HL-LHC sensitivity to an ultraheavy $S_{uu}$ diquark in the $uχ$ channel

TL;DR

This paper investigates HL-LHC sensitivity to an ultraheavy color-sextet diquark $S_{uu}$ produced in up-quark fusion and decaying via $S_{uu} \to u\chi$, with $\chi$ decaying hadronically to $Wb$, $Zt$, or $h^0 t$. It employs a consistent Monte Carlo setup at $\sqrt{s}=13.6$ TeV and a Random Forest classifier to distinguish signal from multijet backgrounds in fully hadronic final states, exploring varying $m_\chi$, $M_S$, and Yukawa couplings. The results show 95% C.L. exclusions around $M_S \approx 8.2$ TeV for $m_\chi = 1.5$–$2$ TeV and modest couplings, extending to $\approx 9.25$ TeV when the Yukawa coupling $y_{u\chi}$ is large ($0.5$); decays to $Zt$ and $h^0 t$ yield somewhat lower reach. These findings complement prior analyses of the $S_{uu} \to \chi\chi$ six-jet channel and motivate a global, multi-channel approach to constraining ultraheavy diquarks at the HL-LHC, including future refinements such as NLO corrections and PDF uncertainties.

Abstract

We study the HL-LHC sensitivity to an ultraheavy diquark $S_{uu}$ produced in up-quark fusion and decaying as $S_{uu}\to uχ$, $χ\to Wb, Zt, h^0t$. For fully hadronic decays of the W, Z and top quark, this gives rise to multijet final states. Within the same model framework used previously for the $S_{uu}\toχχ$ six-jet channel, we consider $S_{uu}$ masses in the multi-TeV range and vectorlike quark masses of order a few TeV, and simulate proton-proton collisions at $\sqrt s = 13.6$ TeV with integrated luminosities up to the HL-LHC target. The analysis strategy employs a machine-learning-based discriminant adapted from the six-jet study to the new four-jet topology, which we use to derive the corresponding discovery reaches and exclusion limits. We find that this topology improves the overall sensitivity to $S_{uu}$ in regions where the branching ratio $B(S_{uu}\to uχ)$ is sizable and provides a complementary signature for studying ultraheavy diquarks at the HL-LHC.

Figures (8)

• Figure 1: $pp\rightarrow S_{uu}\rightarrow u\chi\rightarrow u(Wb)\rightarrow u(jjb)$ cross section as a function of $M_S$ and $y_{uu}$, $y_{u\chi}$ at $\sqrt{s}=13.6$ TeV.
• Figure 2: Fully hadronic cross sections for different $\chi$ decay modes in $S_{uu}\rightarrow u\chi$ at $\sqrt{s}=13.6$ TeV, for $y_{uu}=0.2$ and $y_{u\chi}=0.1$.
• Figure 3: 95% C.L. Upper Limit on the signal strength multiplier $\mu$ times the ML event yield $S_{ev}$ for $S_{uu} \rightarrow u \chi \rightarrow u (Wb)$ fully hadronic channel with $m_\chi = 1.5$ TeV and $D=0.9$.
• Figure 4: 95% C.L. Upper Limit on the signal strength multiplier $\mu$ times the ML event yield $S_{ev}$ for $S_{uu} \rightarrow u \chi \rightarrow u (Wb)$ fully hadronic channel with $m_\chi = 2$ TeV and $D=0.9$.
• Figure 5: 95% C.L. Upper Limit on the signal strength multiplier $\mu$ times the ML event yield $S_{ev}$ for $S_{uu} \rightarrow u \chi \rightarrow u (Wb)$ fully hadronic channel for the conservative $y_{u\chi}=0.1$ case and the $y_{u\chi}=0.5$ discovery channel.