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 (e or µ) that have the same electric charge, at least two jets and large missing transverse momentum. The data are selected from pp collisions at √ s = 7 TeV recorded by the ATLAS detector and correspond to an integrated luminosity of 1.04 fb −1 . 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 W t.
Keywords: Hadron-Hadron ScatteringOpen Access, Copyright CERN, for the benefit of the ATLAS collaboration
Conclusion 19The ATLAS collaboration 24
IntroductionThe Standard Model (SM) of the electroweak and strong interactions is extremely successful in explaining most of the measurements in particle physics at energies accessible today. Its predicted behaviour at high energies, however, presents some theoretical problems which have motivated a large variety of theories encompassing and extending it. Due to the large variety of models proposed, signature-based searches are often useful when exploring the consequences of these theories in an economical way. In hadron collisions it is useful to group final states by the number of charged leptons (electrons or muons). Within this classification, a signal with two leptons of the same electric charge (same-sign leptons) is Figure 1. Production of same-sign top-quark pairs via the production of a heavy vector boson (such as color-triplet Q interesting since it has a low background rate in the Standard Model, and potentially large contributions from new theories, for example new flavour-changing Z ′ bosons, proposed [1] to explain the forward-backward asymmetry (A FB ) measured at the Tevatron [2, 3], or new heavy quarks [4,5]. In this paper we present a search for events characterised by two isolated same-sign leptons in association with at least two jets and large missing transverse momentum (E miss T ). Two specific signal processes are considered, same-sign top-quark production [6][7][8] and pair production of down-type heavy quarks of charge −1/3 [9]. Feynman diagrams of these processes are shown in figures 1 and 2, respectively. The uu → tt process illustrated in figure 1 can be mediated at the tree level by the exchange of a s-channel resonance (left), or a t-channel resonance (right). In the case of new vector bosons exchanged in the s-channel, the new particle must be a colour-triplet or colour-sextet (respectively labelled as Q 5 µ , Y 5 µ ) with charge 4/3, while for t-channel exchange it can be a colour-singlet Z ′ or colour-octet g ′ , both with zero charge. For resonance m...
