Quark
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For other uses, see Quark (disambiguation).
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Quarks (IPA: /kw?rk/) are a class of fundamental particle that, together with leptons form the group known as fermions, which are the constituents of matter. Quarks are confined within and the main constituents of composite particles called hadrons, thus free quarks have never been observed but their existence has been deduced. Quarks are the only fundamental particles that interact through all four of the fundamental forces. There are six different types of quarks, known as flavors: up, down, charm, strange, top and bottom.
The up and down varieties of quarks for a group called the "first generation" and they are by far the most prevalent. They primarily combine in groups of three to form the composite particles called nucleons, namely protons and neutrons. A proton is made up of two up quarks and one down quark; while a neutron contains one up quark and two down quarks. The electron is a lepton and is not a composite particle and thus, is not made of quarks.
Some theorized exotic quark composites are the tetraquark and pentaquark. An exotic environment where a free quark would exist would be a quark-gluon plasma during the quark epoch. Some stars that might demonstrate the physical existence of the quark are the quark star and quark-nova.
