Search for Supersymmetry in Hadronic Final States using MT2 in pp collisions at √s = 8 TeV and Evolution Studies of the CMS Electromagnetic Calorimeter Endcap Signals

Over the past decades, the standard model of particle physics has been proven to accurately describe the vast majority of the experimental observations within particle physics. The discovery of a boson at a mass of about 125 GeV seems to provide the last missing piece of the standard model, the Higgs boson. Despite this success, there are some phenomena, for which the description of the standard model is insufficient. In order to surmount these shortcomings, new-physics models have been advanced. One popular model is supersymmetry, which solves several of the deficiencies of the standard model. Supersymmetry extends the description of the standard model by adding a symmetry between fermions and bosons: the elementary particle spectrum is at least doubled. In this dissertation, a search for supersymmetry in fully hadronic final states is presented. The search analyzes proton-proton collision data, collected at $\sqrt{s} = 8\,\text{TeV}$ with the Compact Muon Solenoid experiment at the Large Hadron Collider. The data correspond to an integrated luminosity of $19.5\,\text{fb}^{-1}$. The search uses the variable $M_\mathrm{T2}$ to discriminate between events coming from standard model processes and signal events. The variable is a generalization of the transverse mass for events containing two pair-produced particles, where both particles decay at least to one detected and one undetectable particle. Selecting events with high values of $M_\mathrm{T2}$ reduces the contribution from standard model processes, in particular multijet events, and enhances the sensitivity for signal events for a large variety of supersymmetric models. The signal regions of this search are defined by the jet and b-tagged jet multiplicities, the hadronic energy in the event, and the value of $M_\mathrm{T2}$. The combined information of multiple signal regions yields a high sensitivity for the production of the partner particles of both gluons and quarks, regardless of the quark flavor, for a wide range of particle masses. The event yields in the signal regions stemming from standard model processes are estimated by prediction methods that use data from control regions orthogonal to the signal regions definition. No significant excess over the expected numbers of background events is observed. The comparison of data yields and predictions is used to set exclusion limits on various models of supersymmetry. Beyond this search in hadronic final states, the potential of a search for the pair production of supersymmetric partners of the top quark is assessed using events containing two charged leptons. Several signal discriminating variables have been tested, such as variants of the $M_\mathrm{T2}$ variable or a discriminant based on the examination of the event kinematics. The second part of this dissertation contains two studies related to the upgrade program of the Compact Muon Solenoid detector. The physics performance of the pixel detector upgrade, which is foreseen to happen in 2017, is evaluated in the first study. Based on the event selection of the so-called $M_\mathrm{T2}b$ search for supersymmetry, performed in 2011, the focus of the study is the improvement of identifying jets, which originate from bottom quark hadronization. For the tested model, an increase of 20% in the signal selection efficiency is observed for a fixed rate of misidentifying light-quark or gluon jets as bottom-quark jets. In the final study presented in this dissertation, the long-term evolution of signals produced in the forward part of the electromagnetic calorimeter of the Compact Muon Solenoid experiment is analyzed. The lead-tungstate crystals, which act both as absorber and scintillator, are the heart of the calorimeter. The scintillation light is read out by vacuum phototriodes in the forward part of the detector. The transparency of the crystals decreases under radiation. In order to measure and correct for these transparency changes, a light monitoring system has been installed within the calorimeter. Several sources are known to contribute to the transparency decrease. The goal of this study is to disentangle various contributions to the signal change. In order to achieve this, measurements, performed during quality assurance tests prior to the construction of the detector, are correlated with signal changes observed in monitoring data during proton-proton collision data taking. Pseudorapidity-dependent contributions are singled out: ionizing damages to crystals, damages to the vacuum phototriodes, and possible cumulative damages to crystals. The results of this study are compared with predictions, which are based on laboratory and testbeam results and are used in studies for possible detector upgrades. The comparison indicates that the modeling of the predictions is satisfactory.