A theory embedding the Standard Model gauge factors in a larger gauge group. Particular embeddings can relate couplings and matter multiplets or predict proton decay. The unification hypothesis alone does not specify a unique mass spectrum or an experimentally established extension.
A quark-lepton unification framework with conventional minimal gauge group . leptons supply a fourth color charge, and a right-handed neutrino completes each family. fermion masses additionally depend on symmetry breaking and Yukawa couplings.
The charge relation combines left and right weak isospin with the generator distinguishing quarks from leptons inside the extended color charge group.
For the right-handed neutrino, and , so . Its colour-singlet and left-weak-singlet assignments make it sterile under the Standard Model after symmetry breaking.
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The Grand Unified Theory (GUT) is a theoretical framework in particle physics that attempts to unify the three fundamental forces of the Standard Model—electromagnetism, the weak nuclear force, and the strong nuclear force—into a single force. The idea behind GUT is that at high energy levels, these three forces are manifestations of a single underlying force, much as different types of magnetism can be seen as different aspects of the same magnetic force.
Appears to be an unsolved physics problem. TODO why? Don't they all fit into the Standard Model already? So why is strong force less unified with electroweak, than electromagnetic + weak is unified in electroweak?