Abstract
We explore the extent to which future precision measurements of the Standard Model (SM) observables at the proposed Z-factories and Higgs factories may have impacts on new physics beyond the Standard Model, as illustrated by studying the Type-I Two-Higgs-doublet model (Type-I 2HDM). We include the contributions from the heavy Higgs bosons at the tree-level and at the one-loop level in a full model-parameter space. While only small tan β region is strongly constrained at tree level, the large tan β region gets constrained at loop level due to tan β enhanced tri-Higgs couplings. We perform a multiple variable χ2 fit with non-alignment and non-degenerate masses. We find that the allowed parameter ranges could be tightly constrained by the future Higgs precision measurements, especially for small and large values of tan β. Indirect limits on the masses of heavy Higgs bosons can be obtained, which can be complementary to the direct searches of the heavy Higgs bosons at hadron colliders. We also find that the expected accuracies at the Z-pole and at a Higgs factory are quite complementary in constraining mass splittings of heavy Higgs bosons. The typical results are | cos(β − α)| < 0.05, |∆mΦ| < 200 GeV, and tan β ≳ 0.3. The reaches from CEPC, Fcc-ee and ILC are also compared, for both Higgs and Z-pole precision measurements. Comparing to the Type-II 2HDM, the 95% C.L. allowed range of cos(β − α) is larger, especially for large values of tan β.
Highlights
We explore the extent to which future precision measurements of the Standard Model (SM) observables at the proposed Z-factories and Higgs factories may have impacts on new physics beyond the Standard Model, as illustrated by studying the Type-I TwoHiggs-doublet model (Type-I 2HDM)
We find that the expected accuracies at the Zpole and at a Higgs factory are quite complementary in constraining mass splittings of heavy Higgs bosons
To compare the sensitivities of different Higgs factory machine options, in figure 13, we show the reach in cos(β − α) − tan β plane for Circular Electron Positron Collider (CEPC), Fcc-ee and International Linear Collider (ILC) for mφ = 800 GeV and 2000 GeV
Summary
The SM has been tested to a high precision from the measurements at the Z-pole from LEP-I [34], at the Tevatron [36] and the LHC [37]. We list the running scenarios of various machines in terms of their center of mass energies and the corresponding integrated luminosities, as well as the estimated precisions of relevant Higgs measurements that are used in our global analyses in table 3 These expected results in the table serve as the input values for the later studies in this paper in constraining the theoretical parameters in the BSM Higgs sector. In the context of 2HDM, the BSM Higgs sector would be more readily probed at those machines by direct searches via processes like e+e− → Z∗ → bbA/H, AH and H+H−, etc Those studies would be interesting and important, but the analyses of the signals and backgrounds would be quite a different task from the current focus based on the Higgs and EW precision measurements on the SM parameters. The reach in the charged Higgs boson mass and tan β is comparable to the Higgs precision measurements [53], which are complementary to the oblique corrections that are more sensitive to the mass differences between the charged Higgs and the neutral ones
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