This article proposes a novel method for unbiased PDF updating by using the forward-backward asymmetry $(A_{FB})$ in the Drell-Yan $pp rightarrow Z/gamma^{*} rightarrow ell^+ell^-$ process. The $A_{FB}$ spectrum, as a function of the dilepton mass, is not only governed by the electroweak (EW) fundamental parameter, i.e. the weak mixing angle $sin^2 theta_{W}$, but also sensitive to the parton distribution functions (PDFs). When performing simultaneous or iterative fittings for the PDF updating and EW parameter extraction with the same $A_{FB}$, the strong correlations between them may induce large bias into these two sectors. From our studies, it was found that the sensitivity of $A_{FB}$ on $sin^2 theta_{W}$ is dominated by its average value around the $Z$ pole region, while the shape (or gradient) of the $A_{FB}$ spectrum is insensitive to $sin^2 theta_{W}$ but highly sensitive to the PDF modeling. Accordingly, a new observable related to the gradient of the spectrum is defined, and demonstrated to have the capability of significantly reducing the correlation and potential bias between the PDF updating and electroweak measurement. Moreover, the well-defined observable will provide unique information on the sea-valence PDF ratios of the first generation quarks.
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