Search for the Higgs Boson in the Channel H→ZZ(*)→4l with the ATLAS Detector

The decay channel H→ZZ(∗)→4ℓ provides a clean signature for the Higgs boson in the mass range between ∼120 GeV/c2 and 2MZ, above which the gold-plated channel with two real Z bosons in the final state opens up. The signal cross section is several orders of magnitude smaller than those for the backgrounds, therefore a thorough understanding of the multi-lepton processes is needed to obtain a high background rejection. Crucial for this channel is a very good understanding of the trigger and detector response for lepton identification and reconstruction. In order to compute an exclusion limit, a good knowledge of the back- grounds as well as of the signal selection efficiency after all analysis cut is needed. The observability of the signal on top of the reducible t¯t, Zb¯b and W Z and of the irreducible Z Z backgrounds is discussed in detail, with particular emphasis on the lepton reconstruction. We evaluate the ATLAS discovery potential for the H → 4ℓ channel, including the most realistic and up-to-date description of the detector performance.