  The decay of a Higgs boson into a pair of W bosons h --> W^+W^-, is a
dominant mode for Higgs boson masses above 135 GeV. At hadron colliders,
searches for this decay focus on channels in which both W bosons decay
leptonically into charged leptons, h --> W^+ W^- --> l^+ l^- plus missing
energy. We show that semileptonic decays of heavy flavors are an important
background to this signal. Lepton isolation provides too little suppression of
heavy flavor contributions, and an additional 4 to 8 orders-of-magnitude
suppression must come from physics cuts. An increase of the cut on the the
minimum transverse momentum of non-leading leptons in multilepton events is one
effective way to achieve the needed suppression, without appreciable loss of
the Higgs boson signal.
