Two-loop helicity amplitudes for the production of two off-shell   electroweak bosons in quark-antiquark collisions
  Knowledge of two-loop QCD amplitudes for processes $q\bar q \to V_1 V_2 \to (l_1\bar l'_1)(l_2 \bar l'_2)$ is important for improving the theoretical description of four-lepton production in hadron collisions. In this paper we compute these helicity amplitudes for all intermediate vector bosons, $V_1 V_2 = \gamma^*\gamma^*, WW, ZZ, WZ, W \gamma^*$, including off-shell effects and decays to leptons. 
Fabrizio Caola, Johannes M. Henn, Kirill Melnikov, Alexander V.   Smirnov and Vladimir A. Smirnov
hep-ph