A search for supersymmetry with a single lepton in the final state using hadronic variables

Supersymmetry (SUSY) is a theory for physics beyond the Standard Model (SM),which attempts to eliminate some of its shortcomings. It postulates a symmetry between bosons and fermions and thereby provides a mechanism to solve the hierarchyproblem, which is closely related to the stability of the Higgs boson mass. However,this mechanism imposes constraints on the mass spectra of the supersymmetric partners of SM particles. In particular, the masses of the lighter 3rd generation squarksshould be smaller than 700 GeV, one of the neutralinos is expected to be lighter than350 GeV and the gluino mass must not exceed 1.5 TeV. SUSY theories that are realizedwithin these mass limits are commonly referred to as “natural” SUSY. The search fornew physics, in particular for SUSY, is one of the primary goals of the Large HadronCollider (LHC), where the allowed mass ranges of natural SUSY are within reach.This thesis presents results from a search for gluino-pair production, with each gluinodecaying to two top quarks and a neutralino via virtual or on-shell top squarks. “Simplified models” are used to describe this topology as a function of the supersymmetricparticle masses mg , m ˜0 and mt . This topology is well motivated by the mass hierarchy˜˜of natural SUSY.Events are selected from samples of proton-proton collisions, recorded by the CMSppexperiment at s = 7 TeV during 2011 and s = 8 TeV during 2012. The size of thesamples corresponds to 4.98 fb 1 (2011) and 19.4 fb 1 (2012). This thesis focuses onthe analysis of the larger dataset and the results are compared to those obtained fromthe sample collected in 2011, where a similar analysis strategy was used.The event selection of the analysis is designed to retain a high efficiency for the targeted signal, while rejecting as much SM background as possible. Selected events arecharacterized by the presence of a single isolated electron or muon, missing transversemomentum, hadronic activity and a minimum of six reconstructed jets, of which atleast one must be identified as a b-quark jet. The dominant SM background component in this selection is tt production, followed by a small contribution of W +jets andsingle-t events. The requirement of a single isolated electron or muon, together withmissing transverse momentum, provides a strong suppression of backgrounds from multijet production, but preserves a high efficiency for signal events. The signal topology,with four top quarks in the final state, produces high jet multiplicities including up tofour b-quark jets. Therefore, the most sensitive signal regions are those defined by aminimum of three identified b-quark jets.Since the tails of the missing transverse momentum spectra of the SM backgroundcontributions are not necessarily well described by simulation, a model for the spectrumis developed to perform an estimation from data. The model component for tt decays isparametrized as function of the hadronic activity and the number of identified b-quarkjets. It is complemented by a static component to account for the small contributionof W +jets production. This component is extracted from simulation, but its relativenormalization is estimated from data as function of the hadronic activity and theiii