O(N_f alpha^2) Electromagnetic Charge Renormalization in the Standard   Model
  The O(N_f alpha^2) electroweak radiative corrections to the Thomson scattering matrix element are calculated for a general renormalization scheme with massless fermions. All integrals can be evaluated exactly in dimensional regularization which in several cases yields new results that are remarkably simple in form. A number of stringent internal consistency checks are performed. The Z-gamma mixing complicates the calculation of 1-particle reducible diagrams considerably at this order and a general treatment of this problem is given. Conditions satisfied by O(N_f alpha^2) counterterms are derived and may be applied to other calculations at this order. 
Paresh Malde and Robin G. Stuart (Randall Laboratory of Physics)