Beyond the Standard Model effective field theory: The singlet extended Standard Model

One of the assumptions of simplified models is that there are a few new particles and interactions accessible at the LHC and all other new particles are heavy and decoupled. The effective field theory (EFT) method provides a consistent method to test this assumption. Simplified models can be augmented with higher order operators involving the new particles accessible at the LHC. Any UV completion of the simplified model will be able to match onto these beyond the Standard Model EFTS (BSM-EFT). In this paper, we study the simplest simplified model: the Standard Model extended by a real gauge singlet scalar. In addition to the usual renormalizable interactions, we include dimension-5 interactions of the singlet scalar with Standard Model particles. As we will show, even when the cutoff scale is 3 TeV, these new effective interactions can drastically change the interpretation of Higgs precision measurements and scalar searches. In addition, we discuss how power counting in a BSM-EFT depends strongly on the processes and parameter space under consideration. Finally, we propose a chi(2) method to consistently combine the limits from new particle searches with measurements of the Standard Model. Unlike imposing a hard cutoff on heavy resonance rates, our method allows fluctuations in individual channels that are consistent with global fits.

Automated summary

The assumption of simplified models is that only a few new particles and interactions are accessible at the LHC, with all other new particles being heavy and decoupled. However, even when the cutoff scale is 3 TeV, new effective interactions involving Standard Model particles can significantly impact interpretations of Higgs precision measurements and scalar searches. The power counting in BSM-EFT strongly depends on the processes and parameter space being considered.