The final state with two b jets and two leptons at the LHC as a probe of the scalar sector

The data from the LHC produced in 2012 at 8 TeV and collected by the CMS experiment are used to probe the scalar sector of the Standard Model (SM) and beyond. Two analyses have been conducted based on an integrated luminosity of 19.5-19.8 fb-1 in the final state with two b jets and two leptons. The decay of the SM Higgs boson to a pair of b quarks is challenging at the LHC. In this thesis we propose the use of a Matrix Element (ME) technique to search for the process Z(ll)h(bb). The ME weights for the background and signal hypotheses are combined thanks to neural networks to search for the presence of the signal. The sensitivity of the analysis does not allow to conclude on the presence or absence of the signal and an upper limit on μ=σ/σSM of 1.6 is derived compatible at 2 standard deviation (s.d.) with the presence of the signal. A search to the well-known ZZ process in the same final state is also conducted giving a result compatible from the expectation from other CMS measurements. The impact of the pile-up on the results of this analysis is also discussed. Several observations are still unexplained by the SM. In this context a search in the frame of two-Higgs-doublets models (2HDMs) is presented. Such models can arise in several beyond standard model theories. The H/A → Z(ll)A/H(bb) processes have been chosen to potentially discover two new resonances. It constitutes the first search at the LHC for these processes. A generic analysis was conducted in order to allow possible recasting of the results in a wide range of models. One interesting excess with a local (global) significance of 2.6 (1.5) s.d. was observed, compatible both in shape and in amplitude with the probed signal with mA = 104 GeV and mH = 270 GeV. Generic limits as a function of mA and mH are derived and used to test a type II 2HDM. Limits for this model is then obtained as a function of mA, mH, cos(β-α) and tanβ. Perspectives to explore low mA in the boosted regime are also discussed.(SC - Sciences) -- UCL, 201