In physics, the quark model is a classification scheme for hadrons in terms of their valence quarks—the quarks and antiquarks which give rise to the quantum numbers of the hadrons.
The quark model was originally just a very good classification scheme to organize the depressingly large number of hadrons that were being discovered starting in the 1950s and continuing through the 1960s but it received experimental verification beginning in the late 1960s and continuing to the present. Hadrons are not "fundamental", but their "valence quarks" are thought to be, the quarks and antiquarks which give rise to the quantum numbers of the hadrons.
These quantum numbers are labels identifying the hadrons, and are of two kinds. One set comes from the Poincaré symmetry—JPC, where J, P and C stand for the total angular momentum, P-symmetry, and C-symmetry respectively. The remainder are flavour quantum numbers such as the isospin, strangeness, charm, and so on. The quark model is the follow-up to the Eightfold Way classification scheme.
All quarks are assigned a baryon number of 1⁄3. Up, charm and top quarks have an electric charge of +2⁄3, while the down, strange, and bottom quarks have an electric charge of −1⁄3. Antiquarks have the opposite quantum numbers. Quarks are also spin-1⁄2 particles, meaning they are fermions.
Mesons are made of a valence quark−antiquark pair (thus have a baryon number of 0), while baryons are made of three quarks (thus have a baryon number of 1). This article discusses the quark model for the up, down, and strange flavours of quark (which form an approximate SU(3) symmetry). There are generalizations to larger number of flavours.
Read more about Quark Model: History, Mesons, Baryons, States Outside The Quark Model
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