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The generic name of a class of particles which interact strongly with one another. Examples of hadrons are protons, neutrons, the &pgr;, K, and D mesons, and their antiparticles. Protons and neutrons, which are the constituents of ordinary nuclei, are members of a hadronic subclass called baryons, as are strange and charmed baryons. Baryons have half-integral spin, obey Fermi-Dirac statistics, and are known as fermions. Mesons, the other subclass of hadrons, have zero or integral spin, obey Bose-Einstein statistics, and are known as bosons. The electric charges of baryons and mesons are either zero or ±1 times the charge on the electron. Masses of the known mesons and baryons cover a wide range, extending from the pi meson, with a mass approximately one-seventh that of the proton, to values of the order of 10 times the proton mass. The spectrum of meson and baryon masses is not understood. See Baryon, Bose-Einstein statistics, Fermi-Dirac statistics, Meson, Neutron, Proton
Based on an enormous body of data, hadrons are now thought to consist of elementary fermion constituents known as quarks which have electric charges of + |e| and |e|, where |e| is the absolute value of the electron charge. For example, a quark-antiquark pair makes up a meson, while three quarks constitute a baryon. See Elementary particle, Quarks