On helicity and spin on the light cone

and refer to [2] and [3] for more background information. Here, M is the wanted eigenvalue of the invariant mass squared operator, with associated eigenfunction ψ ≡ Ψqq̄. It is the probability amplitude for finding in the qq̄–space a quark with the effective (constituent) mass m1, longitudinal momentum fraction x, transversal momentum ~k⊥ and helicity λ1, and correspondingly for the anti– quark with m2, 1− x, −~k⊥ and λ2. The spinor factor Sλ1λ2;λ1λ2 is tabulated explicitly in [5]. It is defined in terms of LepageBrodsky spinors [4,5],