In physics, the Pati–Salam model is a Grand Unified Theory (GUT) proposed in 1974 by Abdus Salam and Jogesh Pati.
Like other GUTs, its goal is to explain the seeming arbitrariness and complexity of the Standard Model in terms of a simpler, more fundamental theory that unifies what are in the Standard Model disparate particles and forces.
The Pati–Salam unification is based on there being four quark color charges, dubbed red, green, blue and violet (or originally lilac), instead of the conventional three, with the new "violet" quark being identified with the leptons.
The model also has left–right symmetry and predicts the existence of a high energy right handed weak interaction with heavy W' and Z' bosons and right-handed neutrinos.
Originally the fourth color was labelled "lilac" to alliterate with "lepton".
Of course, calling the representations things like (4, 1, 2) and (6, 1, 1) is purely a physicist's convention(source?
), not a mathematician's convention, where representations are either labelled by Young tableaux or Dynkin diagrams with numbers on their vertices, but still, it is standard among GUT theorists.
The weak hypercharge, Y, is the sum of the two matrices: It is possible to extend the Pati–Salam group so that it has two connected components.
This explains the name left and right and is one of the main motivations for originally studying this model.
This extra "left-right symmetry" restores the concept of parity which had been shown not to hold at low energy scales for the weak interaction.
This is the simplest extension of the minimal left-right model unifying QCD with B−L.
This model doesn't predict gauge mediated proton decay (unless it is embedded within an even larger GUT group).