“…where D S () stands for the (2S + 1)-dimensional representation of the group SL(2,C) universal covering of the Lorentz group, ΣS are the (2S + 1) × (2S + 1) matrices representing the Lorentz generators, [p] is the so-called standard Lorentz transformation which takes the unit time vector t into the direction of the four-momentum p (36). The spin density matrix in eq.…”