Isolated Leptons and Single Top at HERA

TL;DR

This paper reports a comprehensive search for isolated leptons and single top quark production in ep collisions at HERA, finding results consistent with the Standard Model and setting limits on anomalous top production.

Contribution

It provides the first combined analysis of H1 and ZEUS data for isolated leptons and sets new upper limits on anomalous single top production cross section.

Findings

Observed 81 events vs. 87.8 ± 11.0 SM prediction

Measured single W production cross section as 1.06 ± 0.16 (stat.) ± 0.07 (sys.)

Set upper limit on anomalous single top cross section < 0.25 pb

Abstract

The search for events containing isolated leptons and missing transverse momentum produced in ep collisions is performed individually and in a common phase space with the H1 and ZEUS detectors at HERA. The combined H1 and ZEUS data sample corresponds to an integrated luminosity of 0.98 fb^-1 and comprises the complete high energy data from the HERA programme. A total of 81 events are observed in the data, compared to a Standard Model prediction of 87.8 \pm 11.0, which is dominated by single W production. At large hadronic transverse momentum P_T^X > 25 GeV in the e+p data, integrated luminosity 0.58 fb^-1, 23 data events are observed compared to a SM prediction of 14.0 \pm 1.9. The total single W production cross section is measured as 1.06 \pm 0.16 (stat.) \pm 0.07 (sys.) in agreement with a SM expectation of 1.26 \pm 0.19. The isolated lepton events are examined in the context of a search for anomalous single top production, where the hadronic decays of the W are additionally considered. Although several top-like candidates are present in the H1 data no clear signal is observed and an upper limit on the anomalous single top production cross section of < 0.25 pb is established at the 95% confidence level. This limit corresponds to an upper bound on the anomalous magnetic coupling of < 0.18 assuming a top mass of 175 GeV.