In particle physics, a hadron (pronounced /ˈhædrɒn/, from the Greek: ἁδρός, hadrós, "stout, thick") is a composite particle made of quarks held together by the strong force (as atoms and molecules are held together by the electromagnetic force). Hadrons are categorized into two families: baryons (made of three quarks), and mesons (made of one quark and one antiquark).
The best-known hadrons are protons and neutrons (both baryons), which are components of atomic nuclei. All hadrons except protons are unstable and undergo particle decay–however neutrons are stable inside atomic nuclei. The best-known mesons are the pion and the kaon, which were discovered during cosmic ray experiments in the late 1940s and early 1950s. However these are not the only hadrons; a great number of them have been discovered and continue to be discovered (see list of baryons and list of mesons).
Other types of hadron may exist, such as tetraquarks (or, more generally, exotic mesons) and pentaquarks (exotic baryons), but no evidence currently conclusively suggests their existence.
The term hadron was introduced by Lev B. Okun in a plenary talk at the 1962 International Conference on High Energy Physics. In this talk he said:
Not withstanding the fact that this report deals with weak interactions, we shall frequently have to speak of strongly interacting particles. These particles pose not only numerous scientific problems, but also a terminological problem. The point is that "strongly interacting particles" is a very clumsy term which does not yield itself to the formation of an adjective. For this reason, to take but one instance, decays into strongly interacting particles are called non-leptonic. This definition is not exact because "non-leptonic" may also signify "photonic". In this report I shall call strongly interacting particles "hadrons", and the corresponding decays "hadronic" (the Greek ἁδρός signifies "large", "massive", in contrast to λεπτός which means "small", "light"). I hope that this terminology will prove to be convenient.
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