2. Quantum Chromodynamics

The quark model

In 1964, Gell-Mann and Feynman’s PhD student George Zweig, who was working at CERN, proposed that the baryons and mesons are bound states of the hypothetical triplet particles (Gell-Mann 1964, Zweig 1964). Gell-Mann called the triplet particles “quarks”, using a word that had been introduced by James Joyce in his novel Finnegans Wake.

Since the quarks form an SU(3) triplet, there must be three quarks: a u quark (charge 2/3), a d quark (charge –1/3) and an s quark (charge –1/3). The proton is a bound state of two u quarks and one d quark (uud). Inside the neutron are two d quarks and one u quark (ddu). The Λ hyperon has the internal structure uds. The three Σ hyperons contain one s quark and two u or two d quarks (uus or dds). The Ξ hyperons are the bound states uss and dss. The Ω is a bound state of three s quarks: sss. The eight mesons are bound states of a quark and an antiquark.

In the quark model, the breaking of the SU(3)-symmetry can be arranged by the mass term for the quarks. The mass of the strange quark is larger than the masses of the two non-strange quarks. This explains the mass differences inside the baryon octet, the baryon decuplet and the meson octet.

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