The major issue and the forefront research activity in particle physics todayis the exploration of the mechanism that generates the elementary particlemasses. In the Standard Model that describes three of the four basic forces innature - the electromagnetic, weak and strong interactions - this fundamentalmechanism leads to the existence of a new type of particle, the Higgs boson,which has escaped detection so far. The discovery of this particle, which willhave a paramount importance and far-reaching implications, is the major goal ofthe CERN Large Hadron Collider which recently started operation after 20 yearsof preparation. The observation of the Higgs boson at the LHC and thedetermination of its fundamental properties will be the essential issueaddressed by the present research project. A comprehensive investigation of thevarious Higgs boson detection channels at the LHC, production mechanisms anddecay modes, as well as the major sources of backgrounds will be performed ina way that is as close as possible to the experimental conditions. Precisetheoretical predictions, including higher order quantum effects, will beprovided and the associated uncertainties will be assessed. The implications ofobserving the Higgs particle for the Standard Model and for new physics beyondit, such as supersymmetric theories and models with extra space-timedimensions, will be investigated in detail. Besides the Principal Investigatorwho will devote 80% of his time on the project, the research team will beformed by theoretical physicists from three laboratories in the Paris area, LPTOrsay, LPTHE Jussieu and IPhT Saclay, as well as a staff member of the CERNTheory Unit. This group will be completed by the six postdoctoral fellows andtwo PhD students that will be appointed. The duration of the project, fiveyears, will crucially coincide with the period in which the LHC is expected tomake major breakthroughs in the field under investigation.