Z-pole observables in an effective theory

TL;DR

This paper investigates Z-pole observables related to bottom quark pair production, exploring how new physics operators could explain observed deviations from the Standard Model, with implications for future collider experiments.

Contribution

It identifies specific effective operators involving right-chiral quarks that can account for Z-pole anomalies and discusses their testability at the LHC.

Findings

Certain effective operators can explain the Z-pole discrepancies.

Operators involving right-chiral quarks are less constrained by flavor physics.

Predicted signatures of these operators could be observed at the upgraded LHC.

Abstract

There are two Z-peak observables related to the pair production of bottom quarks that show a deviation of about 2.5\sigma\ each from Standard Model expectations. While the discrepancy in the forward-backward asymmetry is a long-standing one, the tension for the second observable, namely the ratio of the partial width for a Z decaying to a pair of bottom quarks to the total hadronic decay width of the Z, has recently gone up due to a full two-loop evaluation of the Standard Model contributions. We show how both these discrepancies may be explained in the framework of new physics that couples only to the third generation of quarks. In the paradigm of effective operators, the Wilson coefficients of some of the possible operators are already very tightly constrained by flavour physics data. However, there still remain certain operators, particularly those involving right-chiral quark fields, which can successfully explain the anomalies. We also show how the footprints of such operators may be observed at the upgraded LHC.