[mod edit] This question was originally asked at https://physics.stackexchange.com/questions/115053/whats-a-lepto-diquark This questions refers to Slansky's Group theory for unified model building, page 106 of chapter 7. He assigns the weight (1)(01), which is stepwise projected from E6 to SU(2)×SU(3), to a state which he calls a 4/3-charged antilepto-diquark. I assume that the diquark comes from the 2×2/3 for the charge and the lepton from the fact that it will probably be contained in a multiplet together with the leptons... What I don't understand: Where does the charge assignement 4/3 comes from? On the basis of the weight I would have called it a anti-up quark, since it is the upper component of the doublet, the 1 in the first brackets, and transforms as a 3¯ under SU(3) (since (01) is the corresponding fundamental weight). Hence I would assign it a charge 1/3, which later on will be used to determined the charges of the other particles. How does he comes to the conclusion that it must be a diquark? He comments that it mediates the proton decay. I thought that usually a force is mediated by a boson... Does he means that since the quarks will be together with the leptons in a multiplet, the baryon number does not need to be conserved. Which unable the proton to decay? Is this diquark something realistic or is it an out-dated object, i.e. ruled out by experiments? When and why was it postulated?