Multi-top signals of vectorlike quarks at the LHC

TL;DR

This work analyzes a minimal SM extension with a weak-singlet vectorlike quark $t'$ (charge $+2/3$) and a gauge-singlet complex scalar $\phi$ that contains a pseudoscalar $a_t$ and a scalar $φ_t$. Mixing between the SM top and $t'$ produces long cascade decays when $t'\to t a_t$ or $t φ_t$ is kinematically allowed, with $a_t\to t\bar t$ and $φ_t\to a_t a_t$ often driving multi-top final states. The authors show that such cascades can yield $6t$, $8t$, and even $10t$ events at the LHC, with the $8t$ channel potentially dominating in substantial regions of parameter space; cross sections can reach up to a few fb for Run 3 and higher at HL-LHC. The final-state topology includes spectacular multi-lepton and multi-$b$-jet signatures, enabling mass measurements of all three new states if a discovery occurs, and motivating dedicated searches for hybrid and multi-top signals beyond conventional vectorlike-quark channels.

Abstract

We point out that events with 6 or more top quarks may be observed at the LHC if certain particles exist at the TeV scale. In a model where a vectorlike quark of charge 2/3 decays into a top quark and a pseudoscalar particle, which subsequently decays into a top-antitop pair, the LHC production cross section for events with 6 top quarks may be above 10 fb. If the pseudoscalar is part of a complex scalar field, then longer cascade decays, involving the scalar partner, may lead to events with 8 or even 10 top quarks. We show that for a region of parameter space the dominant LHC signal in this model is 8 top quarks (i.e., four $t\bar t $ pairs). The ensuing signals would be spectacular, including many leptons and $b$ jets. A discovery in that case would allow several cross section measurements that may determine the masses of all three new particles.

Figures (7)

• Figure 1: Upper limits on the $s_L$ mixing due to the unitarity limit (\ref{['eq:upper']}) with $y_{\rm max} = 2.3$ (dashed black line), the electroweak parameter $T < 0.12$ (solid blue line), the measured $|V_{tb}|$ element (dotted red line), and the measured Higgs-top coupling with a coupling modifier $\kappa_t > 0.83$ (dot-dashed orange line). The ruled out region lies above the solid blue line and it is shaded.
• Figure 2: Cross section for $t' \bar{t}'$ production in proton-proton collisions, computed at NLO with MadGraphAlwall:2014hca at a center-of-mass energy of $13.6$ TeV (solid red line) or $14$ TeV (dashed blue line). The scale uncertainty is indicated by envelopes around each line (see main text for details).
• Figure 3: $t'\bar{t}'$ production at the LHC leading to a 6-top (left) or 10-top (right) final state.
• Figure 4: $t'\bar{t}'$ production at the LHC leading to an 8-top final state.
• Figure 5: Cross section for $t' \bar{t}'$ production at the 13.6 TeV LHC multiplied by the branching fractions for both $t'$ and $\bar{t}'$ to decay into a top quark and a new spin-0 particle, which can be either the pseudoscalar $a_t$ or the scalar $\varphi_t$. The possible final states are $t\bar{t} a_t a_t \to 3(t\bar{t})$ (dashed red line), $t\bar{t} \, a_t \varphi_t \to t \bar{t} \, a_t a_t a_t \to 4(t\bar{t})$ (solid black line), or $t\bar{t} \, \varphi_t \varphi_t \to t \bar{t} \, a_t a_t a_t a_t \to 5(t\bar{t})$ (dotted blue line). The $a_t$ and $\varphi_t$ masses are fixed at 400 GeV and 1 TeV, respectively, and $\mathcal{B}(\varphi_t \to a_t a_t)$ is taken to be 1.