  We report a search for single top quark production with the CDF II detector
using 2.1 fb-1 of integrated luminosity of pbar p collisions at sqrt{s}=1.96
TeV. The data selected consist of events characterized by large energy
imbalance in the transverse plane and hadronic jets, and no identified
electrons and muons, so the sample is enriched in W -> tau nu decays. In order
to suppress backgrounds, additional kinematic and topological requirements are
imposed through a neural network, and at least one of the jets must be
identified as a b-quark jet. We measure an excess of signal-like events in
agreement with the standard model prediction, but inconsistent with a model
without single top quark production by 2.1 standard deviations (sigma), with a
median expected sensitivity of 1.4 sigma. Assuming a top quark mass of 175
GeV/c2 and ascribing the excess to single top quark production, the cross
section is measured to be 4.9+2.5-2.2(stat+syst)pb, consistent with
measurements performed in independent datasets and with the standard model
prediction.
