Scattering of electrons on positrons .
For one obtains in first order perturbation theory for the differential cross-section in the centre-of-mass system (c.m.s.):
with
Bhabha scattering is used in e+e- colliders for monitoring and measuring the luminosity.
.
For 
one obtains in first order perturbation theory for the differential cross-section in the centre-of-mass system (c.m.s.):
