The Vacuum Polarization: Power Corrections beyond OPE ?
  We compute the vacuum polarization on the lattice using non-perturbatively O(a) improved Wilson fermions. The result is compared with the operator product expansion (OPE). 
M. Gockeler, R. Horsley, W. Kurzinger, V. Linke, D. Pleiter, P.E.L.   Rakow and G. Schierholz