The ATLAS experiment at CERN is a large multi-purpose detector operating with outstand- ing performances. For its upgrade phase during the High Luminosity LHC, the tracking system will be replaced with a new system completely made of silicon detectors called ITk. Novel detectors, based on the CMOS process, have been considered for the outer layer of the ITk pixel subsystem. This thesis presents measurements obtained with prototype devices produced in the CMOS technology, and discusses the possibility of including these sensors in the ITk. One of the biggest achievements of the ATLAS experiment is the discovery of the Higgs boson in 2012. While being a keystone for the validation of the Standard Model, the Higgs boson can also be a probe for new physics, especially by investigating its coupling to invisible particles. In this context, the llνν final state can play a major role. This thesis introduces a data-driven method for estimating the yield of the Z + jets background, one of the most important background processes affecting this analysis. The results obtained with Monte Carlo events prove that the method can accurately reproduce the background yield. This is an important step to improve the evaluation of this difficult background.