Effective field theory and scalar extensions of the top quark sector

Abstract

Effective field theory (EFT) approaches are widely used at the LHC, such that it is important to study their validity, and ease of matching to specific new physics models. In this paper, we consider a generic extension of the SM in which a top quark couples to a new heavy scalar. We find the dimension six operators generated by this theory at low energy, and match the EFT to the full theory up to NLO precision in the simplified model coupling. We then examine the range of validity of the EFT description in top pair production, finding excellent validity even if the scalar mass is only slightly above LHC energies, provided NLO corrections are included. In the absence of the latter, the LO EFT overestimates kinematic distributions, such that over-optimistic constraints on BSM contributions are obtained. We next examine the constraints on the EFT and full models that are expected to be obtained from both top pair and four top production at the LHC, finding for low scalar masses that both processes show similar exclusion power. However, for larger masses, estimated LHC uncertainties push constraints into the non-perturbative regime, where the full model is difficult to analyse, and thus not perturbatively matchable to the EFT. This highlights the necessity to improve uncertainties of SM hypotheses in top final states.

Measuring the top energy asymmetry at the LHC: QCD and SMEFT interpretations

“…Bounds on top operator coefficients from perturbative unitarity have been discussed in refs. [48,49]. They are much weaker than our predictions and thus will not affect the interpretation of an LHC analysis.…”

Extended Higgs boson sectors, effective field theory, and Higgs boson phenomenology

“…This is the limit where our results are most relevant: BSM degrees of freedom with non-trivial QCD interactions that are integrated out to arrive at C Gφ,S , C Gφ,S = 0 can typically can be more efficiently constrained by direct searches at hadron colliders, see e.g. the recent discussion of [60,61].…”

Landscaping CP-violating BSM scenarios

“…In the light of expected uncertainties and hadron collider energy coverage, the underlying EFT assumption of an absence of direct evidence of new propagating degrees of freedom often pushes constraints on the Wilson coefficients (WCs) C i /Λ 2 in Eq. ( 1) to a regime where perturbative matching to concrete ultraviolet (UV) scenarios becomes challenged or impossible (for a recent discussion see [16]). This does not constitute a breakdown of EFT methods, but demonstrates a lack of a particular analysis' sensitivity to motivated UV completions of the SM, which is not visible from the constraint on the WCs themselves directly.…”

“…Refs. [16,18]. More concretely: Specific UV scenarios impose hierarchies among WCs that are also reflected in their evolution over a broad range of momentum transfer scales that typically informs a LHC measurement result.…”