Baryon

Baryon (pronounced bar-ee-on)

(1) In physics, a proton, neutron, or any elementary particle that decays into a set of particles that includes a proton; any of a class of elementary particles that have a mass greater than or equal to that of the proton, participate in strong interactions, and have a spin of 1/2 . Baryons are either nucleons or hyperons. The baryon number (baryogensis) is the number of baryons in a system minus the number of antibaryons.

(2) In astronomy, a specific description for objects in the universe composed of conventional atomic matter.

1950–1955: From the Ancient Greek βαρύς (barús & barý(s)) (heavy) +‎ -on (the Ancient Greek -ον (-on) suffix ending neuter nouns and adjectives).  The word was coined by Dutch-American physicist Abraham Pais (1918–2000) and chosen because at the time, most known elementary particles had lower masses than baryons. Baryons are significant in cosmology because the standard model assumes the Big Bang produced a state with equal amounts of baryons and antibaryons.  Baryon is a noun & baryonic is an adjective; the noun plural is baryons.

Of matter

For experts only: The baryon number.

By definition, baryonic matter should only include matter composed of baryons. It should include protons, neutrons and all the objects composed of them (ie atomic nuclei), but exclude things such as electrons and neutrinos which are actually leptons.  Astronomers however apply the term ‘baryonic matter’ rather more liberally, arguing that on astronomical scales, protons and neutrons are always accompanied by electrons (in appropriate numbers for astronomical objects to possess all but zero net charge).  Astronomers therefore use ‘baryonic’ to refer to all objects made of normal atomic matter, essentially ignoring the presence of electrons (which represent less than 0.0005% of the mass).  Neutrinos, on the other hand, are considered non-baryonic by astronomers and physicists alike.  Another of astronomy’s quirks is that black holes are often included as baryonic matter although physicists insist that while most of the matter from which black holes form is baryonic matter, once swallowed by the black hole, this distinction is lost and, although still subject to speculation, the current orthodoxy is that black holes cannot possess properties such as baryonic or non-baryonic.  Objects of baryonic matter include clouds of cold gas, planets, comets, asteroids, stars, neutron stars and (possibly) black holes.  Non-baryonic matter includes neutrinos, free electrons, dark matter, supersymmetric particles, axions, and (possibly) black holes.