Monotop signature from a fermionic top partner

We investigate monotop signatures arising from phenomenological models of fermionic top partners, which are degenerate in mass and decay into a bosonic dark matter candidate, either spin 0 or spin 1. Such a model provides a monotop signature as a smoking gun, while conventional searches with $t\overline{t}+$ missing transverse momentum are limited. Two such scenarios, (i) a phenomenological third generation extradimensional model with excited top and electroweak sectors, and (ii) a model where only a top partner and a dark matter particle are added to the standard model, are studied in the degenerate mass regime. We find that in the case of extra dimension a number of different processes give rise to effectively the same monotop final state, and a great gain can be obtained in the sensitivity for this channel. We show that the monotop search can explore top-partner masses up to 630 and 300 GeV for the third generation extradimensional model and the minimal fermionic top-partner model, respectively, at the high luminosity LHC.

Figure 1

Representative set of Feynman diagrams resulting in the monotop signature in the third generation extradimensional model.

Figure 2

Schematic monotop event display. The grey dashed lines represent invisible particles, whereas the thin grey lines depict soft particles that do not pass the minimum selection criteria.

Figure 3

The missing energy ${\cancel{E}}_T$ distributions for the SM background (black) and the monotop signal in the third generation extradimensional model [case (i), blue]. The distribution for the SUSY case with the $\tilde{t}t{\tilde{\chi }}_1^0$ channel is also shown in red. For illustration purposes, we show the SUSY and UED distributions with the same top-partner masses. We assume the 13 TeV LHC with $3{\mathrm{ab}}^{-1}$ of integrated luminosity.

Figure 4

Sensitivity lines $\mathcal{S}/\sqrt{\mathcal{B}}$ (solid) and $\mathcal{S}/\mathcal{B}$ (dashed) with $\int \mathcal{L}dt=3{\mathrm{ab}}^{-1}$ as functions of the top-partner mass $m_{t^{(1)}}$ in the third generation extradimensional model. The mass splitting $m_{t^{(1)}}-m_{X^{(1)}}=8\mathrm{GeV}$ is assumed. The results for different missing energy selections, ${\cancel{E}}_T/\mathrm{GeV}>550$ (blue), 600 (green), and 650 (orange), are shown.

Figure 5

Current bound on the mass of the fermionic top partner from the monojet study at 95% C.L [68]. We display the results for different missing energy selections.

Figure 6

The current and projected sensitivities on $\xi$ [introduced in Eq. (8)]. The integrated luminosity of 36 300, and $3000{\mathrm{fb}}^{-1}$ is considered.