ABSTRACT
Although the standard model is completely successful in accounting for low-energy phenomena, it leaves unexplained why the gauge group of strong and electroweak interactions is SU(3) × SU(2) × U(1), with very different gauge couplings and why the fermion quantum numbers have their particular values. The idea of Grand Unified Theories (GUTS) is that the SU(3), SU(2) and U(l) are subgroups of a larger gauge symmetry group G and that quarks and leptons belong to the same multiplets of G. This higher symmetry is unbroken above some very large mass scale M X .
