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

Effective field theory and scalar extensions of the top quark sector