Electroweak scalar effects beyond dimension-six in SMEFT

The Standard Model effective field theory (SMEFT) provides a robust framework for probing deviations in the couplings of Standard Model particles from their theoretical predictions. This framework relies on an expansion in higher-dimensional operators, often truncated at dimension-six. In this work, we compute the effective dimension-eight operators generated by integrating out heavy scalar fields at one-loop order in the Green’s basis within two extended scalar sector models: the two Higgs doublet model and the complex triplet scalar model. In this work, we consider loops that consist of heavy field propagators only. We also investigate the impact of heavy scalar fields on the fermion sector, deriving the fermionic effective operators up to dimension-eight for these models, and detail how contributions can be mapped onto nonredundant bases. To assess the importance of higher-order contributions in the SMEFT expansion, we analyze dimension-eight effects for electroweak precision observables at the next frontier of precision lepton machines such as GigaZ.