Weak decays of charm hadrons: the next lesson on QCD - and possibly more!

Second-generation theoretical technologies -- heavy quark expansions, QCD sum rules and QCD simulations on the lattice -- are arriving on the scene, allowing a treatment of charm decays that is genuinely based on QCD. The availability of those theoretical tools strengthens the case for the need for comprehensive precision measurements of charm decays. It is pointed out that the decays of charm mesons as well as of charm baryons have to be systematically analysed to gain control over theoretical uncertainties. A $\tau$-charm factory with $E_{c.m.}\leq 5.5$ GeV is optimally suited for such a program. It would lead to a deeper understanding of QCD that would also prepare us better for exploiting the discovery potential anticipated in beauty decays. In addition it is quite conceivable (though certainly not guaranteed) that there arise fundamental surprises, such as non-canonical rare $D$ decays, $D^0$-$\bar D^0$ oscillations and CP violation in charm decays which would dramatically alter our perspective on Nature's fundamental forces.