Observability of an ultraheavy diquark decaying into vectorlike quarks at the LHC

TL;DR

The paper investigates the discovery potential and exclusion reach for an ultraheavy diquark scalar $S_{uu}$ decaying to a pair of vectorlike quarks $\chi$ at the HL-LHC, focusing on the fully hadronic six-jet final state. It employs MC simulations at $\sqrt{s}=13.6$ TeV and two signal benchmarks $y_{uu}B(\chi\to Wb)=0.1$ and $0.2$ with $m_{\chi}=1.5$ or $2$ TeV, using a Random Forest classifier to optimize signal–background separation and a discriminator threshold $D=0.9$. The analysis implements two statistical frameworks—RooFit/RooStats with a Poisson likelihood and log-normal nuisances, and a toy-based $CL_s$ approach with asymptotic limits—to derive local $p$-values and $CL_s$ upper limits on the signal strength $\mu$. Results indicate discovery potential near $M_S\sim7$--$7.2$ TeV for favorable couplings and significant exclusion reach up to $M_S\sim8.2$--$9.3$ TeV depending on $m_{\chi}$ and $y_{uu}$, with consistent conclusions across methods and a stability of limits against the ML discriminator. The work highlights how larger Yukawa couplings extend the mass reach while heavier VLQs modestly reduce sensitivity, and it points to future studies including additional decay channels and combined analyses to further characterize the $S_{uu}$ phenomenology at the HL-LHC.

Abstract

We present a comprehensive analysis of the discovery reach and exclusion limits for an ultraheavy diquark scalar (7 - 8.75 TeV) decaying into two vectorlike quarks (1.5 - 2 TeV) at the HL-LHC, improving upon the statistical results reported in our previous six-jet final state study. The statistical analysis was performed using the output of Machine Learning algorithms trained to discriminate signal from background. The mass regions of interest were determined through scans of the local $p$-values, $CL_s$, and the upper limits on the model-independent signal strength $μ$. The results indicate a promising sensitivity to ultraheavy diquark scalars within the explored mass range, suggesting that the HL-LHC could either discover or set stringent exclusion limits on such particles.

Figures (8)

• Figure 1: Cross-section for $pp\to S_{uu}\rightarrow \chi\chi$ at $\sqrt{s}=13.6$ TeV, when $\chi \rightarrow W^+b \rightarrow (jj)b$, $y_{uu}=0.2$ and $m_\chi = 1.5$ and 2 TeV.
• Figure 2: Local $p$-value quantifying the excess above the observed values of the test statistic $\tilde{q}_{\mu}^{\text{obs}}$, for two D values.
• Figure 3: Local $p$-value quantifying the excess above the observed values of the test statistic $\tilde{q}_{\mu}^{\text{obs}}$, for two $m_\chi$.
• Figure 4: $CL_s$ scan in terms of $M_S$ for a VLQ mass of 1.5 TeV.
• Figure 5: $CL_s$ scan in terms of $M_S$ for a VLQ mass of 2 TeV.