Interference effects for H --> WW --> lvqq and H --> ZZ --> llqq searches in gluon fusion at the LHC

Signal-background interference effects are studied for H --> WW and H --> ZZ searches in gluon fusion at the LHC. More specifically, the interference in the channels with semileptonic weak boson pair decay is analysed for light and heavy Higgs masses with minimal and realistic experimental selection cuts. In the semileptonic decay modes, the interference is affected by tree-level background contributions enhanced by 1/e^2 relative to the gluon-fusion continuum background in the fully leptonic decay modes. We find that for both light and heavy Higgs masses the interference with the loop-induced weak-boson pair background dominates over the interference with the tree-level weak-boson plus jets background for a range of selection cuts. We therefore conclude that higher-order background contributions can induce leading interference effects. With appropriate background suppression cuts the interference can be reduced to the 10% level for heavy Higgs masses, and to the per mille level for the light SM Higgs.