Search for Down-Type Fourth Generation Quarks with the ATLAS Detector in Events with One Lepton and Hadronically Decaying $W$ Bosons

This Letter presents a search for pair production of heavy down-type quarks decaying via $b^{\prime }\to Wt$ in the $\mathrm{\text{lepton}}+\mathrm{\text{jets}}$ channel, as $b^{\prime }{\overline{b}}^{\prime }\to W^{-}t{W}^{+}\overline{t}\to b\overline{b}{W}^{+}{W}^{-}{W}^{+}{W}^{-}\to l^{\pm }\nu b\overline{b}q\overline{q}q\overline{q}q\overline{q}$. In addition to requiring exactly one lepton, large missing transverse momentum, and at least six jets, the invariant mass of nearby jet pairs is used to identify high transverse momentum $W$ bosons. In data corresponding to an integrated luminosity of $1.04{\mathrm{fb}}^{-1}$ from $pp$ collisions at $\sqrt{s}=7\mathrm{TeV}$ recorded with the ATLAS detector, a heavy down-type quark with mass less than 480 GeV can be excluded at the 95% confidence level.

Figure 1

Jet multiplicity distribution for signal and backgrounds for events with at least one jet. The shaded SM backgrounds are stacked on one another, while the $b^{\prime }$ signal histograms are not. In this figure and those following, the bottom plot shows the relative difference between the SM prediction and the data together with the uncertainty (shaded band) due to statistics, jet energy scale, and $W+\mathrm{\text{jets}}$ normalization.

Figure 2

The invariant mass distribution of jet pairs with $\Delta R<1.0$ for data and simulation in a control sample of events with exactly three to five jets.

Figure 3

The efficiency for jet pairs from a simulated $W$ boson decay to have $\Delta R<1.0$ and a dijet mass within 70–100 GeV, for simulated $t\overline{t}$ and signal events. Events are required to have exactly six jets.

Figure 4

Distribution of the numbers of events observed in the data and expected from SM processes for jet multiplicity $N_{\mathrm{jets}}=6$, 7, $\ge 8$ with hadronic $W$ multiplicity $N_W=0$, 1, $\ge 2$. The expected $b^{\prime }$ signals for two masses are also shown, stacked on top of the backgrounds.

Figure 5

Expected and observed cross section exclusion upper limits at 95% C.L. for a fourth generation $b^{\prime }$ quark. Systematic uncertainties on the expected limit are shown with shaded bands. Previously published limits from CDF [3, 4], CMS [5], and ATLAS [7] are also shown.