  A search for a manifestly exotic S=-2 baryon state decaying to Xi- pi-, and
its neutral partner decaying to Xi- pi+, has been performed using 220 pb^(-1)
of pp= collisions at sqrt(s)=1.96 TeV collected by the Collider Detector at
Fermilab. The Xi trajectories were measured in a silicon tracker before their
decay, resulting in a sample with low background and excellent position
resolution. No evidence was found for S=-2 pentaquark candidates in the
invariant mass range of 1600--2100 MeV/c^2 Upper limits on the product of
pentaquark production cross section times its branching fraction to Xi-
pi^{+,-}, relative to the cross section of the well established Xi(1530)
resonance, are presented for neutral and doubly negative candidates with pT>2
GeV/c and |y|<1 as a function of pentaquark mass. At 1862 MeV/c^2, these upper
limits for neutral and doubly negative final states were found to be 3.2% and
1.7% at the 90% confidence level, respectively.
