CERN-THESIS-2012-153 26/07/2012 - CERN Document Server

Chapter 1
Introduction
The top quark is the heaviest elementary particle known, and it has a small lifetime which does not allow
bound-states of the top quark to be formed. Because of its large mass, mt = 173.2 ± 0.9 GeV [2], the top
quark has a strong coupling to the electroweak symmetry breaking sector, becoming an interesting channel
to test the Standard Model (SM) of particle physics, providing potentially a window onto physics beyond the
SM. If deviations from the SM predictions of the production and decay properties of the top quark are found,
they will provide model-independent evidence for physics beyond the Standard Model. According to the
SM, the top quark decays almost exclusively through the mode t → Wb. Flavor changing neutral currents
(FCNC) are highly suppressed in the SM by the GIM mechanism which yields a branching ratio of the order
of 10 −14 . Several SM extensions predict a higher branching ratio for top quark FCNC decays. In these
models, the branching ratio is typically many orders of magnitude larger than the SM branching ratio, and
can be as high as ∼ 1 × 10 −4 in certain quark-singlet models [3]. Previous measurements done at the Teva-
tron [4] put an experimental limit on the branching ratio of this FCNC top quark decay to be less than 3.2%.
For this thesis, I performed a search for FCNC in 2.1 fb −1 of 2011 pp collision data collected by ATLAS.
The search is done in t¯t events, where one of the top quarks is assumed to follow the dominant SM decay
t → Wb and the other the FCNC decay t → Zq. Since only leptonic decays of the W-boson and decays of
the Z-boson into a pair of charged leptons are used as signal, the final state topology includes three isolated
charged leptons, at least two jets, one of them coming from a b-quark, and missing transverse momentum
from the undetected neutrino. Events with two identified leptons (electrons and/or muons) and one high-
quality inner detector track (track-lepton) are searched for. These events are denoted “2ID+TL events”.
The track-lepton selection was first introduced in a t¯t cross section measurement done at the Tevatron by
the CDF experiment [5]. In ATLAS, it has been used for the t¯t cross section measurement as well, with
both 2010 and 2011 data [6,7]. A FCNC search in an orthogonal channel, where three identified leptons are
looked for, is also performed by other members of the collaboration. This selection is denoted “3ID”. I will
be briefly introduce it here, since the final result to be presented combines them both.
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