Search for New Phenomena in ttbar Events with Large Missing Transverse Momentum in Proton-Proton Collisions at sqrt(s) = 7 TeV with the ATLAS Detector

Abstract

A search for new phenomena in ttbar events with large missing transverse momentum in proton-proton collisions at a center-of-mass energy of 7 TeV is presented. The measurement is based on 1.04 fb^-1 of data collected with the ATLAS detector at the LHC. The search is carried out in the single-lepton channel, characterized by an isolated lepton of high transverse momentum, four or more jets and large missing transverse momentum. Contributions to this final state may arise from a number of Standard Model extensions. The results are interpreted in terms of a model where new top-quark partners are pair-produced and each decay to an on-shell top (or antitop) quark and a long-lived undetected neutral particle. The data are found to be consistent with Standard Model expectations. A limit at 95% confidence level is set excluding a cross-section times branching ratio of 1.1 pb for a top-partner mass of 420 GeV and a neutral particle mass less than 10 GeV. In a model of exotic fourth generation quarks, top-partner masses are excluded up to 420 GeV and neutral particle masses up to 140 GeV.

Figures (3)

• Figure 1: (a) Transverse mass of the lepton and missing energy and (b) $E_{\mathrm{T}}^{\mathrm{miss}}$ after applying all selection criteria except the cut on the variable shown. MC background contributions are stacked on top of each other and normalized according to the data-driven corrections discussed in the text. The lines with the arrows indicate the selection criteria that define the signal region ($m_{\mathrm{T}}>150$ GeV and $E_{\mathrm{T}}^{\mathrm{miss}}>100$ GeV). 'Other Backgrounds' includes both multi-jet backgrounds and $Z$+jets, single top and diboson production. Expectations from two signal mass points are stacked separately on top of the SM background. The last bin includes the overflow.
• Figure 2: Excluded region (under the curve) at the 95% confidence level as a function of $T$ and $A_0$ masses, compared with the CDF exclusion Aaltonen:2011rrAaltonen:2011na. Theoretical uncertainties on the $T\overline{T}$ cross-section are not included in the limit, but the effect of these uncertainties is shown. The gray contours show the excluded cross-section times branching ratio as a function of the two masses.
• Figure 3: Cross-section times branching ratio excluded at the 95% confidence level versus $T$ mass for an $A_0$ mass of 10 GeV. Theoretical predictions for both spin-$\frac{1}{2}$ and scalar $T$ pair production are also shown.