A Search For Disappearing Tracks In Proton Proton Collisions At Sqrt S
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A Search for Disappearing Tracks in Proton-proton Collisions at Sqrt(s)
This thesis presents a search for evidence of new supersymmetric particles in proton-proton collision data collected by the Compact Muon Solenoid at the Large Hadron Collider located in Geneva, Switzerland. The data was collected in 2012 at a center of mass energy of 8 TeV and corresponds to an integrated luminosity of 19.5 inverse femtobarns. The search examines events that produce the experimental signature of a disappearing track. Disappearing tracks are identifed as those with little or no associated calorimeter energy deposits and with several missing hits in the outer layers of the tracker. This search is motived by anomaly-mediated supersymmetry which postulates that a long-lived chargino decaying to a neutralino and pion with low momentum could give rise to a disappearing track. The sources of background in this search arise from leptons which fail to be reconstructed and false trajectories determined by the track reconstruction algorithm. The number of observed events is in agreement with the background expectation, and limits are set on the cross section of direct electroweak chargino production in terms of the chargino mass and mean proper lifetime. At 95 % confidence level, anomaly-mediated supersymmetry breaking models with a chargino mass less than 260 GeV, corresponding to a mean proper lifetime of 0.2 ns, are excluded.
Search for Dark Matter with ATLAS
This thesis describes in detail a search for weakly interacting massive particles as possible dark matter candidates, making use of so-called mono-jet events. It includes a detailed description of the run-1 system, important operational challenges, and the upgrade for run-2. The nature of dark matter, which accounts for roughly 25% of the energy-matter content of the universe, is one of the biggest open questions in fundamental science. The analysis is based on the full set of proton-proton collisions collected by the ATLAS experiment at the Large Hadron Collider at √s = 8 TeV. Special attention is given to the experimental challenges and analysis techniques, as well as the overall scientific context beyond particle physics. The results complement those of non-collider experiments and yield some of the strongest exclusion bounds on parameters of dark matter models by the end of the Large Hadron Collider run-1. Details of the upgrade of the ATLAS Central Trigger for run-2 are also included.
Heavy Neutral Particle Decays to Tau Pairs
Author: Michail Bachtis
language: en
Publisher: Springer Science & Business Media
Release Date: 2013-12-13
The work presented in this thesis spans a wide range of experimental particle physics subjects, starting from level-1 trigger electronics to the final results of the search for Higgs boson decay and to tau lepton pairs. The thesis describes an innovative reconstruction algorithm for tau decays and details how it was instrumental in providing a measurement of Z decay to tau lepton pairs. The reliability of the analysis is fully established by this measurement before the Higgs boson decay to tau lepton pairs is considered. The work described here continues to serve as a model for analysing CMS Higgs to tau leptons measurements.