Abstract : This thesis presents a three-year work in the CMS experiment, in the context of the first LHC proton-proton collisions. In particular, my studies concentrated on Higgs boson searches. This particle, whose existence is predicted by the Standard Model, has not been observed yet, and constitutes the main Physics target of many high-energy colliders. In the recent past, the LEP and Tevatron experiments allowed to exclude, at 95% confidence level, the existence of the Higgs boson for masses below 114 GeV/c2 and in the [147, 179] GeV/c2 mass range. Therefore, at the moment the subject of this thesis was being decided, the very low and very high mass ranges emerged as the most interesting and promising regions. With collision data collected in 2011 I have studied the response stability and uniformity of the electromagnetic calorimeter (ECAL). Isolated electrons from W boson decays into electron- neutrino pairs have been used to characterize the ECAL response (local uniformity corrections, crystal transparency corrections, ageing of the readout channels). These studies are of great rel- evance for Higgs boson searches in the di-photon decay channel, which is the reference analysis for low-mass Higgs hypotheses and demands a supreme energy resolution to profit from the very narrow Higgs resonance. The other experimentally-allowed Higgs mass range has been tackled with the Physics analysis channel described in this thesis. In particular, I have studied the decay channel H → WW > lνqq, for Higgs mass hypotheses well above the 2mW threshold. This channel has the largest effective cross section (σ × B) for the Higgs boson, although the final state is contaminated by large background events from Standard Model sources. A complete analysis strategy has been defined, optimizing the selections that allow to enhance the signal presence in the final set of events. I have studied the performances of lepton reconstruction and identification and contributed to the development and characterization of a trigger, specifically designed for this channel. Finally, all sources of systematic uncertainties that affect the statistical interpretation of the result have been addressed. With no significant excess observed on the expected background yield, an upper limit on the Higgs boson production cross-section has been settled, for Higgs masses between about 320 and 400 GeV/c2.