Optimal integration of Zh angular observables into a global analysis of the Higgs self-coupling λ_hhh

Identify and validate the optimal methodology to incorporate angular observables from e+e− → Zh production into a global analysis of the Higgs self-coupling λ_hhh, comparing and assessing approaches such as template fits to differential cross sections, compression into effective hZZ couplings constrained by allowed Lorentz structures, and statistically optimal observables.

Background

The authors discuss two complementary strategies to determine the Higgs self-coupling at e+e− colliders: direct di-Higgs production at high energies and indirect constraints from single-Higgs processes such as Zh production at 240–350 GeV. In the single-Higgs approach, loop-level sensitivity to λ_hhh can be enhanced using differential information, including angular distributions.

However, they explicitly state that the best way to include such angular observables in a global fit remains unclear, and they list several candidate techniques. Establishing the optimal method is necessary to robustly disentangle λ_hhh from other parameters in SMEFT or model-based analyses.

References

It is not very clear what is the best way to incorporate the angular observables in the global analysis for λhhh.

— Focus topics for the ECFA study on Higgs / Top / EW factories  (2401.07564 - Blas et al., 2024) in Section Hself — Single-Higgs: lifting the degeneracies

Evidence is developing today as if $H(125)$ were indeed the textbook Higgs boson, but the case is not yet closed. We have not yet measured the $H$ self-interactions to determine the shape of the Higgs potential.

— From the November Revolution toward the Millennium  (2608.27279 - Quigg, 27 Aug 2026) in Section 4, “The Higgs Sector”