Abstract
We perform a global effective-field-theory analysis to assess the combined precision of Higgs couplings, triple gauge-boson couplings, and top-quark couplings, at future circular e+e− colliders, with a focus on runs below the production threshold. Deviations in the top-quark sector entering as one-loop corrections are consistently taken into account in the Higgs and diboson processes. We find that future lepton colliders running at center-of-mass energies below the production threshold can still provide useful information on top-quark couplings, by measuring virtual top-quark effects. With rate and differential measurements, the indirect individual sensitivity achievable is better than at the high-luminosity LHC. However, strong correlations between the extracted top-quark and Higgs couplings are also present and lead to much weaker global constraints on top-quark couplings. This implies that a direct probe of top-quark couplings above the production threshold is also helpful for the determination of Higgs and triple-gauge-boson couplings. In addition, we find that below the production threshold, the top-quark Yukawa coupling can be determined by its loop corrections to all Higgs production and decay channels. Degeneracy with the ggh coupling can be resolved, and even a global limit is competitive with the prospects of a linear collider above the threshold. This provides an additional means of determining the top-quark Yukawa coupling indirectly at lepton colliders.
Highlights
After the discovery of the Higgs boson [1, 2], understanding the electroweak symmetry breaking mechanism remains one of the major challenges in particle physics
Prospects have been widely studied through global analyses in the standardmodel effective field theory (SMEFT) and revealed that improvements of up to several orders of magnitude can be achieved compared to present limits [3,4,5,6,7,8,9]
In the absence of clear signs of physics beyond the standard model (BSM), a common approach for testing the SM and identifying possible deviations is provided by the SMEFT [20,21,22]
Summary
After the discovery of the Higgs boson [1, 2], understanding the electroweak symmetry breaking mechanism remains one of the major challenges in particle physics. Given the expected precision of measurements at future lepton colliders, next-to-leading-order (NLO) theory predictions in the SMEFT can potentially be relevant. These corrections can involve effective operators which do not appear at leading order, and provide new opportunities in the exploration of physics beyond the standard model (SM). The only missing ingredient was the theory prediction for W +W − production at the same order This process is notably sensitive to the triple gauge-boson couplings (TGC) which can be generated by operators affecting Higgs interactions. If future lepton colliders only run below the tt (tth) threshold, can they still determine top-quark– gauge-boson (top-quark Yukawa) couplings with high precision?.
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