Calculation of the Two-Loop Heavy-Flavor Contribution to Bhabha   Scattering
  We describe in detail the calculation of the two-loop corrections to the QED Bhabha scattering cross section due to the vacuum polarization by heavy fermions. Our approach eliminates one mass scale from the most challenging part of the calculation and allows us to obtain the corrections in a closed analytical form. The result is valid for arbitrary values of the heavy fermion mass and the Mandelstam invariants, as long as s,t,u >> m_e^2. 
R. Bonciani, A. Ferroglia, and A.A. Penin