Search for a Heavy Toplike Quark in $p\overline{p}$ Collisions at $\sqrt{s}=1.96\mathrm{TeV}$

We present the results of a search for pair production of a heavy toplike ($t^{\prime }$) quark decaying to $Wq$ final states using data corresponding to an integrated luminosity of $5.6{\mathrm{fb}}^{-1}$ collected by the CDF II detector in $p\overline{p}$ collisions at $\sqrt{s}=1.96\mathrm{TeV}$. We perform parallel searches for $t^{\prime }\to Wb$ and $t^{\prime }\to Wq$ (where $q$ is a generic down-type quark) in events containing a lepton and four or more jets. By performing a fit to the two-dimensional distribution of total transverse energy versus reconstructed $t^{\prime }$ quark mass, we set upper limits on the $t^{\prime }{\overline{t}}^{\prime }$ production cross section and exclude a standard model fourth-generation $t^{\prime }$ quark decaying to $Wb$ ($Wq$) with mass below 358 $(340)\mathrm{GeV}/c^2$ at 95% C.L.

Figure 1

Observed and expected 95% C.L. upper limits as a function of the mass of the $t^{\prime }$ quark, for a $t^{\prime }$ decaying to $Wb$ (upper) and $Wq$ (lower) with 100% branching ratio. The light and dark gray areas show the $\pm 1\sigma$ and $\pm 2\sigma$ areas around the expected limits. The dashed line is the theory expectation.

Figure 2

Log scale distributions of $H_T$ and $M_{\mathrm{reco}}$ comparing data (dots) with backgrounds (filled histograms) and signal (empty histogram). The $t^{\prime }{\overline{t}}^{\prime }$ signal is for a $t^{\prime }$ mass $360\mathrm{GeV}/c^2$ and a $t^{\prime }{\overline{t}}^{\prime }$ cross section corresponding to the 95% C.L. upper limit. The amounts of all backgrounds are set to their fitted results from the fit assuming $t^{\prime }$ decays to $Wb$. In the lower plot the points are the difference between the data and the sum of all the backgrounds, the histograms are the signal contribution.

Figure 3

Log scale distributions of $H_T$ and $M_{\mathrm{reco}}$ comparing data (dots) with backgrounds (filled histograms) and signal (empty histogram). The $t^{\prime }{\overline{t}}^{\prime }$ signal is for a $t^{\prime }$ mass $350\mathrm{GeV}/c^2$ and a $t^{\prime }{\overline{t}}^{\prime }$ cross section corresponding to the 95% C.L. upper limit. The amounts of all backgrounds are set to their fitted results from the fit assuming $t^{\prime }$ decays to $Wq$. In the lower plot the points are the difference between the data and the sum of all the backgrounds, the histograms are the signal contribution.