Resumen de Recerca de nous fenòmens en estats finals amb jets energètics i molta energia transversa faltant en el lhc usant el detector atlas
This thesis comprises the results of the so-called monojet analysis, as performed with a dataset of proton-proton collisions at a center-of-mass energy of 13 TeV recorded by the ATLAS detector between 2015 and 2018, corresponding to a total integrated luminosity of 139 fb⁻¹. The analysis looks for events with at least one high energetic jet, large amount of missing transverse momentum (ETmiss) and no leptons in the final state. A total of 13 signal regions is considered, starting at ETmiss=200 GeV. Background contributions are estimated with simulated Monte Carlo samples, whose normalization is optimized via a simultaneous likelihood fit to the data in dedicated control regions using the ETmiss distribution. Overall, good agreement is observed between the data and the Standard Model (SM) predictions, with a level of uncertainty ranging between 1.5% and 4.7% across the ETmiss spectrum. Using the CLs method, such agreement is then used to set model-independent limits at 95% of confidence-level (CL) on the visible cross section of new phenomena in the range between 736 fb and 0.3 fb. Limits are also set on the parameter space of a number of models beyond the SM, including two simplified dark matter production models, four SUSY quark-pair production models in compressed scenarios and the ADD extra dimensions model, among others. In addition, 95% CL limits are set to the branching ratio of a Higgs boson decaying into the dark sector. In all cases, the obtained limits represent a significant improvement in comparison with respect to previous ATLAS monojet results. Finally, a number of studies on the performance of the Pixel ATLAS detector is presented.
