Direct bounds on heavy toplike quarks with standard and exotic decays

Heavy vectorlike quarks with electric charge $Q=2/3$ (also called heavy tops) appear naturally in many extensions of the Standard Model. Although these typically predict the existence of further particles below the TeV scale, direct searches for heavy tops have been performed assuming that they decay only into SM particles. The aim of this paper is to overcome this situation. We consider the most constraining experimental LHC searches for vectorlike quarks, including analyses of the $36{\mathrm{fb}}^{-1}$ of data collected in the latest run at 13 TeV of center of mass energy, as well as searches sensitive to heavy tops decaying into a new scalar, $S$. Combining all these, we derive bounds for arbitrary values of the heavy top branching ratios. A simple code that automatizes this process is also provided. At the physics level, we demonstrate that bounds on heavy tops are not inevitably weaker in the presence of new light scalars. We find that heavy tops with masses below $\sim 900\mathrm{GeV}$ are excluded by direct searches, independently of whether they decay into $Zt$, $Ht$, $Wb$ or $St$ (with $S$ giving either missing energy of bottom quarks) or into any combination of them.

Figure 1

NNLO cross sections for pair production of top partners [33, 35] at 8 (left) and 13 (right) TeV of c.m.e.

Figure 2

(left) Region of the plane of branching ratios excluded by the $Zt+X$ analysis of Ref. [36] for $M=800\mathrm{GeV}$. The dashed green lines represent the constraints provided by the experimental collaboration, while the orange area is the one we obtain with the simplified method outlined in the text. (middle) Same as (left) but for $M=1000\mathrm{GeV}$. (right) Same as (left) but for $M=1100\mathrm{GeV}$.

Figure 3

(left) Region of the plane of branching ratios excluded by the $Ht+X$ analysis of Ref. [33] for $M=750\mathrm{GeV}$. The dashed green lines represent the constraints provided by the experimental collaboration, while the orange area is the one we obtain with the simplified method outlined in the text. (middle) Same as (left) but for the $Wb+X$ analysis. (right) Same as (left) but for the combination of both analyses.

Figure 4

(left) Normalized distribution of $m_{bb}^{\min \mathrm{\Delta }R}$ for $M=600\mathrm{GeV}$ and $m_S=200\mathrm{GeV}$ in the $Ht+Wb$ (thick solid orange) and the $St+Wb$ (thin solid green) channels. The cut on $m_{bb}^{\min \mathrm{\Delta }R}>100\mathrm{GeV}$ is depicted with a vertical dashed line. (right) Same as (left) but for $M=800\mathrm{GeV}$ and $m_S=300\mathrm{GeV}$.

Figure 5

(left) Excluded region in the stop-neutralino mass plane by the CMS search of Ref. [37]. The area enclosed by the solid (dashed) orange (green) line is obtained with the recast analysis (by the experimental collaboration). (right) Bound on $\mathrm{BR}(T\to St)$ for $m_S=100$ (dashed green), 200 (dashed orange), 300 (solid orange) and 400 (solid green) GeV.

Figure 6

Excluded regions in the space of branching ratios for $M=700\mathrm{GeV}$ and $m_S=100\mathrm{GeV}$. The gray area results from a combined statistical analysis of all signal regions of Table 2. In the green area, the $St+X$ events are disregarded. The red region is excluded by the analysis of Ref. [35]. Likewise, the blue region is excluded by the analysis of Ref. [34].

Figure 7

Same as Fig. 6 but for $M=800\mathrm{GeV}$.

Figure 8

Same as Fig. 6 but for $M=900\mathrm{GeV}$.

Figure 9

Same as Fig. 6 but for $M=1000\mathrm{GeV}$.