Article Outline
Hadron, family of elementary particles. All known particles of matter and energy are classified as hadrons, leptons, or force carriers. Scientists believe leptons and force carriers are fundamental particles, meaning they cannot be divided into smaller particles. Hadrons, on the other hand, have an underlying structure of smaller particles called quarks. The most common hadrons are protons and neutrons, the building blocks of the nucleus in an atom.
Any particle composed of quarks is a hadron. There are two known classes of hadrons: baryons and mesons. Baryons are composed of three quarks, while mesons consist of a quark and an antiquark, the antiparticle counterpart of a quark. There are six known types of quarks: up, down, charm, strange, top, and bottom. Antiquarks are named antiup, antidown, anticharm, antistrange, antitop, and antibottom.
Theoretically a baryon can consist of any three quarks, but only certain combinations have been observed. For example, two up quarks and one down quark make a proton, while two down quarks and an up quark make a neutron. Baryons composed of up and down quarks are the most common and most stable, but in experiments physicists have observed baryons containing other quarks. The omega particle, for example, contains three strange quarks, while the charmed lambda contains a charm, an up, and a down quark. A baryon called the beauty lambda, composed of a bottom, an up, and a down quark, was observed by scientists in 1981. Baryons containing the less common quark combinations tend to decay into other particles very quickly.
Mesons consist of one quark and one antiquark. The pi-plus meson, for example, is made of an up quark and an antidown quark. As with baryons, there are mesons with strange quarks (K-mesons), charm quarks (D-mesons), and bottom quarks (B-mesons). When physicists at the Fermi National Accelerator Laboratory (Fermilab) in Illinois discovered the top quark in 1995, they found it as part of a meson containing a top and an antitop quark. Mesons quickly decay into other particles.
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