We present a search for electroweak production of single top quarks in appr
oximate to 90 pb(-1) of data collected with the D(null set) detector at the
Fermilab Tevatron collider. Using arrays of neural networks to separate si
gnals from backgrounds, we set upper limits on the cross sections of 17 pb
for the s-channel process p (p) over bar --> tb + X, and 22 pb for the t-ch
annel process p (p) over bar --> tqb + X, both at the 95% confidence level.
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