  Extensions of the MSSM often predict the existence of new fermions and their
scalar superpartners which are vectorlike with respect to the standard model
gauge group but may be chiral under additional gauge factors. In this paper we
explore the production and decay of an important example, i.e., a heavy
isosinglet charge -1/3 quark and its scalar partner, using the charge
assignments of a 27-plet of E6 for illustration. We emphasize that, depending
on the symmetries of the low energy theory, such exotic particles may decay by
the mixing of the fermion with the d, s, or b quarks; may decay by leptoquark
or diquark couplings (which may nevertheless preserve a form of R-parity); or
may be stable with respect to renormalizable couplings but decay by
higher-dimension operators on cosmological times scales. We discuss the latter
two possibilities in detail for various assumptions concerning the relative
masses of the exotic fermions, scalars, and the lightest neutralino, and
emphasize the necessity of considering the collider signatures in conjunction
with the normal MSSM processes. Existing and projected constraints from
colliders, indirect experiments, proton decay, and big bang nucleosynthesis are
considered.
