Abstract
A search for single top-quark production via flavour changing neutral current processes from initial states involving a gluon and up or charm quarks 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 signaland background-like candidates using a neural network. No signal is observed and an upper limit on the production cross-section multiplied by the t→Wb branching fraction is set. The observed 95 % CL limit is σqg→t ×B(t →Wb) < 3.4 pb and the expected 95 % CL limit is σqg→t ×B(t → Wb) < 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 κugt/Λ < 5.8×10−3 TeV−1 and κcgt/Λ < 13×10−3 TeV−1 and on the branching fractions B(t → ug) < 4×10−5 and B(t→ cg)< 17×10−5.
Highlights
The top quark is the heaviest elementary particle, with a mass mtop = 173.3 ± 0.8 GeV [1] close to the electroweak symmetry breaking scale, making it a very good object to test the Standard Model (SM) and search for beyond SM phenomena
They can be produced singly via the weak interaction though three different subprocesses; the t-channel process involving the exchange of a space-like W boson, the Wt production involving the production of a real W boson and the s-channel process involving the production of a time-like W boson
The flavour changing neutral currents (FCNC) decay of a top quark with a gluon as the neutral current t → ug, where u is a charm or an up quark and g is gluon, is very difficult to seperate from the multi-jet background through quantum chromodynamic (QCD) processes
Summary
At the Large Hadron Collider (LHC), the top quarks are predominantly produced in pairs via the strong interaction. They can be produced singly via the weak interaction though three different subprocesses; the t-channel process involving the exchange of a space-like W boson, the Wt production involving the production of a real W boson and the s-channel process involving the production of a time-like W boson. The Lagrangian holds for both FCNC production and decay of top quarks
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