Search for Heavy Long-Lived Particles in ATLAS

This thesis presents a search for long-lived particles through a measuremt of their mass performed on a data sample of \lumi from proton-proton collisions at a centre-of-mass energy $\sqrt{s} = 7~\TeV$ collected by the ATLAS detector at the LHC in 2011. The mass of the long-lived particles is estimated from a measurement of their speed, $\beta$, and their momentum measurements using the relation $m=p/\beta\gamma$. Two distinct types of LLPs are sought after; sleptons, the supersymmetric partners of the Standard Model leptons, and \rhads, colorless bound states that include a colored supersymmetric long-lived particle in addition to colored Standard Model particles. Sleptons are expected to interact with the detector as if they were heavy muons, charged throughout the detector and penetrating. In contrast, \rhads~may change charge in the detector due to the strong interactions between the light quarks in the \rhads~with the detector material. Thus, \rhads~may be charged in the ID and neutral in the MS or vice versa, and therefore the signature of a colored long-lived colored particle might be different than the signature of a non-colored long-lived particle. No excess is observed above the estimated background and upper limits, at 95\% confidence level, are set on the mass of the long-lived particles in different scenarios, based on their possible interactions in the inner detector, the calorimeters and the muon spectrometer. Long-lived staus in gauge-mediated SUSY-breaking models are excluded up to a mass of 300~\GeV for $\tan\beta=5\mbox{--}20$. Directly produced long-lived sleptons are excluded up to a mass of 278~\GeV. Gluino (stop, sbottom) \rhads are excluded up to a mass of $\rhadronGluinoFullMassExclusion$ ($\rhadronStopFullMassExclusion$, $\rhadronSbottomFullMassExclusion$) when using a generic interaction model.