Dimension-8 SMEFT analysis of minimal scalar field extensions of the Standard Model

Abstract

We analyze the constraints obtainable from present data using the Standard Model Effective Field Theory (SMEFT) on extensions of the Standard Model with additional electroweak singlet or triplet scalar fields. We compare results obtained using only contributions that are linear in dimension-6 operator coefficients with those obtained including terms quadratic in these coefficients as well as contributions that are linear in dimension-8 operator coefficients. We also implement theoretical constraints arising from the stability of the electroweak vacuum and perturbative unitarity. Analyzing the models at the dimension-8 level constrains scalar couplings that are not bounded at the dimension-6 level. The strongest experimental constraints on the singlet model are provided by Higgs coupling measurements, whereas electroweak precision observables provide the strongest constraints on the triplet model. In the singlet model the present di-Higgs constraints already play a significant role. We find that the current constraints on model parameters are already competitive with those anticipated from future di- and tri-Higgs measurements. We compare our results with calculations in the full model, exhibiting the improvements when higher-order SMEFT terms are included. We also identify regions in parameter space where the SMEFT approximation appears to break down. We find that the combination of current constraints with the theoretical bounds still admits regions where the SMEFT approach is not valid, particularly for lower scalar boson masses.

Higgs decays to two leptons and a photon beyond leading order in the SMEFT

“…The state of the art for matching onto the SMEFT is generally dimension-eight. Examples include [22,24,30,52,53]. The process of matching at tree level to an effective Lagrangian beyond leading order in the EFT expansion is in general not particularly difficult.…”

Leading directions in the SMEFT

“…There are also several important aspects on the phenomenological side worth mentioning, which could serve as promising future directions. As already pointed out in section 3.2, including RGE [7,[173][174][175], one-loop effects [176][177][178][179][180][181][182][183] and going beyond the dimension-6 operators [184][185][186] would be of great importance for improving the phenomenological analyses behind the results presented in figure 1. In addition, a complementary study focusing on the direct resonance searches for all proposed single-field extensions would allow us to compare against the EFT bounds obtained in this work.…”

Higgs associated production with a vector decaying to two fermions in the geoSMEFT

“…3. There are known instances where the dimension-eight terms have significant impacts on the infrared physics that would be missed by only including dimension-six results [14,[21][22][23][24]. This should, for completeness, be contrasted with cases where dimensioneight has little phenomenological impact [12].…”