Measurement of the forward--backward asymmetry of muons coming from the decay of $W$ bosons with the D\O\ Detector

We present a measurement of the forward--backward asymmetry of muons coming from the decay of $W$ bosons produced in $p\bar{p}$ collisions at $\sqrt{s} = $1.96 TeV collected with the D\O\ Detector at Fermilab. We discuss the prospects for using the asymmetry to constrain the ratio of $d$ and $u$ quark momentum distributions in the proton.