Electroweak Scalar Effects Beyond Dimension-6 in SMEFT

TL;DR

This paper calculates dimension-eight operators in SMEFT arising from heavy scalars in extended models, analyzing their effects on electroweak precision observables to improve the understanding of potential deviations from the Standard Model.

Contribution

It provides the first detailed computation of dimension-eight operators from heavy scalars in two extended scalar sector models within SMEFT, including their impact on fermionic sectors and precision observables.

Findings

Dimension-eight operators generated by heavy scalars are computed at one-loop.

Heavy scalars significantly affect electroweak precision observables.

Higher-order SMEFT contributions are relevant for future precision measurements.

Abstract

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. 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 non-redundant bases. To assess the importance of higher-order contributions in the SMEFT expansion, we analyze the dimension-eight effects for electroweak precision observables at the next frontier of precision lepton machines such as GigaZ.