Abstract
A search for the production of a single top quark in association with a Z
boson is presented, both to identify the expected standard model process and to
search for flavour-changing neutral current interactions. The data sample
corresponds to an integrated luminosity of 19.7 inverse-femtobarns recorded by
the CMS experiment at the LHC in proton-proton collisions at sqrt(s) = 8 TeV.
Final states with three leptons (electrons or muons) and at least one jet are
investigated. An events yield compatible with tZq standard model production is
observed, and the corresponding cross section is measured to be sigma( p p -> t
Z q -> l nu b l+ l- q ) = 10 +8/-7 fb with a significance of 2.4 standard
deviations. No presence of flavour-changing neutral current production of tZq
is observed. Exclusion limits at 95% confidence level on the branching
fractions of a top quark decaying to a Z boson and an up or a charm quark are
found to be B( t -> Z u ) < 0.022% and B( t -> Z c ) < 0.049%.
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