Lam-Tung relation breaking in $Z$ boson production as a probe of SMEFT effects

TL;DR

This paper investigates the Lam-Tung relation violation in Z boson production at the LHC, proposing SMEFT effects as a possible explanation and a means to constrain new physics through detailed analysis of operator contributions.

Contribution

It provides a model-independent SMEFT analysis showing quadratic effects of dimension-6 dipole operators dominate the Lam-Tung relation breaking, offering new insights into potential new physics signals.

Findings

SMEFT contributions appear at 1/Λ^4 order with O(α_s) accuracy.

Quadratic effects of Z-boson dipole operators dominate the violation.

Breaking effects can constrain Z-boson dipole interactions.

Abstract

The violation of Lam-Tung relation in the high-$p_T^{\ell\ell}$ region of the Drell-Yan process at the LHC presents a long-standing discrepancy with the standard model prediction at $\mathcal{O}(\alpha_s^3)$ accuracy. In this Letter, we employed a model-independent analysis to investigate this phenomenon within the framework of the Standard Model Effective Field Theory (SMEFT). Our findings revealed that the leading contributions from SMEFT to this violation appear at the $1/\Lambda^4$ order with $\mathcal{O}(\alpha_s)$ accuracy in QCD interaction. Notably, we demonstrated that the quadratic effect of dimension-6 dipole operators, associated with the $Z$ boson, dominates the breaking effects induced by various dimension-6 and dimension-8 operators. This provides a possible explanation for the observed discrepancy with the Standard Model predictions at the LHC. Furthermore, the breaking effects could also serve as a powerful tool for constraining $Z$-boson dipole interactions, highlighting their importance among potential sources of new physics in the Drell-Yan process.