Abstract : The most promising channels for intermediate mass Higgs boson discovery at LHC are leptonic and photonic decays. Therefore, a good uniformity of response of electromagnetic calorimeter is required to reach the 0.7% constant term needed.
This thesis deals with the absolute calibration of this detector. An electrical description of the calibration system, the detector and its read-out chain has been made for a better comprehension of the signal pulse shapes. A method, using a convolution of the calibration waveforms, has been developped to predict physics reponse, leading to absolute calibration. The level of accuracy obtained allows to reach the 0.3% contribution to the constant term required. Test beam analysis of a prototype module showed the performance of the electromagnetic calorimeter in terms of local resolution and linearity. A uniformity study has been made, leading to a 0.8% dispersion on a Delta nu * Delta phi = 1.2*0.75 area.
In a second part, the observability of an invisible Higgs boson produced via weak boson fusion at the LHC is presented. A level 1 trigger strategy for this purely jet and missing E_T final states is discussed. A method to measure the level of background using physics events is presented. This analysis shows that an invisible branching ratio of 25% could be reached at 95% CL with only 30 fb-1 for a Higgs boson mass of 120 GeV/c2.