thumb|upright=1.5|A hadron is a [[Composite particle|composite subatomic particle. Every hadron must fall into one of the two fundamental classes of particle, bosons and fermions.]]
In particle physics, a hadron is a composite subatomic particle made of two or more quarks held together by the strong nuclear force. Pronounced , the name is derived . They are analogous to molecules, which are held together by the electric force. Most of the mass of ordinary matter comes from two hadrons: the proton and the neutron, while most of the mass of the protons and neutrons is in turn due to the binding energy of their constituent quarks, due to the strong force.
Hadrons are categorized into two broad families: baryons, made of an odd number of quarks (usually three), and mesons, made of an even number of quarks (usually two: one quark and one antiquark). Two pentaquark states (exotic baryons), named and , were discovered in 2015 by the LHCb collaboration. (see Free neutron decay).
Hadron physics is studied by colliding hadrons, e.g. protons, with each other or the nuclei of dense, heavy elements, such as lead (Pb) or gold (Au), and detecting the debris in the produced particle showers. A similar process occurs in the natural environment, in the extreme upper-atmosphere, where muons and mesons such as pions are produced by the collisions of cosmic rays with rarefied gas particles in the outer atmosphere.
Terminology and etymology
The term "hadron" is a new Greek word introduced by L. B. Okun in a plenary talk at the 1962 International Conference on High Energy Physics at CERN.
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