The role of the top Yukawa coupling in triple Higgs production at the LHC

TL;DR

The paper investigates how the top Yukawa coupling influences triple-Higgs production at the LHC. It employs a deconstruction method to express the HHH cross-section as a sum of terms with distinct coupling dependencies, enabling reweighting across parameter points using precomputed reduced cross-sections. The results show that varying y_t within its experimental uncertainties can dramatically change the LO cross-section (e.g., σ(HHH) from 0.034 fb to 0.12 fb when kt=1.2, and to 0.007 fb when kt=0.8) while the differential shapes like the three-Higgs invariant mass are only mildly affected, indicating a mainly rescaling effect. These findings underline the importance of precise top-Yukawa measurements for observing HHH at the HL-LHC and for tightening constraints on the Higgs self-couplings, and demonstrate the deconstruction approach as a practical tool for exploring coupling-structure in multi-Higgs processes.

Abstract

In this letter I quantify the effects of varying the top Yukawa coupling in the process of production of three Higgs bosons at the LHC. Modifications of the coupling within experimentally observed ranges have a sizeable impact on the total cross-section, but do not have large effects on the distributions of the invariant mass of the three-Higgs system and other global observables.

Figures (4)

• Figure 1: Examples of interference topologies associated with the deconstruction term $(\delta y_t)^2(\delta \lambda_3)(\delta \lambda_4)\hat{\sigma}_{211}$. The couplings which receive modifications are explicitly indicated.
• Figure 2: Bounds for the $\kappa_3$ and $\kappa_4$ coupling modifiers for variations of the top Yukawa couplings within the 2$\sigma$ windows of the ATLAS and CMS fits ATLAS:2022vkfCMS:2022dwdParticleDataGroup:2024cfk, for the triple-Higgs production process, using the 59 fb upper limit on the $HHH$ cross-section from ATLAS:2024xcs. An estimate of the limits for $\kappa_3$ as function of the top Yukawa, using the results of ATLAS:2024ish are shown as vertical lines. For both $HHH$ and $HH$, the bounds obtained assuming $k_t=1$ (unmodified SM value) are shown for reference. The bounds on $\kappa_3$ obtained by ATLAS by combining $H$ and $HH$ measurements with relaxed assumptions about all $\kappa$ modifiers ATLAS:2022jtk are also shown. The perturbative unitarity regionDiLuzio:2017tfnStylianou:2023tgg is also overlaid.
• Figure 3: Normalised differential distributions for the invariant mass of the tri-Higgs system (left) and the scalar sum of their transverse momenta (right), for different values of the top Yukawa coupling around $\pm$20% of its SM value, and with all other couplings set to the SM value.
• Figure 4: Normalised differential distributions for the scalar sum of transverse momenta of hadronic objects in the final state after Higgs decays for $\kappa_t=1\pm0.2$ and $\kappa_{b,3,4}=1$.