QED box amplitude in heavy fermion production

We evaluate the two-photon box contribution to heavy fermion production in electron positron annihilation, that provides O(α2) electromagnetic corrections to the Born cross section. The study of its non-relativistic expansion, relevant at energies close to the threshold of production, is also performed. We also verify that the threshold expansion of the one-loop integrals correctly reproduces our results, thus extending the applicability of this technique to heavy fermion production diagrams. (orig.)