Lam-Tung relation breaking effects and weak dipole moments at lepton colliders

The breaking of the Lam-Tung relation in the Drell-Yan process at the LHC exhibits a long-standing tension with the Standard Model (SM) prediction at O ( α s 3 ) accuracy. This tension could be explained by weak dipole interactions of leptons and quarks, associated with the Z -boson within the framework of the Standard Model Effective Field Theory (SMEFT). In this paper, we propose to cross-check these weak dipole interactions by measuring the violation effects of the Lam-Tung relation at future lepton colliders through the processes e + e − → Z γ → ℓ ℓ ¯ γ and e + e − → Z γ → q q ¯ γ . By considering different decay modes of the Z -boson, these channels exhibit distinct sensitivities to various dipole operators, providing a way to disentangle their individual effects. Additionally, the high flavor-tagging efficiencies at lepton colliders could provide strong constraints on the dipole interactions of heavy quarks, such as b and c quarks, which are challenging to probe in the Drell-Yan process at the LHC due to the suppression of parton distribution functions.