NLO tools and the background to Hrightarrow WW

A theory input is needed for the estimation of the largest background for the $H\rightarrow WW$ search at the LHC. This is the shape of the $m_{\ell\ell}$ spectrum from continuum $WW$ production. We find this to depend on how NLO matrix elements are merged with parton showers, and we compare the results from a number of different implementations. The results suggest that both the size of the background estimate and its uncertainty may have been underestimated. This conclusion is reinforced by a "Note Added", which comments on the LHC results released on November 14 2012.