Assessing uncertainties in the determination of the trilinear Higgs self-coupling from single-Higgs observables
Henning Bahl, Philip Bechtle, Johannes Braathen, Sven Heinemeyer, Jenny List, Murillo Vellasco, Georg Weiglein
TL;DR
The paper addresses the challenge of indirectly determining the trilinear Higgs self-coupling $λ_{hhh}$ through loop effects in single-Higgs observables at future circular $e^+e^-$ colliders within a SMEFT framework, using the Inert Doublet Model (IDM) as a concrete BSM example. It employs global SMEFT fits (via HEPfit) with IDM pseudo-data, mapping IDM effects onto the Warsaw-basis Wilson coefficients and exploring several EFT truncation schemes to quantify theoretical uncertainties. A key contribution is the explicit estimation of truncation-induced theory errors $ε_{theo}$ and their propagation as new nuisance parameters in the fits, showing that these uncertainties can substantially degrade the precision on $κ_λ$ compared to naïve one-loop expectations. The work underscores the importance of accounting for EFT truncation in precision Higgs studies and suggests that direct di-Higgs measurements at a future linear collider may offer more robust access to $λ_{hhh}$, while also outlining future avenues to refine uncertainty estimates (including $C_1$-coefficient uncertainties).
Abstract
Circular $e^{+}e^{-}$ colliders operating at energies below the di-Higgs production threshold can provide information on the trilinear Higgs self-coupling $λ_{hhh}$ via its loop contributions to single Higgs production processes and electroweak precision observables. We investigate how well a non-SM value of $λ_{hhh}$ can be determined indirectly via its loop contributions within a global EFT fit. Using an inert doublet extension of the SM Higgs sector as an example for a scenario of physics beyond the SM that could be realised in nature, we find that theoretical uncertainties related to the treatment of loop contributions and the truncation of the EFT expansion, which are usually neglected, play an important role in determining the sensitivity to $λ_{hhh}$ in a global fit. The results obtained from such an indirect determination of $λ_{hhh}$ without taking these additional uncertainties into account would be too optimistic, leading to an artificially high resulting precision for $λ_{hhh}$. They could therefore be misleading in the quest to precisely identify the underlying physics of electroweak symmetry breaking.