Combination of the Searches for Pair-Produced Vectorlike Partners of the Third-Generation Quarks at $\sqrt{s}=13\mathrm{TeV}$ with the ATLAS Detector

A combination of the searches for pair-produced vectorlike partners of the top and bottom quarks in various decay channels ($T\to Zt/Wb/Ht$, $B\to Zb/Wt/Hb$) is performed using $36.1{\mathrm{fb}}^{-1}$ of $pp$ collision data at $\sqrt{s}=13\mathrm{TeV}$ with the ATLAS detector at the Large Hadron Collider. The observed data are found to be in good agreement with the standard model background prediction in all individual searches. Therefore, combined 95% confidence-level upper limits are set on the production cross section for a range of vectorlike quark scenarios, significantly improving upon the reach of the individual searches. Model-independent limits are set assuming the vectorlike quarks decay to standard model particles. A singlet $T$ is excluded for masses below 1.31 TeV and a singlet $B$ is excluded for masses below 1.22 TeV. Assuming a weak isospin $(T,B)$ doublet and $|V_{Tb}|\ll |V_{tB}|$, $T$ and $B$ masses below 1.37 TeV are excluded.

Figure 1

Representative leading-order Feynman diagrams for (a) $T\overline{T}$ and (b) $B\overline{B}$ pair production. The studied VLQ decays are also displayed.

Figure 2

Observed (solid lines) and expected (dashed line) 95% C.L. upper limits on the $T\overline{T}$ cross section versus mass for the combination and the standalone analyses in black and colored lines, respectively. The (a) singlet and (b) doublet scenarios [8] are displayed. The shaded bands correspond to $\pm 1$ and $\pm 2$ standard deviations around the combined expected limit. The rapidly falling thin red line and band show the theory prediction and corresponding uncertainty [13], respectively.

Figure 3

Observed (solid lines) and expected (dashed line) 95% C.L. upper limits on the $B\overline{B}$ cross section versus mass for the combination and the standalone analyses in black and colored lines, respectively. The (a) singlet, (b) $(T,B)$ doublet, and (c) $(B,Y)$ doublet scenarios [8] are displayed. The shaded bands correspond to $\pm 1$ and $\pm 2$ standard deviations around the combined expected limit. The rapidly falling thin red line and band show the theory prediction and corresponding uncertainty [13], respectively.

Figure 4

Observed lower limits at 95% C.L. on the mass of the (a) $T$ and (b) $B$ as a function of branching ratio assuming $\mathcal{B}(T\to Ht)+\mathcal{B}(T\to Zt)+\mathcal{B}(T\to Wb)=1$ and $\mathcal{B}(B\to Hb)+\mathcal{B}(B\to Zb)+\mathcal{B}(B\to Wt)=1$. The yellow markers indicate the branching ratios for the SU(2) singlet and doublet scenarios where the branching ratios become approximately independent of the VLQ mass [8].