Enhanced di-Higgs production from TeV-scale heavy neutral leptons at future lepton colliders

TL;DR

This work investigates how TeV-scale heavy neutral leptons (HNLs) in a low-scale Inverse Seesaw framework (ISS(3,3)) can enhance di-Higgs production in $\ell^+\ell^- \to hh$ at future lepton colliders. The authors compute all one-loop HNL contributions, emphasizing box diagrams, and analyze several collider scenarios (FCC-ee, Linear Collider setups, plasma wake-field accelerators, and Muon Colliders). They demonstrate that HNLs can boost the cross-section by up to $\sim 60\times$ at $\sqrt{s}=10$ TeV for $e^+e^-$, with substantial but smaller enhancements for $\mu^+\mu^-$, and that the peak occurs near $M_0 \approx \sqrt{s}/2$. The study also examines correlations with electroweak precision observables, finding potential shifts in $\Gamma(Z\to\text{inv})$ and the $T$ parameter, while some observables like $h\to\mu\mu$ and $Z\to\ell\ell$ can show modest deviations; this highlights the complementarity of di-Higgs channels to precision EW tests and cLFV searches. Overall, di-Higgs production at very-high-energy leptonic colliders provides a novel and complementary probe of low-scale seesaw scenarios and heavy neutral leptons.

Abstract

Within the context of heavy neutral lepton extensions of the Standard Model, we consider the rare di-Higgs production mode $\ell^+\ell^-\to hh$ at future high-energy lepton colliders. As a concrete example, we study the impact of a low-scale Inverse Seesaw realisation on the prospects for di-Higgs production. Our results show that the presence of TeV-scale heavy neutral leptons can enhance the cross-section by up to factor 60. We further comment on the interplay with electroweak precision observables, showing that bounds on the di-Higgs production cross-section at future high-energy lepton colliders could serve as complementary probes of low-scale seesaw scenarios.

Figures (9)

• Figure 1: Feynman diagrams of the Standard Model one-loop contribution to $\ell^+\ell^- \to hh$ (displayed in Unitary gauge).
• Figure 2: Cross-section predictions for several SM di-Higgs production modes at lepton colliders with respect to the centre-of-mass energy, $\sqrt{s}$.
• Figure 3: On the left, pseudo-rapidity ($\eta$) distribution of $h$ in $e^+e^-\to hh$ for various fixed $\sqrt{s}$; on the right panel, transverse-momentum ($p_T$) distribution of $h$ in $e^+e^-\to hh$. We consider different possibilities for $\sqrt s$. The differential cross-sections have been normalised to their total integrated values.
• Figure 4: Feynman diagrams of the one-loop contribution to $\ell^+\ell^- \to hh$ in the presence of HNL (displayed in Unitary gauge).
• Figure 5: Cross-section for $\ell \ell \to hh$ production (fb) as a function of the degenerate heavy neutrino mass ($M_0$, in GeV), for $\sqrt{s}=365$ GeV, 1 TeV, 3 TeV and 10 TeV, and colliding electron or muon beams. Full (dashed) lines correspond to the total (SM) contributions to the di-Higgs production cross-section.