The Z b b ¯ $$ Zb\overline{b} $$ couplings at future e + e − colliders

Abstract

Many new physics models predict sizable modifications to the SM Zbb couplings, while the corresponding measurements at LEP and SLC exhibit some discrepancy with the SM predictions. After updating the current results on the Zbb coupling constraints from global fits, we list the observables that are most important for improving the Zbb coupling constraints and estimate the expected precision reach of three proposed future e + e − colliders, CEPC, ILC and FCC-ee. We consider both the case that the results are SM-like and the one that the Zbb couplings deviate significantly from the SM predictions. We show that, if we assume the value of the Zbb couplings to be within 68% CL of the current measurements, any one of the three colliders will be able to rule out the SM with more than 99.9999% CL (5σ). We study the implications of the improved Zbb coupling constraints on new physics models, and point out their complementarity with the constraints from the direct search of new physics particles at the LHC, as well as with Higgs precision measurements. Our results provide a further motivation for the construction of future e + e − colliders.

Electroweak phase transition with composite Higgs models: calculability, gravitational waves and collider searches

“…In contrast, those mixings are present in 15 B . Since the Zb LbL couplings are stringently constrained by LEP [73,98], 15 B is strongly disfavored by the experiment and we will not use it to build the NMCHM in this paper.…”

Type-II 2HDM under the precision measurements at the Z-pole and a Higgs factory

“…The EW precision measurements are not only important in understanding the SM physics, but also can impose strong constraints on new physics models [30,31]. The benchmark scenarios of several proposed future e + e − machines and the projected precisions on Z-pole and Higgs measurements are summarized below.…”

Bottom-quark forward-backward asymmetry, dark matter, and the LHC

“…The main motivation behind this model is the observed 3 σ deviation of the bottom-quark forward-backward asymmetry A b FB measured at the LEP experiment at CERN. It is well known that this asymmetry may be modified by varying the right-handed bottom coupling to the Z-boson [5,10,[14][15][16][17][18]. In general, the modification of the couplings produces other effects that have relevant implications on the precision electroweak observables, which should be considered simultaneously.…”