discovered nearly 300 kinds of hadrons by this means. In 2015, scientists discovered a rare, unstable type of hadron consisting of five smaller particles, called the pentaquark. A pentaquark contains a baryon (three quarks) and a meson (a quark and an antiquark).
All hadrons are nearly the same size, ranging from about 0.7 to 1.7 femtometers (quadrillionths of a meter) in diameter. But hadrons vary widely in mass. The lightest, the pions, have only about 1/7 of an atomic mass unit (see Atom (Relative atomic mass)). The heaviest, the upsilons, carry more than 10 atomic mass units.
See also Baryon; Gluon; Meson; Quark; Subatomic particle.
