Hadron
Written by Jean Tate
Large Hadron Collider
If a particle contains quarks, and is held together by the strong force, it is a hadron. As far as we know, only two combinations of quarks – and thus two kinds of hadron – are possible: three quarks, which is called a baryon (e.g. proton, neutron), and a quark and anti-quark, which is called a meson (e.g. pion). The name is derived from the ancient Greek word hadros, meaning thick.
Perhaps the best known use of the word hadron is in the Large Hadron Collider, which aims to smash protons (a kind of hadron, remember) together at very high energies; it is located near Geneva, and straddles the French-Swiss border.
The quarks with comprise a hadron determine its properties; for example, the electric charge of a hadron is an integer multiple of the electron's charge (-2, -1, 0, 1, or 2), depending on the quarks which comprise it (so the proton's charge is +1, as it is made up of two up quarks, with +2/3 charge each, and one down, with -1/3 charge). A hadron's mass has, however, little to do with the masses of the quarks of which it is composed; rather, most of the mass arises from the energy associated with the strong interaction (mediated by gluons) which keeps the quarks together in the hadron.
Just as an atom may become excited – one of its electrons moves into a higher energy state – so hadrons may become excited too; these states are called resonances, and there are hundreds and hundreds of them. In atoms, excited states decay electromagnetically (usually by emission of a photon); in hadrons, by the strong force (also sometimes called the strong nuclear force, or the color force); resonances have extremely short half-lives.
At a sufficiently high temperature and pressure, hadrons may cease to exist as independent particles; matter in this form would be a quark-gluon plasma, analogous to an ordinary plasma … the color force would play the role of electromagnetism (and the range of distinct phenomena extremely rich). Study of these phenomena is built on the theory of quantum chromodynamics, the strong interaction counterpart to quantum electrodynamics (USQCD is one collaboration involved in one kind of QCD research).
Aside from stories about the LHC (e.g. Particles Injected into Large Hadron Collider, Will the Large Hadron Collider Destroy the Earth?), Universe Today articles on hadrons include Forget Neutron Stars, Quark Stars Might be the Densest Bodies in the Universe, and the light-hearted The Particle Zoo: Collecting Your Own Subatomic Particles.
Two Astronomy Cast episodes are worth a listen, re hadrons, Nucleosynthesis: Elements from Stars, and The Search for the Theory of Everything.
Filed under: Astronomy
Tags: fundamental particles, hadrons, Physics
