Search for supersymmetry with a compressed mass spectrum using the ATLAS detector

In this dissertation a search for supersymmetric partners of the Higgs and gauge bosons (charginos and neutralinos) is presented, using the data from the ATLAS experiment, at the Large Hadron Collider (LHC). Considerations on the naturalness of the Higgs boson mass suggest that the two lightest neutralinos might be a mix of the partners of the Higgs and could have a similar mass. Using this scenario as reference, in the first analysis presented in this thesis, I am searching for final states with pairs of electrons and muons with low transverse momentums coming from the decay of the second lightest neutralino in the lightest, high missing transvers momentum generated by the lightest neutralinos which are non detectable, and an energetic jet coming from QCD initial state radiation. Results are shown for the analysis obtained with the data collected during 2015 and 2016, where a total integrated luminosity of 36.6 fb^-1 where collected. No statistically significant excess was observed. In order to improve the sensitivity of the analysis at even smaller mass splitting between supersymmetric states, a new final state has been investigated where the second lepton is substituted with a track, in order to be able to access a region of the phase space with even lower momentum of the leptons. This is presented in the second analysis in the thesis, using a total integrated luminosity of 139 fb^-1 collected during the whole Run 2, the period of time from 2015 to 2018. Again, no significant excess was observed and limits on the supersymmetric particle masses were set. The second lightest neutralino has been excluded up to a mass of 175 GeV for signals with a difference in mass between the second lightest and the lightest neutralino of 9 GeV, while signals with a mass of the neutralino of 100 GeV have been excluded down to 2.4 GeV in mass difference. Moreover, in this thesis a new simulation that takes into account the effects of the radiation damage on the silicon pixel sensors of the ATLAS detector is presented. Silicon pixel detectors are at the core of the current and planned upgrade of the ATLAS detector at the Large Hadron Collider (LHC). As the closest detector component to the interaction point, these detectors are subjected to a significant amount of radiation over their lifetime. Simulating radiation damage is therefore critical in order to make accurate predictions for current and future detector performance that will enable searches for new particles and forces as well as precision measurements of Standard Model particles such as the Higgs boson. In this thesis are compared the predictions of the evolution of key parameters with real data collected at LHC.