Approaching robust EFT limits for CP violation in the Higgs sector

Constraining CP-violating interactions in effective field theory of dimension six faces two challenges. First, degeneracies in the multidimensional space of Wilson coefficients have to be lifted. Second, quadratic contributions of CP-odd dimension-six operators are difficult to disentangle from squared contributions of CP-even dimension-six operators and from linear contributions of dimension-eight operators. Both of these problems are present when new sources of CP violation are present in the interactions between the Higgs boson and heavy strongly interacting fermions. We show that degeneracies in the Wilson coefficients can be removed by combining measurements of Higgs-plus-two-jet production via gluon fusion with measurements of top-pair associated Higgs production. In addition, we demonstrate that the sensitivity of the analysis can be improved by exploiting the top-quark threshold in the gluon fusion process. Finally, we substantiate a perturbative argument about the validity of effective field theory by comparing the quadratic and linear contributions from CP-odd dimension-six operators and use this to show explicitly that high statistics measurements at future colliders enable the extraction of perturbatively robust constraints on the associated Wilson coefficients.