Light pair correction to Bhabha scattering at small angle.

This work deals with the computation of electron pair correction to small angle Bhabha scattering, in order to contribute to the improvement of luminometry precision at LEP/SLC below 0.1% theoretical accuracy. The exact QED four-fermion matrix element for $e^+e^-\to e^+e^-e^+e^-$, including all diagrams and mass terms, is computed and different Feynman graph topologies are studied to quantify the error of approximate calculations present in the literature. Several numerical results, obtained by a Monte Carlo program with full matrix element, initial-state radiation via collinear structure functions, and realistic event selections, are shown and critically compared with the existing ones. The present calculation, together with recent progress in the sector of $O(\alpha^2)$ purely photonic corrections, contributes to achieve a total theoretical error in luminometry at the 0.05% level, close to the current experimental precision and important in view of the final analysis of the electroweak precision data.