  We explore the possible exotic particle content beyond the standard model by
examining all its scalar bilinear combinations. We categorize these exotic
scalar fields and show that without the suppression of (A) their Yukawa
couplings with the known quarks and leptons, and (B) the trilinear couplings
among themselves, most are already constrained to be very heavy from the
nonobservation of proton decay and neutron-antineutron oscillations, the
smallness of $K^0 - \bar {K^0}$, $D^0 - \bar {D^0}$ and $B_d^0 - \bar {B_d^0}$
mixing, as well as the requirement of a nonzero baryon asymmetry of the
universe. On the other hand, assumption (B) may be naturally violated in many
models, especially in supersymmetry, hence certain exotic scalars are allowed
to be below a few TeV in mass and would be easily detectable at planned future
hadron colliders. In particular, large cross sections for the distinctive
processes like $\bar p p \to tt,\bar t c$ and $p p \to t t, b b$ would be
expected at the Fermilab Tevatron and CERN LHC, respectively.
