Diquark Color States, Schur’s Lemma and Casimir Charges

We present a systematic method, called the “tensor method”, to find the set of Hermitian projectors on the irreps of a parton system. Here it is explained in the simplest case of a quark pair, but it can be applied to any parton system (as we shall see in following chapters). We also state the extremely useful Schur’s lemma, define transition operators between equivalent irreps, and write the pictorial expression of the quadratic Casimir operator of a parton system in a given irrep.