Prospects for observing an invisibly decaying Higgs boson in the ttH production at the LHC

The prospects for observing an invisibly decaying Higgs boson in the ttH production at LHC are discussed. An isolated lepton, reconstructed hadronic top-quark decay, two identified b-jets and a large missing transverse energy are proposed as the final state signature for event selection. Only the standard model backgrounds are taken into account. It is shown that the ttZ, ttW, bbZ and bbW backgrounds can individually be suppressed below the signal expectation. The dominant source of background remains the tt production. The key for the observability will be an experimental selection which allows further suppression to be achieved of the contributions from the tt events with one of the top quarks decaying into a tau lepton. Depending on the details of the final analysis, an excess of the signal events above the standard model background of about 10% to 100% can be achieved in the mass range m/sub H/ = 100-200 GeV. (15 refs).