Prospects for observing an invisibly decaying Higgs boson in the ${t bar t H}$ production at the LHC

The prospects for observing an invisibly decaying Higgs boson in the 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 , , and backgrounds can individually be suppressed below the signal expectation. The dominant source of background remains the production. The key for the observability will be an experimental selection which allows further suppression to be achieved of the contributions from the 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_H= 100-200 GeV.Received: 1 July 2002, Revised: 16 May 2003, Published online: 3 July 2003E. Richter-Was: Supported in part by Polish Government grant KBN 2P03B11819