I have a proton and an antiproton scattering, via a pion exchange.
The matrix element has the form:
M=g*(\bar{u}_{1}\gamma ^{5}u_{2})\frac {1} {q^2-m^2}( \bar{v}_{1}\gamma ^{5}v_2)
Wher g is my coupling constant, and q the 4-momentum of the pion.
The problem is that when I compute the...