Reduction of the electroweak correlation in the PDF updating by using the forward-backward asymmetry of Drell-Yan process

Abstract

We propose a new observable for the measurement of the forward-backward asymmetry $(A_{FB})$ in Drell-Yan lepton production. At hadron colliders, the $A_{FB}$ distribution is sensitive to both the electroweak (EW) fundamental parameter ${\sin }^2\hat{I}{¸}_W$, the weak mixing angle, and the parton distribution functions (PDFs). Hence, the determination of ${\sin }^2\hat{I}{¸}_W$ and the updating of PDFs by directly using the same $A_{FB}$ spectrum are strongly correlated. This correlation would introduce large bias or uncertainty into both precise measurements of EW and PDF sectors. In this article, we show that the sensitivity of $A_{FB}$ on ${\sin }^2\hat{I}{¸}_W$ is dominated by its average value around the $Z$ pole region, while the shape (or gradient) of the $A_{FB}$ spectrum is insensitive to ${\sin }^2\hat{I}{¸}_W$ and contains important information on the PDF modeling. Accordingly, a new observable related to the gradient of the spectrum is introduced, and demonstrated to be able to significantly reduce the potential bias on the determination of ${\sin }^2\hat{I}{¸}_W$ when updating the PDFs using the same $A_{FB}$ data.