Search for Pair Production of a Heavy Up-Type Quark Decaying to a $W$ Boson and a $b$ Quark in the $\mathrm{\text{lepton}}+\mathrm{\text{jets}}$ Channel with the ATLAS Detector

A search is presented for production of a heavy up-type quark ($t^{\prime }$) together with its antiparticle, assuming subsequent decay to a $W$ boson and a $b$ quark, $t^{\prime }{\overline{t}}^{\prime }\to W^{+}b{W}^{-}\overline{b}$. The search is based on $1.04{\mathrm{fb}}^{-1}$ of proton-proton collisions at $\sqrt{s}=7\mathrm{TeV}$ collected by the ATLAS detector at the CERN Large Hadron Collider. Data are analyzed in the $\mathrm{\text{lepton}}+\mathrm{\text{jets}}$ final state, characterized by a high transverse momentum isolated electron or muon, high missing transverse momentum, and at least three jets. No significant excess of events above the background expectation is observed. A 95% C.L. lower limit of 404 GeV is set for the mass of the $t^{\prime }$ quark.

Figure 1

$m_{\mathrm{reco}}$ distribution in the combined (a) $e/\mu +3$ jets and (b) $e/\mu +\ge 4$ jets channels. The data (points) are compared to the SM background predictions using the values of the nuisance parameters obtained from the fit to data under the background-only hypothesis (stacked histograms). In the top panels, the bin contents have been divided by bin width. The bottom panels show the background-subtracted data distribution. The underflow and overflow have been folded into the first and last bins, respectively. Also shown is the expected contribution from a signal with mass $m_{t^{\prime }}=400\mathrm{GeV}$ (histogram).

Figure 2

Observed (solid line) and expected (dashed line) 95% C.L. upper limits on the $t^{\prime }{\overline{t}}^{\prime }$ cross section as a function of the $t^{\prime }$ mass. The surrounding shaded bands correspond to the 1 and 2 standard deviations (s.d.) around the expected limit. The thin line shows the theoretical prediction including its 1 s.d. uncertainty band. The shaded area is the mass region previously excluded by the CDF experiment [4].