Hadron

Computer illustration of two spheres of red light, containing smaller lighted objects within them, interaction through swirls of red light.
Atomic interactions, conceptual image. Computer artwork representing the interactions between atomic and sub-atomic particles. Richard Kail/Getty Images

A hadron is the name for a family of composite particles made of quarks in a bound state. Quarks are bound together into hadrons through the strong nuclear force, mediated by gauge bosons called gluons. Hadrons are classified into a variety of different types, based upon the exact physical structure of quarks within the hadron in question.

Families of Hadrons

Hadrons come in two classes: Baryons and Mesons.

Baryons are made up of three quarks bound together by gluons, while a meson consists of a quark and anti-quark bound together.

Mesons are unstable particles that rapidly decay into other particles, such as electrons, neutrinos, and photons. Meanwhile, baryons such as protons and neutrons make up the nucleus of every atom in the universe, forming the building blocks of all visible matter in the universe.

Spin of Hadrons

A significant physical property of particles is their quantum spin, which determines whether the particle is a fermion or a boson. Hadrons are composed of quarks and gluons.

Since the spin of a composite particle is determined by the sum of the individual components that compose the particle, this gives us a method for determining the spin of baryons and mesons. The quarks are fermions with a half-integer spin, while gluons are bosons with a spin of 1.

This means that a baryon, consisting of 3 quarks, will always have a half integer spin.

Any combination of adding positive or negative one-halves will result in a non-integer spin, such as 1/2 + 1/2 + -1/2 = +1/2 or -1/2 + -1/2 + 1/2 = 1/2 or -1/2 + -1/2 + -1/2 = -3/2. The presence of spin-1 gluons will still result in the baryon having a non-integer spin. This means that baryons will always be fermions.

Similar reasoning leads to the conclusion that mesons will always be bosons. If you add the spins of any two quarks together, you'll arrive at results of either +1, 0, or -1. Again, the gluons will modify the total spin by integer values, but that won't ever change it from an integer to a non-integer. The integer spin result means that all mesons are bosons.

Types of Baryons

Baryons are further divided into nucleons and hyperons. 

Nucleons are the family of particles that make up an atomic nucleus. At present, there are only two known nucleon particles: protons and neutrons. These particles were originally believed to be fundamental particles until research in the 1960's indicated that they were composed of quarks.

Hyperons, on the other hand, are a family of particles that contain a strange quark, but which contain no top, charm, or bottom quarks. Hyperons were studied as early as the 1950s, but specific types were predicted by the work of Murray Gell-Man and Yuval Ne'eman in their model called "the Eightfold Way" and later discovered in 1964 at Brookhaven National Laboratory.  

Types of Hadrons: baryons, mesons, nucleons, hyperons