Once the Higgs boson coupling to the Z, gHZZ, has been determined, the measurement of the cross sections for each exclusive Higgs boson decay, H → XX, gives access to all other copious decays (down to a branching ratio of a few 10−4), and to the corresponding couplings in a model-independent, absolute, way. For example, the ratio of the WW- fusion-to-Higgstrahlung cross sections for the same Higgs boson decay, proportional to g2HWW / g2HZZ , yields gHWW, and the Higgsstrahlung rate with the H → ZZ∗ decay, proportional to g4HZZ provides a first determination of the Higgs boson total decay width.
The H → ZZ∗ decay gives rise to a ZZZ∗ final state, either fully leptonic (six charged leptons), two-third leptonic (four leptons), one-third leptonic (two leptons), or fully hadronic (no leptons). The first two configurations should be essentially background free, and lead to a “straightforward”, albeit statistically limited, determination of the Higgs boson width. The requirements on the detector design to achieve a background-free and highly efficient analysis for final states with at least four leptons will be studied. The other two configurations are potentially more abundant, but are also contaminated by a much larger background, in particular from the ten times more copious H → WW∗ decay, yielding a ZWW∗ final state. These final states, however, are kinematically over- constrained, with the knowledge of the Higgs boson mass (one constraint), of the Z and W masses (two constraints), and four total energy-momentum conservation equations (four constraints), allowing the determination of the jet energies to rely on jet directions rather than on direct energy measurements. The requirements on the detector design and on jet clustering algorithms for jet directions, and the development of 7C kinematic fits, to achieve an effective separation between the H → ZZ∗ and H → WW∗ decays, will be among the important outcomes of this study.