Question

Breit-Wigner vs propagator?

Answer 1

I know for the second that we should use the Breit-wigner curve, but what about the first? If we go about using the propagator: 1q~2−m2zc2, (+ - - -) signature then we a singularity when q~2=m2zc2 which is subverted when the Breit-Wigner curve is used. Thus my question is as follows: When is it appropriate to use the propagator as given above and when to use the Breit-Wigner curve.