Baryon << BAR ee on >> is a unit of matter smaller than an atom composed of three still smaller particles called quarks. A baryon is a type of composite particle known as a hadron. The other main type of hadron is called a meson. The name baryon comes from the Greek word for heavy. The lightest baryon, the proton, has a mass that is 1,836 times that of an electron. In general, the heavier the baryon, the more unstable it is—that is, the more rapidly it will decay (break down) into smaller particles.
The most familiar baryons include protons and neutrons. Protons and neutrons are called nucleons because they form an atom’s nucleus. Protons are the lightest and most stable baryons. They will last at least a trillion trillion times the present age of the universe. Neutrons are heavier and less stable. The forces that bind neutrons to a nucleus can make neutrons stable. Freed from a nucleus, however, a neutron will decay into a proton and two other units called an electron and an electron-antineutrino. Free neutrons have an average life span of about 15 minutes before they decay.
There are many other types of baryons besides protons and neutrons, all heavier than the nucleons. Unlike nucleons, the heavier types do not occur naturally in atoms. Scientists create them in high-energy devices called particle accelerators. The heavier baryons are highly unstable, with average life spans shorter than one-billionth of a second. Most of these baryons contain heavy quarks, which quickly decay. It may take several decays to produce quarks that form nucleons. A powerful force called the strong interaction binds the quarks in baryons, as well as the nucleons in nuclei.
Scientists discovered the proton in 1919 and the neutron in 1932. They discovered the first unstable baryons in the early 1950’s and have catalogued more than 100 varieties since.
See also Neutron; Proton; Subatomic particle.
