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

TL;DR

This paper proposes using exclusive Z boson decays to Upsilon and gamma to probe anomalous Zbar{b} couplings at future colliders, providing a new method to constrain these couplings beyond existing measurements.

Contribution

It introduces a novel approach to constrain Zbar{b} couplings using exclusive Z decays, with NLO calculations and collider sensitivity analysis.

Findings

The partial decay width calculated at NLO agrees with light-cone distribution amplitude results.

HL-LHC can distinguish Zbar{b} couplings if signal efficiency improves by a factor of 1.7.

Exclusive Z decays are sensitive to the axial-vector component of the Zbar{b} coupling.

Abstract

We propose to utilize the exclusive $Z$-boson rare decays $Z\to \Upsilon(ns)+\gamma$ to constrain the $Zb\bar{b}$ couplings at the HL-LHC and 100 TeV proton-proton collider. We demonstrate that the event yield of the proposed processes is sensitive to the axial-vector component of the $Zb\bar{b}$ coupling and can provide complementary information to the jet-charge weighted single-spin asymmetry measurement at the EIC and the $gg\to Zh$ production rate measurement at the LHC. By applying the NRQCD factorization formalism, we calculate the partial decay width of $Z\to \Upsilon(ns)+\gamma$ to the NLO accuracy in strong interaction, which is found to agree with those obtained from the light-cone distribution amplitude approach. We show that the HL-LHC can break the degeneracy of the $Zb\bar{b}$ couplings, as implied by the precision electroweak data at LEP and SLC, if the signal efficiency can be improved by a factor of 1.7 from the present ATLAS analysis at the 13 TeV LHC with an integrated luminosity of $36.1~{\rm fb}^{-1}$.