Search for invisible Higgs boson decays in e+e− collisions at centre-of-mass energies up to 184 GeV

In a data sample of 78.3 pb^-1 collected in 1996 and 1997 by the ALEPH detector at centre-of-mass energies from 161 to 184 GeV, invisible decays of a Higgs boson have been searched for in the reaction e+e- --> hZ, where the Z can decay into e+e-, mu+mu- or qqbar. No evidence for a signal is found and limits on the production cross section are derived as a function of the Higgs boson mass. These results are combined with those obtained in an update of the analyses of the ALEPH data taken at LEP 1. For a production cross section equal to that of the minimal standard model Higgs boson, masses below 80 GeV/c^2 are excluded at 95% C.L.