Anomalous Zbb¯ Couplings: From LEP to LHC

Abstract

The bottom quark forward-backward asymmetry (A b F B ) data at LEP exhibits a long-standing discrepancy with the standard model prediction. We propose a novel method to probe the Zb b interactions through gg → Zh production at the LHC, which is sensitive to the axial-vector component of the Zb b couplings. We demonstrate that the Zh data collected at the 13 TeV LHC can already resolve the apparent degeneracy of the anomalous Zb b couplings implied by the LEP precision electroweak measurements, with a strong dependence on the observed distribution of the Z boson transverse momentum. We also show the potential of the HL-LHC to either verify or exclude the anomalous Zb b couplings observed at LEP through measuring the Zh production rate at the HL-LHC, and this conclusion is not sensitive to possible new physics contribution induced by top quark or Higgs boson anomalous couplings in the loop.

Probing the $Zb\bar{b}$ anomalous couplings via exclusive $Z$ boson decay

“…Here, we focus on the parameter space with κ V,A > 0, since the off Z-pole A b F B measurements have excluded the region with κ V,A < 0 [4]. In the same figure, we also show the constraints from the R b (blue region) and (A b FB , A b ) (red region) measurements at the Z-pole, as well as the expected constraint from the measurement of the gg → Zh production at the HL-LHC (gray region) [8]. To derive the expected constraint from the gg → Zh measurement, we have taken into account a factor of 2, as compared to that presented in Ref.…”

“…To derive the expected constraint from the gg → Zh measurement, we have taken into account a factor of 2, as compared to that presented in Ref. [8], to include both the ATLAS and CMS contributions, which results in a factor of √ 2 reduction in its error band size. The similar results, but for the 100 TeV pp collider, are shown in Fig.…”

“…Recently, several approaches have been proposed in the literature to break the above mentioned degeneracy and to further test the Zb b couplings at the LHC and future colliders [8][9][10]. For example, the axial-vector component of the Zb b coupling can be determined by the precision measurement of the gg → Zh production at the LHC and high-luminosity LHC (HL-LHC), a proton-proton collider to operate at a centerof-mass energy of 14 TeV with an integrated luminosity of 3 ab −1 [8]. It can also be better constrained by the average jet charge weighted single-spin asymmetry to be measured at the upcoming Electron-Ion Collider (EIC) [10], while the vector-component of the Zb b coupling can be determined by the measurement of the single-spin asymmetry, of the polarized electron-proton cross section in neutral current deeply inelastic scattering processes with one b-tagged jet in the final state, at the HERA and the EIC [9].…”

“…It is also well know that the Zb b anomalous couplings are not fully determined by the electroweak precision measurements since the appearance of the degeneracy under the global analysis [4]. Recently, several approaches have been proposed in the literature to break the above mentioned degeneracy and to further test the Zb b couplings at the LHC and future colliders [8][9][10]. For example, the axial-vector component of the Zb b coupling can be determined by the precision measurement of the gg → Zh production at the LHC and high-luminosity LHC (HL-LHC), a proton-proton collider to operate at a centerof-mass energy of 14 TeV with an integrated luminosity of 3 ab −1 [8].…”

Jet Charge: A new tool to probe the anomalous $Zb\bar{b}$ couplings at the EIC

“…Breaking the above-mentioned degeneracy and as well as confirming or excluding the anomaly of A b FB measurement at LEP is one of the major tasks of the particle physics at the current and future colliders. Recently, we demonstrated that the axial-vector component of Zb b coupling can be determined by the precision measurement of gg → Zh production at the Large Hadron Collider (LHC) and the high-luminosity LHC (HL-LHC) [6], while its vector component can be better constrained by the measurement of single-spin asymmetry (SSA) in the polarized leptonhadron collisions, such as at HERA and the upcoming Electron-Ion Collider (EIC) [7].…”