Search for same-sign top-quark production and fourth-generation down-type quarks in pp collisions at sqrt(s) = 7 TeV with the ATLAS detector

TL;DR

This paper reports a search for new physics involving same-sign top-quark pairs and heavy down-type quarks in proton-proton collisions at 7 TeV, using ATLAS data, setting limits on their production cross sections and masses.

Contribution

It provides the first limits on same-sign top-quark production and heavy down-type quark masses at 7 TeV using ATLAS data with specific event selection criteria.

Findings

No significant excess observed over Standard Model expectations.

Upper limits set on cross sections for new physics processes.

Heavy down-type quarks with masses below 450 GeV are excluded.

Abstract

A search is presented for same-sign top-quark production and down-type heavy quarks of charge -1/3 in events with two isolated leptons (electrons or muons) that have the same electric charge, at least two jets and large missing transverse momentum. The data are selected from pp collisions at sqrt(s)=7 TeV recorded by the ATLAS detector and correspond to an integrated luminosity of 1.04 inverse femtobarns. The observed data are consistent with expectations from Standard Model processes. Upper limits are set at 95% confidence level on the cross section of new sources of same-sign top-quark pair production of 1.4-2.0 pb depending on the assumed mediator mass. Upper limits are also set on the pair-production cross-section for new heavy down-type quarks; a lower limit of 450 GeV is set at 95% confidence level on the mass of heavy down-type quarks under the assumption that they decay 100% of the time to Wt.