Search for Single Top Quark Production in ep Collisions at HERA

TL;DR

This paper searches for single top quark production in electron-proton collisions at HERA, using data analysis techniques to identify potential anomalous production signals and setting upper limits on production cross sections and couplings.

Contribution

It introduces a search for anomalous top quark production via flavor changing neutral currents at HERA, applying both cut-based and multivariate analyses to set new upper limits.

Findings

Observed 5 events in leptonic channel, expected 1.31 from background.

No excess in hadronic channel above Standard Model expectations.

Established upper limits of 0.55 pb on cross section and 0.27 on t-u-gamma coupling.

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 t-u-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 -> e t X) = 0.29 +0.15 -0.14 pb at \sqrt{s} = 319 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 t-u-gamma coupling \kappa_{t-u-gamma} are established at the 95% confidence level.