Search for anomalous Wtb couplings and flavour-changing neutral currents in t-channel single top quark production in pp collisions at sqrt(s) = 7 and 8 TeV

TL;DR

This paper sets limits on anomalous Wtb couplings and searches for top quark FCNC interactions using single top quark events in proton-proton collisions at 7 and 8 TeV, employing Bayesian neural networks for discrimination.

Contribution

It provides the first combined limits on anomalous Wtb couplings and FCNC interactions in t-channel single top production at these energies.

Findings

95% CL limits on anomalous Wtb couplings

Upper limits on FCNC coupling strengths and branching fractions

Consistent with Standard Model predictions

Abstract

Single top quark events produced in the t channel are used to set limits on anomalous Wtb couplings and to search for top quark flavour-changing neutral current (FCNC) interactions. The data taken with the CMS detector at the LHC in proton-proton collisions at sqrt(s) = 7 and 8 TeV correspond to integrated luminosities of 5.0 and 19.7 inverse femtobarns, respectively. The analysis is performed using events with one muon and two or three jets. A Bayesian neural network technique is used to discriminate between the signal and backgrounds, which are observed to be consistent with the standard model prediction. The 95% confidence level (CL) exclusion limits on anomalous right-handed vector, and left- and right-handed tensor Wtb couplings are measured to be |f[V]^R| < 0.16, |f[T]^L| < 0.057, and -0.049 < f[T]^R < 0.048, respectively. For the FCNC couplings kappa[tug] and kappa[tcg], the 95% CL upper limits on coupling strengths are |kappa[tug]|/Lambda < 4.1E-3 TeV-1 and |kappa[tcg]|/Lambda < 1.8E-2 TeV-1, where Lambda is the scale for new physics, and correspond to upper limits on the branching fractions of 2.0E-5 and 4.1E-4 for the decays t to ug and t to cg, respectively.