Invisible Higgs boson decay into massive neutrinos of fourth generation

Results from several recent experiments provide indirect evidence in favor of the existence of a fourth generation neutrino. Such a neutrino with a mass m of about 50 GeV is compatible with current physical and astrophysical constraints and well motivated in the framework of superstring phenomenology. If sufficiently stable, the existence of such a neutrino leads to a drastic change in Higgs boson physics: for a wide range of Higgs boson masses the dominant mode of Higgs boson decay is invisible and the branching ratios for the most promising modes of Higgs boson search are significantly reduced. The proper strategy of Higgs boson searches in such a framework is discussed. It is shown that in the same framework the absence of a signal in the search for invisible Higgs boson decay at the CERN LEP means that the mass of the Higgs boson is greater than 113.5 GeV if the neutrino mass is about 50 GeV.