  It has been proposed that the recently discovered archetypical "exotic"
meson, X(3872), with M(X(3872))=3871.68+-0.17 MeV/c^2, and an extremely narrow
width, Gamma(X(3872))<1.2 MeV, is a hadronic molecule of bound D^0 and D^*0
mesons. If true, this would establish a new species of hadrons, distinct from
qqbar mesons and qqq baryons. It is put to an important experimental test by
making a high precision measurement of the proposed molecule's binding energy.
Using 818 pb-1 of e+e- annihilation data taken with the CLEO-c detector at
psi(3770), the decays D0 -> KS K+ K- and D^0(Dbar0) -> K+- pi-+ pi+ pi- have
been studied to make the highest precision measurement of D0 mass,
M(D0)=1864.851+-0.020+-0.019+-0.054 MeV/c^2, where the first error is
statistical, the second error is systematic, and the third error is due to
uncertainty in kaon masses, or 1864.851+-0.061 MeV/c^2 with all errors added in
quadrature. This leads to M(D0 + D*0)=3871.822+-0.140 MeV/c^2, and the binding
energy BE(X(3872)) = M(D0+D*0)-M(X(3872)) = +142+-220 keV. At the 90%
confidence level this leads to the conclusion that X(3872) is either unbound by
as much as 140 keV, or it is bound by less than 420 keV. If bound, X(3872) has
a very large radius; the central value of binding energy corresponds to a
radius of 12 fm, and the lower limit to 7 fm, both being uncomfortably large
for a molecule.
