Probing New Physics Scales from Higgs and Electroweak Observables at $e^+ e^-$ Higgs Factory

Abstract

New physics beyond the standard model (SM) can be model-independently formulated via dimension-6 effective operators, whose coefficients (cutoffs) characterize the scales of new physics. We study the probe of new physics scales from the electroweak precision observables (EWPO) and the Higgs observables (HO) at the future $e^+e^-$ Higgs factory (such as CEPC). To optimize constraints of new physics from all available observables, we establish a scheme-independent approach. With this formulation, we treat the SM electroweak parameters and the coefficients of dimension-6 operators on equal footing, which can be fitted simultaneously by the same $χ^2$ function. As deviations from the SM are generally small, we can expand the new physics parameters up to linear order and perform an analytical $χ^2$ fit to derive the potential reach of the new physics scales. We find that the HO from both Higgs produnction and decay rates can probe the new physics scales up to 10TeV (and to 44TeV for the case of gluon-involved operator $\mathcal{O}_g$), and the new physics scales of Yukawa-type operators can be probed by the precision Higgs coupling measurements up to (13-25)TeV. Further including the EWPO can push the limit up to 35TeV. From this prospect, we demonstrate that the EWPO measured in the early phase of a Higgs factory can be as important as the Higgs observables. These indirect probes of new physics scales at the Higgs factory can mainly cover the energy range to be directly explored by the next generation hadron colliders of pp(50-100TeV), such as the SPPC and FCC-hh.