Search for single top-quark production via flavour changing neutral currents at 8 TeV with the ATLAS detector

TL;DR

This paper reports a search for single top-quark production via flavor-changing neutral currents at 8 TeV using ATLAS data, setting upper limits on production cross-section, couplings, and branching fractions, with no signal observed.

Contribution

First search at 8 TeV for single top-quark FCNC production using neural networks and setting new upper limits on couplings and branching fractions.

Findings

No signal observed for FCNC single top production.

Set upper limits on cross-section and couplings.

Derived constraints on top-quark FCNC branching ratios.

Abstract

A search for single top-quark production via flavour-changing neutral current processes from gluon plus up- or charm-quark initial states in proton-proton collisions at the LHC is presented. Data collected with the ATLAS detector in 2012 at a centre-of-mass energy of 8 TeV and corresponding to an integrated luminosity of 20.3 fb$^{-1}$ are used. Candidate events for a top quark decaying into a lepton, a neutrino and a jet are selected and classified into signal- and background-like candidates using a neural network. No signal is observed and an upper limit on the production cross-section multiplied by the $t \rightarrow \ell\nu b$ branching fraction is set. The observed 95% CL limit is $\sigma_{qg \rightarrow t} \times \mathcal{B}(t \rightarrow Wb) \times \mathcal{B}(W \rightarrow \ell \nu) < 3.4$ pb and the expected 95% CL limit is $\sigma_{qg \rightarrow t} \times \mathcal{B}(t \rightarrow Wb) \times \mathcal{B}(W \rightarrow \ell \nu) < 2.9$ pb. The observed limit can be interpreted as upper limits on the coupling constants of the flavour-changing neutral current interactions divided by the scale of new physics $\kappa_{ugt}/\Lambda < 10 \times 10^{-3}$ TeV$^{-1}$ and $\kappa_{cgt}/\Lambda < 23 \times 10^{-3}$ TeV$^{-1}$ and on the branching fractions $\mathcal{B}(t \rightarrow ug) < 1.2 \times 10^{-4}$ and $\mathcal{B}(t \rightarrow cg) < 6.4 \times 10^{-4}$.