Charge asymmetry has been measured using 19 039 $W$ decays recorded by the Collider Detector at Fermilab during the 1992-93 run of the Tevatron Collider. The asymmetry is sensitive to the ratio of $d$ and $u$ quark distributions to $x<\mathrm{}0.01\mathrm{}$ at $Q^2\approx M_W^2$, where nonperturbative effects are minimal. It is found that of the two current sets of parton distributions, those of Martin, Roberts, and Stirling are favored over the sets most recently produced by the CTEQ Collaboration. The $W$ asymmetry data provide stronger constraints on $\frac{d}{u}$ ratio than the recent measurements of $\frac{F_2^{\mu n}}{F_2^{\mu p}}$ which are limited by uncertainties originating from deuteron corrections.

• Received 12 September 1994

DOI:https://doi.org/10.1103/PhysRevLett.74.850