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Figure 2.8: Dilepton invariant mass (left) and missing transverse energy (right) distributionfor signal events for m H = 185 Gev [2].directly and from “reducible” sources caused by mismeasurement of other particlesas photons. While it is possible to reduce the signal-to-background ratio somewhatthrough kinematic selection and isolation requirements, the final sample will still havesignificantly more background events than signal and relies very strongly on an accurateprediction of the background.H → WW → µµνν Analysis The purely leptonic final states that result from H → WWdecays with a BR of about 4% can be selected very efficiently by looking for appropriatemuon pairs with high p T . Backgrounds mostly come from t¯t and b¯b decays, whichcan be effectively suppressed by discarding events with high-E T jets, and Drell-Yanproduced Z bosons, which can be suppressed by requiring significant missing energyin the event (see Fig. 2.8).H → ZZ → 4l Analysis Finally, the H → ZZ decay with purely leptonic end statesoffers a number of options where the background is primarily through direct ZZ production,which is well-understood and of comparable magnitude to the signal. Themost distinctive such signature is the H → ZZ → 4µ decay, which is often referred toas “gold-plated”.The event selection here requires the existence of two opposite sign muon pairs,each consistent with the Z mass. The backgrounds to this signature caused by bquark decays can be suppressed through isolation requirements, leaving direct ZZproduction as an irreducible background, which is most relevant for m H ≈ 2m Z .However, the signal clearly dominates this background in the relevant mass range (seeFig. 2.9).A similar analysis is possible for states for states with one or two electron pairsinstead of muon pairs. In addition to the discovery potential, this channel can be used12