Physical Review D
Expressions for the nucleon wave functions in the covariant spectator theory are derived. The nucleon is described as a system with a off-mass-shell constituent quark, free to interact with an external probe, and two spectator constituent quarks on their mass shell. Integrating over the internal momentum of the on-mass-shell quark pair allows us to derive an effective nucleon wave function that can be written only in terms of the quark and diquark (quark-pair) variables. The derived nucleon wave function includes contributions from S-, P- and D-waves.
Gross, F., Ramalho, G., & Pena, M. T. (2012). Covariant nucleon wave function with S-, D-, and P-state components. Physical Review D, 85(9), 093005.