SO(10)

A highly elegant and influential Grand Unified Theory (GUT) based on the symmetry of the Special Orthogonal group in 10 dimensions. Its great appeal is that a single 16-dimensional spinor representation of SO(10) can neatly contain all 16 fermions of one standard model generation - the 15 known quarks and leptons (including a right-handed neutrino) - unified as different states of a single fundamental entity. It automatically includes the right-handed neutrino, crucial for generating tiny neutrino masses via the seesaw mechanism. SO(10) predicts the existence of new, ultra-heavy force-carrying particles (X and Y bosons) that mediate proton decay. A key feature is that it can break directly down to the Standard Model in a single step, offering a potentially clean and testable path to unification at an energy scale of around 10^16 GeV.