Gluon-gluon contributions to W⁺W^- production and Higgs interference effects

In this paper we complete our re-assessment of the production of W boson pairs at the LHC, by calculating analytic results for the gg → W ⁺W^? → ?l⁺l^?? process including the effect of massive quarks circulating in the loop. Together with the one-loop amplitudes containing the first two generations of massless quarks propagating in the loop, these diagrams can give a significant contribution with a large flux of gluons. One of the component parts of this calculation is the production of a standard model Higgs boson, gg → H and its subsequent decay, H → W⁺(→ ?l⁺)W?(→ l^??). We will quantify the importance of the interference between the Higgs boson production process and the gluon-induced continuum production in the context of searches for the Higgs boson at the Tevatron and the LHC. For instance, for mH < 140GeV the effect of the interference typically results in around a 10% reduction in the expected number of Higgs signal events. The majority of this interference is due to non-resonant contributions. Therefore cuts on the transverse mass such as those currently used by the ATLAS collaboration reduce the destructive interference to about a 1% effect. We advocate that a cut on the maximum transverse mass be used in future Higgs searches in this channel.