$b \bar b W^+ W^-$ production at hadron colliders. Top signal and   irreducible backgrounds
  We compute complete tree level matrix elements for $gg , q \bar q \rightarrow b \bar b W^+W^-$. We analyze the irreducible backgrounds to top signal at the Tevatron and at the LHC. Their contribution to the total cross section is about $5 \%$ at the LHC, due to single resonant channels. Several distributions with contributions from signal and backgrounds are presented. 
A. Ballestrero, E. Maina and M. Pizzio (U. of Torino and INFN-Torino)