Search for new physics in the supersymmetric and neutrino sectors

The success of the standard model (SM) of particle physics in describing the strong, electromagnetic, and weak fundamental interactions is considered as one of the greatest achievementsof modern physics. Nevertheless, this theoretical framework is believed to be incomplete, as itleaves unanswered major questions at both the particle and cosmological frontiers. New physicsrefers to possible extensions of the SM that provide solutions to these open questions.The thesis presents two searches for new physics in the form of new particles predicted insupersymmetric and heavy neutrino models. The presence of these hypothetical particles issought after in the proton-proton collision data produced at the CERN Large Hadron Collider(LHC) at a centre-of-mass energy of 13 TeV and collected by the CMS experiment.The first search targets the production of supersymmetric particles giving rise to all-hadronicfinal states with large transverse momentum imbalance. The analysis is performed using datacollected during the Run 2 (2016 ??? 2018) of the LHC and corresponding to an integrated luminosity of 137 fb???1 . Models are considered in which supersymmetric particles are produced viathe strong interaction in processes conserving R-parity. The strategy of the search leverageson the high separation power of the MT2 variable between events arising from supersymmetricand from SM background processes. The data are found to agree with the SM backgroundpredictions. The results are used to constrain at 95% confidence level (CL) the mass parametersof various supersymmetric models. For the majority of these models, the most stringent limitsto date are obtained.The second search for new physics presented in the thesis considers, for the first time inCMS, the production of heavy neutrinos in the decays of beauty mesons. This novel analysisexploits a special beauty-hadron-enriched data sample collected in 2018 that contains of order1010 bb events for an integrated luminosity of 41.6 fb???1 . The heavy neutrinos are assumed tobe long lived to study the case in which they mix feebly with the SM neutrinos. No significantexcess of events over the SM background is observed. Limits at 95% CL are presented for parameters describing the mass, lifetime, and mixing of the heavy neutrinos. Multiple scenariosare constrained for both the Majorana and Dirac-like hypotheses and for different mixing patterns. Among the constraints obtained, some are the first ever derived and others are the moststringent to date.Finally, searching for new physics would not be possible without excellent particle reconstruction algorithms at CMS. The last project presented in the thesis consists in the optimisation ofone of the reconstruction algorithms used with the electromagnetic calorimeter, performed inpreparation for the Run 3 of data taking. All the CMS analyses that exploit Run 3 data andthat rely on photons, electrons, jets, or missing transverse momentum benefit from this work.i