Searches for exotic stable massive particles with the ATLAS experiment

This thesis presents two signature-based searches for exotic long-lived particles using experimental data collected by the ATLAS experiment at the Large Hadron Collider (LHC) at CERN. At the LHC, proton-proton collisions take place at the world-record center-of-mass energy of 7 TeV and luminosities of up to $3\times 10^{33}cm^{-2}s^{-1}$. New physics has long been anticipated at the LHC and in this thesis two searches for hitherto unobserved particles are presented. Long-lived heavy hadrons have been predicted by models of supersymmetry, and these were the target of the first physics analysis. Using measurements of timing and energy loss, a search for so-called $R$-hadrons was performed. No excess was observed in the data, and new lower limits were established on the masses of meta-stable supersymmetric particles with color charge: $m_{\tilde{g}} > 562-586$ GeV (depending on choice of scattering model), $m_{\tilde{t}} > 309$ GeV and $m_{\tilde{b}} > 294$ GeV. These constraints are the most stringent limits published to date. In addition, a “blue-sky” search was made for objects possessing high electric charge. Ionization energy loss measurements were combined with variables describing the shape of electromagnetic showers to separate the sought signal from background processes. No event in the experimental data passed the selection and the results are in agreement with a background-only hypothesis. Upper limits between 1-12 pb were computed for the production cross sections of particles with electric charge $6e < q < 17e$ and mass $200 < m < 1000$ GeV. This is the first dedicated search for such particles at the LHC. Finally, a system designed to monitor the LHC beams and timing signals based on beam pick-up detectors is described together with results from early LHC operation.