baryon [Gr.,=heavy], class of elementary particles that includes the proton, the neutron, and a large number of unstable, heavier particles, known as hyperons. From a technical point of view, baryons are strongly interacting fermions; i.e., they experience the strong nuclear force and are described by the Fermi-Dirac statistics, which apply to all particles obeying the Pauli exclusion principle. All members of the baryon family of particles adhere to the law of conservation of baryon family number (see conservation laws, in physics); the baryon family number is +1 for ordinary baryons and -1 for antibaryons (see antiparticle). In any particle interaction, the sum of the baryon family numbers of the interacting particles must equal the sum for the resulting particles. In reactions involving only nucleons, this law requires that the total number of nucleons be the same before and after the reaction. In addition to the nucleons (protons and neutrons), other members of the baryon family include the lambda, sigma, delta, xi, and N particles, as well as a series of higher-mass recurrences of each of these particles. These recurrences may be considered excited states of the lowest-mass member of the series.