  An enhanced antiLambda/antiproton ratio in heavy-ion relative to p+p
collisions has been proposed as one of the signatures for the Quark-Gluon
Plasma (QGP) formation. A significantly large
(antiLambda+antiSigma0+1.1*antiSigma-)/antiproton ratio of 3.5 has been
observed in the mid-rapidity and low transverse momentum region in central
Au+Au collisions at the nucleon-nucleon center-of-mass energy of 4.9 GeV at the
Alternating Gradient Synchrotron (AGS). This is an order of magnitude larger
than the values in peripheral Au+Au collisions and p+p collisions at the
corresponding energy. By using the Ultra-relativistic Quantum Molecular
Dynamics (UrQMD) transport model, we demonstrate that the observed large ratio
can be explained by strong absorption of antiprotons (~99.9%) and antiLambdas
(~99%) in dense nuclear matter created in central collisions. We find within
the model that the initial antiLambda/antiproton ratio, mainly from string
fragmentation, does not depend on the collision centrality, and is consistent
with that observed in p+p collisions. This suggests that the observed large
(antiLambda+antiSigma0+1.1*antiSigma-)/antiproton ratio at the AGS does not
necessarily imply the formation of the QGP. We further study the excitation
function of the ratio in UrQMD, which may help in the search and study of the
QGP.
