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A baryon is any of a number of subatomic particles made up of three smaller particles, called quarks. The category includes the familiar proton and neutron, as well as a number of unusual and short-lived particles. Baryons belong to a larger family, the hadrons, which is comprised of all particles that are made of quarks and which interact with the fundamental force known as the strong force.
Astronomers refer to ordinary matter — the stuff that's all around us — as baryonic matter, because it derives its mass overwhelmingly from baryons: ordinary matter is made mostly of atoms. Atoms are made mostly of protons, neutrons, and electrons; baryonic protons and neutrons are much heavier than non-baryonic electrons. The word baryon comes from the Greek for "heavy," because the particles were once believed to be the most massive of subatomic particles.
All known baryons are triquarks, particles made of three quarks and only three quarks. Since every subatomic particle has an antiparticle, some scientists theorize that baryons could also be made of larger numbers of quarks and antiquarks, as long as the extra particles cancel each other out to result in a net total of three. For instance, some hypothesize the existence of pentaquarks, made of four quarks and an antiquark. The antiquark would cancel out the effects of the fourth quark, resulting in a baryonic structure. These more exotic particles have never been observed in nature.
There are six known kinds of quarks: up, down, strange, charm, top, and bottom. Baryons are classified according to the types of quarks they contain. There are six known families:
The baryon number refers to the number of the particles present in an experimental scenario. Because quarks cannot be "free," but can only exist in combination with other quarks, any reaction in which baryons are decomposed will result in the quarks recombining to form new baryons. For this reason, the number must be the same both before and after every particle reaction.
As with all other particles, every baryon has an antiparticle. Baryons are apparently much more common in the universe than antibaryons. This discrepancy is the result of an unknown process in the early universe which scientists have tentatively labeled baryogenesis. Each antibaryon is assigned a baryon number of negative one; when any particle collides with its antiparticle, both are destroyed in an explosion of energy.