Poincaré invariant coupled channel model for the pion-nucleon system

An exactly Poincaré invariant front form model for the pion-nucleon system is constructed in the space spanned by |B and |B states where B=N,N*, and = . A mass-square operator is constructed in the form M2=M02+V where M0 is a noninteracting mass operator and V is an interaction. Assuming that the spin operator is a free spin operator, the most general form for the interaction V is deduced. A fit to the N elastic scattering amplitudes for pion laboratory kinetic energies up to 700 MeV is carried out, under the assumption that the B-B interactions are separable.