Quark particles were first proposed by Murray Gell-Mann and George Zweig in 1964.
In this model, hadrons such as neutrons and protons are composed of more basic units - quarks. They have a fractional charge, 2/3 or -1/3 times the base charge, and a spin of 1/2.
The spatial scale is the smallest among microscopic particles, about less than 10 -19 meters. In the original quark model, up, down, and strange quarks and their antiparticles could be used to explain the hadrons discovered at that time and predict Ω, which was soon not observed experimentally.
When the quark model was first established, there was no physical evidence to confirm the existence of quarks until SLAC developed the deep inelastic scattering experiment in 1968. Currently, six flavors of quarks have been observed experimentally, and the top quark, which was first observed at Fermilab in 1995, is the last one discovered.
Extended information
There are six kinds of quarks, which form three pairs of each other. The first pair is an up quark and a down quark. The up quark has a charge of 2/3 and the down quark has a charge of -1/3.
They are particles in the quark family that correspond to the electrons and their neutrinos in the lepton family. A proton is made up of two up quarks and one down quark, and a neutron is made up of one up quark and two down quarks.
The next pair of quarks are named strange quarks and charm quarks, and their lepton counterparts are muons and neutrinos. The heaviest pair are the bottom quark and the top quark, and their lepton counterparts are the tau particle and its neutrino.
Every three combinations of these quarks can form other hadrons. Unlike leptons, quarks also have a property called color (an arbitrary name), which is similar to charge, but has three categories.
The final color of all hadrons produced by the proper combination of three quarks is equal to 0 - this is why every baryon, including protons and neutrons, contains three quarks. The color force (or glue force) between quarks is carried by gluons, and the strong nuclear interaction is a side effect of the color force acting between quarks.
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