$A_{FB}$ in the SMEFT: precision Z physics at the LHC

Understand

We study the forward-backward asymmetry $A_{FB}$ in $pp \to \ell^+\ell^-$ at the Z peak within the Standard Model Effective Field Theory (SMEFT).

• We find that this observable provides per mille level constraints on the vertex corrections of the Z boson to quarks,which close a flat direction in the electroweak precision SMEFT fit.
• Moreover, we show that current $A_{FB}$ data is precise enough so that its inclusion in the fit improves significantly LEP bounds even in simple New Physics setups.
• This demonstrates that the LHC can compete with and complement LEP when it comes to precision measurements of the Z boson properties