Abstract
A primary goal of present and future colliders is measuring the Higgs couplings to Standard Model (SM) particles. Any observed deviation from the SM predictions for these couplings is a sign of new physics whose energy scale can be bounded from above by requiring tree-level unitarity. In this paper, we extend previous work on unitarity bounds from the Higgs cubic coupling to Higgs couplings to vector bosons and top quarks. We find that HL-LHC measurements of these couplings compatible with current experimental bounds may point to a scale that can be explored at the HL-LHC or a next-generation collider. Our approach is completely model-independent: we assume only that there are no light degrees of freedom below the scale of new physics, and allow arbitrary values for the infinitely many couplings beyond the SM as long as they are in agreement with current measurements. We also extend and clarify the methodology of this analysis, and show that if the scale of new physics is above the TeV scale, then the deviations can be described by the leading higher-dimension gauge invariant operator, as in the SM effective field theory.
Highlights
The discovery of the Higgs boson at the Large Hadron Collider (LHC) has opened a new chapter in elementary particle physics
We show that marginalizing over unmeasured couplings does not substantially improve the unitarity bound, and we show that if the scale of new physics is high, the quartic Higgs coupling is approximately described by the predictions of the Standard Model effective field theory
We find that measurements at HL-LHC that are consistent with current constraints may point to a scale of new physics in the few TeV range, a scale that can be directly explored at the HL-LHC and future colliders
Summary
The discovery of the Higgs boson at the Large Hadron Collider (LHC) has opened a new chapter in elementary particle physics. As our results will show, upcoming HL-LHC measurements of Higgs couplings probe new physics at the scale of a few TeV or below This scale is not sufficiently large that we can confidently neglect higher-dimension operators in the Standard Model effective field theory (SMEFT). We show that marginalizing over unmeasured couplings does not substantially improve the unitarity bound, and we show that if the scale of new physics is high, the quartic Higgs coupling is approximately described by the predictions of the Standard Model effective field theory. In these cases, we find that measurements at HL-LHC that are consistent with current constraints may point to a scale of new physics in the few TeV range, a scale that can be directly explored at the HL-LHC and future colliders.
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