Search for single top quark production in ep collisions at HERA

Abstract.

A search for single top quark production is performed in $e^{\pm}p$ collisions at HERA. The search exploits data corresponding to an integrated luminosity of 118.3 pb^-1. A model for the anomalous production of top quarks in a flavour changing neutral current process involving a $tu\gamma$ coupling is investigated. Decays of top quarks into a b quark and a W boson are considered in the leptonic and the hadronic decay channels of the W. Both a cut-based analysis and a multivariate likelihood analysis are performed to discriminate anomalous top quark production from Standard Model background processes. In the leptonic channel, 5 events are found while 1.31 $\pm$ 0.22 events are expected from the Standard Model background. In the hadronic channel, no excess above the expectation for Standard Model processes is found. These observations lead to a cross section $\sigma (ep \rightarrow e t X) = 0.29 ^{ + 0.15}_{-0.14} \ {\rm pb}$ at $\sqrt{s} = 319 \ {\rm GeV}$. Alternatively, assuming that the observed events are due to a statistical fluctuation, upper limits of 0.55 pb on the anomalous top production cross section and of 0.27 on the $tu\gamma$ coupling $\kappa_{tu\gamma}$ are established at the 95% confidence level.