Search for Higgs Boson in the channel with missing energy with the detector ALEPH at LEP2

A search for the Standard Model Higgs boson in the channel e⁺e^- → Hνν-bar is presented. An event selection method using Neural Network techniques is developed. The analysis is based on a 254 pb^-1 data sample taken at center-of-mass energies from √s = 161 up to 189 GeV (from 1996 to 1998), by the ALEPH detector within the LEP2 program at CERN. The experimental systematic defects are studied and the corrections and uncertainties are evaluated. The study of this channel alone results in a mass lower limit of the Standard Model Higgs boson of 85.8 GeV/c² with a sensitivity of 89 GeV/c² at the 95% CL. Combined with all studied final states, the limit at 95% CL obtained is: m_H > 90.2 GeV/c² (observed) and m_H > 95.7 GeV/c² (expected). An interpolation of these results within the Minimal Supersymmetric Standard Model is presented along with the combination of all channels, including those originating from the associated production process (e⁺e^- → hA), leading to an exclusion at the 95% CL, for any value of tanβ≥ 1, of the h and A neutral Higgs boson masses of: m_H > 80.1 GeV/c² (observed) and m_A > 81.7 GeV/c² (observed). (author)