Search for supersymmetric particles with compressed mass spectrum in proton-proton collisions at a centre-of-mass energy of 13 TeV

The main subject of this thesis is a search for Supersymmetric (SUSY) particles utilizing the experimental data of proton-proton collisions at a center-of-mass energy of 13 TeV collected by the Compact Muon Solenoid (CMS) experiment during the Run 2 (2016–2018) operation of the Large Hadron Collider (LHC) accelerator, amounting up to 137 fb$^{−1}$ of integrated luminosity. The data analysis focuses on final states with two or three low transverse momentum ($p_T$) leptons (electrons or muons), at least one pair of which has opposite sign charge and the same flavor. The leptons are accompanied by a moderate or large amount of missing transverse energy. Such signatures can be produced in SUSY models, involving either electroweakino or top squark production, with a compressed mass spectrum, i.e. small mass differences between the lightest SUSY particles. The results of the search show agreement between the observed data and the Standard Model predictions. They are used to extract mass exclusion limits for the SUSY particles of the previously mentioned SUSY models. An important part of this thesis is also dedicated to the development of software and firmware algorithms for the Phase 2 upgrade of the CMS Level-1 trigger (L1T) system. First, a set of corrections is derived for L1T muons captured by the Phase 1 demonstrator of the Phase 2 L1T scouting system. In addition, a multivariate algorithm for the L1T Correlator subsystem is developed with the aim of identifying electromagnetic, hadronic and pile-up clusters from the future High Granularity Calorimeter of CMS. One of the L1T Correlator data organization algorithms, the "deregionizer" algorithm, is also discussed and studies on improvements to it are presented. Finally, the development of a firmware module for sorting according to their $p_T$ the electromagnetic objects of the L1T Correlator, along with its corresponding software emulator, is described.