© David Kaplan.
The vertical line of letters in the brackets represents particles of the Standard Model. The d stands for down quark, the e for electron, and the Greek letter (pronounced "nu") for neutrino. Grand unified theories typically group quarks and leptons together into the same grouping of particles. Note that the strong force carrier (the gluon) changes quarks from one color to another. The weak force carrier (here, the W boson) can change an electron to a neutrino. If all of these particles are in the same grouping (technically, "representation" of the symmetry), then there would also be force carriers, X, which change quarks into leptons. (Unit: 2)