Search for electroweak production of supersymmetric particles in compressed mass spectra with the ATLAS detector at the LHC

In this thesis, two analyses searching for the production of supersymmetric particles through the electroweak interaction are presented: the chargino search, targeting the pair production of charginos decaying into W bosons and neutralinos, and the displaced track search, looking for mildly displaced charged tracks arising from the decays of higgsinos into pions. These searches target compressed phase spaces, where the mass difference between the next-to-lightest and lightest supersymmetric particle is relatively small. The searches use proton-proton collision data collected at a centre-of-mass energy of 13 TeV with the ATLAS detector at the LHC. In the chargino search, the targeted difference in mass between charginos and neutralinos is close to the mass of the W boson. In such phase space, the chargino pair production is kinematically similar to the WW background, which makes the chargino signal experimentally challenging to be discriminated from the WW background. A final state with two leptons from the leptonic decays of the W bosons, missing transverse momentum and no hadronic activity is considered, while advanced machine learning techniques are adopted to separate the supersymmetric signal from the backgrounds. Chargino masses up to about 140 GeV are excluded at 95% confidence level in the case of a mass splitting between chargino and neutralino down to about 100 GeV. The results supersede the previous ATLAS results in particularly interesting regions where the chargino pair production could have hidden behind the looking-alike WW background. In the displaced track search, the difference in mass between the produced supersymmetric particles and the lightest neutralinos goes down to 0.3 GeV. The experimental signature is one jet, missing transverse momentum, and a low momentum charged track with an origin displaced from the collision point, the last element being the first time it is used in a search of this kind at a hadron collider. The results show that the analysis has the sensitivity to exclude different signal hypotheses for higgsino masses up to 175 GeV if no excess is observed in data. For lower masses, the larger signal cross-section allows to achieve higher significance values for different mass splitting scenarios. All these signal hypotheses have not been probed by any existing analysis of LHC data.