Search for supersymmetry with tau leptons in the CMS experiment

This thesis is focused on the search for pair production of the supersymmetric $\tau$ lepton partners within the Compact Muon Solenoid (CMS) experiment at the Large Hadron Collider (LHC). The search is performed using the data collected at a center-of-mass-energy $\sqrt{s}$ =13 TeV with the CMS detector in the years 2016-2017. One of the most popular extensions of the Standard Model (SM) is supersymmetry (SUSY). The minimal supersymmetric standard model (MSSM) predicts the existence of fermion superpartners called s fermions. Every SM fermion has two scalar superpartners, and the superpartners of the $\tau$ leptons are called staus ($\bar{\tau}$). The light $\bar{\tau}$ could bring the neutralino relic density to the observed value through co-annihilation with the Lightest Supersymmetric Particle (LSP). Models describing direct $\bar{\tau}$-pair production, where each $\bar{\tau}$ is expected to decay to a $\tau$ lepton and the LSP, as well as neutralino-chargino and chargino pair production with decays to $\tau$ leptons are investigated. Three scenarios of direct $\bar{\tau}$ pair production are considered for interpretation: scenarios in which superpartners of only left-handed or right-handed?? are produced in pairs, and a mass degenerate scenario in which the partners of the left and right handed $\tau$ leptons have the same mass and are produced simultaneously. The analysis selection is based on final states with one hadronically decaying $\tau$ lepton,an electron or muon from the decay of the second ?? , and final states with one electronand one muon from the decay of the $\tau$ leptons. To optimize the sensitivity to the new physics topologies mentioned above, a cut-based approach as well as a multivariate approach (MVA) are applied. The data are compared with the expectation from the SM background processes and with $\bar{\tau}$-pair production signal hypotheses. No evidence for the production of supersymmetric ?? lepton partners is found. Mass and cross section limits are determined within simplified model parameters. In the case of purely left-handed production and a nearly massless neutralino, the strongest limits are obtained for a $\bar{\tau}$ slepton mass of 125 GeV, where 1.14 times the theoretical cross section can be excluded. For the degenerate production scenario, ??e masses up to 150 GeV are excluded when assuming a nearly massless LSP.In addition, the sensitivity of the direct $\bar{\tau}$ production is studied for future experiments,assuming 3 ab$^{-1}$ of proton-proton collision data to be produced by the High Luminosity LHC (HL-LHC) at a center-of-mass energy of 14 TeV as well as assuming 15 ab$^{-1}$ and 27 TeV energy to be delivered by the High Energy LHC (HE-LHC). The analysis is performed using the Delphes simulation of the upgraded CMS detector, where the object reconstruction performance is tuned to the one achieved with CMS Phase-2 full simulation. In the degenerate direct $\bar{\tau}$-pair production scenario, $\bar{\tau}$ masses are excluded below 650 (1150) GeV, with the discovery contour of $\bar{\tau}$ masses reaching up to 470 (810) GeV for HL-LHC (HE-LHC).