Search for a Vectorlike Quark with Charge $2/3$ in $t+Z$ Events from $pp$ Collisions at $\sqrt{s}=7\mathrm{TeV}$

A search for pair-produced heavy vectorlike charge-$2/3$ quarks, $T$, in $pp$ collisions at a center-of-mass energy of 7 TeV, is performed with the CMS detector at the LHC. Events consistent with the flavor-changing-neutral-current decay of a $T$ quark to a top quark and a $Z$ boson are selected by requiring two leptons from the $Z$-boson decay, as well as an additional isolated charged lepton. In a data sample corresponding to an integrated luminosity of $1.14{\mathrm{fb}}^{-1}$, the number of observed events is found to be consistent with the standard model background prediction. Assuming a branching fraction of 100% for the decay $T\to tZ$, a $T$ quark with a mass less than $475\mathrm{GeV}/c^2$ is excluded at the 95% confidence level.

Figure 1

The distributions of the invariant mass of two oppositely charged muons or electrons from data (points) and from Monte Carlo simulations of the backgrounds (colored histograms) and a $350\mathrm{GeV}/c^2$ $T\overline{T}$ signal (open histograms), $M_{{\ell }^{+}{\ell }^{-}}$ (left), jet multiplicity (center), and $R_T$ (right) for events with a reconstructed $Z$-boson candidate and a charged lepton.

Figure 2

The 95% confidence level (C.L.) upper limit on the cross section of the $pp\to T\overline{T}X$ process, as a function of the $T$-quark mass. The branching fraction of $T\to tZ$ is assumed to be 100%. The solid line shows the observed limit. The dotted line corresponds to the expected limit under a background-only hypothesis. The solid (hatched) area shows the $\pm 1$ ($\pm 2$) standard deviation uncertainties on the expected limit. The dot-dashed line shows the value of the theoretical cross section [27] for the $T\overline{T}$ process.