Search for gluino production in final states with an isolated lepton and b-tagged jets using 13 TeV collisions at CMS

A search for supersymmetric particles in proton-proton collisions at a centre-of-mass energy of 13 TeV is presented in this thesis. The data were recorded by the CMS experiment during Run 2 of the LHC, corresponding to an integrated luminosity of 2.3 fb^-1. The signal signature comprises pair produced gluon superpartners, gluinos, decaying each to a top-antitop quark pair and a neutralino - the lightest supersymmetric particle. Since the probability, that only one W boson from the four initial top-quarks decays to a lepton-neutrino pair, is about 40%, events with a single isolated lepton, electron or muon, accompanied by several high-energy hadronic jets and at least one b-quark tagged jet are selected. Signal-enriched search regions are defined based on the azimuthal angle between the lepton and four-vector sum of the missing energy and lepton. The dominant Standard Model backgrounds tend towards lower angles, while the investigated signals are expected to show little dependence on the angle. Further sensitivity to different signal mass scenarios is increased by considering events in categories with different jet and b-tag multiplicities, hadronic and leptonic scales, which are defined as the scalar sums of the jet transverse momenta, and the missing energy and lepton momentum, respectively. The background is estimated by a data-driven approach using background-enriched control regions. Low jet multiplicity data sidebands are used to obtain respective signal-to-control region transfer factors. Contributions from multijet events, which arise from misidentified leptons, are subtracted from these control regions using misidentification probabilities measured in data. No significant deviation from the predicted Standard Model background is observed. This result is interpreted in the framework of simplified models corresponding to the aforementioned signal signature, resulting in exclusion limits for particular gluino-neutralino mass scenarios. Gluinos below 1575 GeV are excluded for light neutralinos, while for scenarios with low neutralino-gluino mass splitting, neutralinos are excluded up to 850 GeV. This extends the limits obtained in Run 1 by about 200 and 250 GeV, respectively. In addition, the upgrade of the CMS outer hadron calorimeter readout system with silicon photomultipliers is presented, describing its motivation and the commissioning of the new hardware. An initial energy calibration with cosmic muons is performed afterwards.