A spurionic interpretation of RG-stable parameter relations in the two-Higgs-doublet model

Recent work on the two-Higgs-doublet model (2HDM) has uncovered basis-invariant relations among the parameters in the scalar potential that are stable under renormalization group (RG) evolution but do not coincide with any of the standard Higgs-family or Generalized-$\mathrm{C}\mathrm{P}$ symmetry classes of the scalar sector. These relations have subsequently been reformulated in terms of so-called GOOFy transformations. In this paper, we show that the same fixed surface admits a conventional interpretation in terms of spurions. The gauge-covariant kinetic terms of the 2HDM possess a Higgs-family $\mathrm{U}(2{)}_H$ symmetry, and the couplings in the potential may be organized into irreducible spurions under its nontrivial $\mathrm{S}\mathrm{U}(2{)}_H$ subgroup. We first explain the one-loop closure in the spurion language and then establish stability to all orders in the scalar and gauge couplings in any mass-independent scheme. General Yukawa couplings introduce additional Higgs-family covariants and can spoil the scalar-sector selection rule. We therefore consider the six conventional Higgs-family and Generalized-$\mathrm{C}\mathrm{P}$ symmetries of the 2HDM, imposed as exact symmetries of the full Lagrangian, and show that closure is controlled by whether the relevant Higgs-family triplet contains an invariant direction. The spurion formulation thus provides a basis-covariant and conventional group-theoretic explanation of the RG stability of the 2HDM parameter relations.