sentences:  The mathematical machinery which is used to ensure this includes gauge symmetry and sometimes also Faddeev–Popov ghosts.According to the optical theorem, the imaginary part of a probability amplitude Im("M") of a 2-body forward scattering is related to the total cross section, up to some numerical factors.Because formula_2 for the forward scattering process is one of the terms that contributes to the total cross section, it cannot exceed the total cross section i.e.