In a plot of the Z resonance from e+ e- collisions, why is photon exchange dominant below the center of mass energy below the Z peak?
Look in the link at the matrix element for the photon diagram, just for e+e- to mu+mu-,the term :
is the photon propagator. Photons have a mass of zero, so it does not appear in the propagator as it should, as -m^2.
The Z has a mass close to 90 GeV. At low energy reactions the Z propagator is small, due to the large Z mass, and the matrix element terms of the Z contributions as long $Q^2$ is much smaller than the mass of the Z, will be very small, and the photon propagator will dominate. As $Q^2$ passes through the resonance Z dominates, as the diagrams show.