We report the first observation of XI- in heavy ion collisions at the
AGS. The lifetime of XI- measured agrees very well with the particle d
ata group value. We present the observed XI- effective mass spectrum a
nd the rapidity distribution after acceptance correction. In the measu
red rapidity region (1.4-2.9) for 14.6 GeV/c Si on Pb, we obtained a y
ield of 0.25 +/- 0.04 XI- per central event and a ratio of N(XI-)/N(LA
MBDA) = 0.12 +/- 0.02 using a fireball+ model (a model which includes
a fireball plus a term simulating a cascade) tor acceptance correction
(see text for systematic error). This result is larger than any casca
de model prediction so far.