  We present a model-independent measurement of $t$-channel electroweak
production of single top quarks in $\ppbar$ collisions at $\sqrt{s}=1.96\;\rm
TeV$. Using $5.4\;\rm fb^{-1}$ of integrated luminosity collected by the D0
detector at the Fermilab Tevatron Collider, and selecting events containing an
isolated electron or muon, missing transverse energy and one or two jets
originating from the fragmentation of $b$ quarks, we measure a cross section
$\sigma({\ppbar}{\rargap}tqb+X) = 2.90 \pm 0.59\;\rm (stat+syst)\; pb$ for a
top quark mass of $172.5\;\rm GeV$. The probability of the background to
fluctuate and produce a signal as large as the one observed is
$1.6\times10^{-8}$, corresponding to a significance of 5.5 standard deviations.
