Search for the Flavor-Changing Neutral Current in Top Pair Events in sqrt(s) = 8 TeV Proton-Proton Collisions at the Large Hadron Collider Using the ATLAS Detector

• Main Author: Brost, Elizabeth
• Other Authors: Majewski, Stephanie
• Language: en_US
• Published: University of Oregon 2016
• Online Access: http://hdl.handle.net/1794/20406

In this dissertation, a search for the flavor-changing neutral current in top-antitop events is presented. The flavor-changing neutral current is forbidden at tree level in the Standard Model and suppressed at higher order due to the GIM mechanism. In the Standard Model, the top quark is expected to decay to a W boson and a bottom quark nearly 100 percent of the time. While the Standard Model branching fractions for flavor-changing neutral currents in top decays are well beyond current experimental reach, there exist theoretical models which predict large enhancements to those branching fractions. Observation of the flavor-changing neutral current in top decays would be an unambiguous confirmation of new physics. This search was conducted in data from proton-proton collisions at the Large Hadron Collider, running at a center-of-mass energy of √s = 8 TeV, which were collected with the ATLAS detector in 2012. These data correspond to an integrated luminosity of 20.3 fb^-1. Candidate events include a lepton (electron or muon), a photon, at least two jets (one of which is b-tagged), and missing transverse energy. As no signal is observed, an observed (expected) upper limit on the branching ratio BR(t → qγ) of 0.063% (0.062%) is presented.