  We present the first direct search for single top quark production using tau
leptons. The search is based on 4.8 fb$^{-1}$ of integrated luminosity
collected in $p\bar{p}$ collisions at $\sqrt{s}$=1.96 TeV with the D0 detector
at the Fermilab Tevatron Collider. We select events with a final state
including an isolated tau lepton, missing transverse energy, two or three jets,
one or two of them $b$ tagged. We use a multivariate technique to discriminate
signal from background. The number of events observed in data in this final
state is consistent with the signal plus background expectation. We set in the
tau+jets channel an upper limit on the single top quark cross section of
\TauLimObs pb at the 95% C.L. This measurement allows a gain of 4% in expected
sensitivity for the observation of single top production when combining it with
electron+jets and muon+jets channels already published by the D0 collaboration
with 2.3 fb$^{-1}$ of data. We measure a combined cross section of
$\SuperCombineXSall$ pb, which is the most precise measurement to date.
