Neutrinos and gauge unification

The approximate unification of gauge couplings is the best indirect evidence for low-energy supersymmetry, although it is not perfect in its simplest realizations. Given the experimental evidence for small nonzero neutrino masses, it is plausible to extend the MSSM with three right-handed neutrino chiral multiplets, with large Majorana masses below the unification scale, so that a seesaw mechanism can be implemented. In this extended MSSM, the unification prediction for the strong gauge coupling constant at Mz can be lowered by up to ∼5%, bringing it closer to the experimental value at 1σ, therefore improving significantly the accuracy of gauge coupling unification.