Exploring Higgs EFT in $t\bar{t}hh$ at High Luminosity LHC

TL;DR

This work explores Higgs EFT (HEFT) effects in non-resonant $t\bar{t}hh$ production at the HL‑LHC, focusing on four couplings $\delta\kappa_\lambda$, $c_2$, $c_{2g}$ and $c_{tg}$. It employs two analysis strategies—cut-based $H_T$ binning and a parametric BDT (PM:BDT) that interpolates across HEFT benchmarks—to derive 95% CL constraints, with the PM:BDT showing substantial gains, especially when combining the single-lepton and dilepton channels. The study fixes $c_g$ and $c_{tg}$ to near-SM values based on external constraints, and finds that the HL‑LHC can provide first sensitivity projections on $c_2$, $c_{2g}$ and $c_{tg}$ in $t\bar{t}hh$ despite the challenging final state. Overall, the results demonstrate the strong potential of $t\bar{t}hh$ at HL‑LHC to probe extended Higgs and top-quark interactions, particularly via multivariate, channel-combined analyses.

Abstract

The non-resonant production of a Higgs boson pair in association with a top-antitop quark pair ($pp\rightarrow t\bar{t}hh$) has only recently begun to be explored at the Large Hadron Collider (LHC) and provides a unique and largely uncharted probe of the top-Higgs sector, offering complementary sensitivity to the Higgs self-coupling and higher-dimensional interactions beyond the Standard Model. In this work, we present a detailed study of this process within the framework of Higgs Effective Field Theory (HEFT) at the High-Luminosity LHC (HL-LHC). A comparative analysis is performed using a traditional cut-based approach in the single-lepton channel and a multivariate parametric boosted decision tree method in both single-lepton and dilepton final states. We derive one- and two-parameter limits at 95\% confidence level on the HEFT couplings $δκ_λ$, $c_2$, $c_{2g}$, and $c_{tg}$. The projected bound on $δκ_λ$ is weaker than current experimental constraints from dedicated Higgs-pair measurement; however, this coupling plays a critical role in shaping the multidimensional allowed parameter space. For the remaining HEFT couplings, where no direct experimental limits currently exist, our results provide the first sensitivity projections in the $t\bar{t}hh$ channel. Overall, this study demonstrates the strong potential of the $t\bar{t}hh$ production process to probe extended Higgs and top-quark interactions beyond the Standard Model through the exploitation of the $t\bar{t}hh$ data at the HL-LHC.

Figures (8)

• Figure 1: Representative Feynman diagrams for gluon-gluon initiated $t\bar{t}hh$ production with the HEFT couplings in Eq. (\ref{['eq:eftLag']}) represented by shaded blobs.
• Figure 2: Normalized distributions for some sensitive variables are shown, for the four main backgrounds $t\bar{t}h$, $t\bar{t}+4b$, $t\bar{t}+2b$ and $t\bar{t}+jets$) and the SM signal $t\bar{t}hh$ processes.
• Figure 3: Production cross section of $t\bar{t}hh$ at LO in QCD as a function of HEFT couplings as given in Eq. (\ref{['eq:prod-cs-HEFT']}). The lines represent the fitted functions, whereas the blobs on the curve represent the benchmark points (BPs) where simulations are performed. The SM prediction is represented by the horizontal dashed black lines.
• Figure 4: Normalized distributions for some HEFT-sensitive variables are shown for the combined background (BKG) and a few selected benchmark points.
• Figure 5: Correlation among the global event variables, reconstructed di-boson topological variables, and global vs. the topological variables are presented for the $t\bar{t}hh$ process with HEFT coupling $c_2=-2.5$.