Semileptonic Charm Decay at a Tau Charm Factory *

The full spectrum of Df3 and Dl4 semileptonic decays can be precisely measured at a Tau Charm Factory. Sensitivety to rare semileptonic decays extends to branching fractions as small as 10m3. Improved measurements of Cabibbo-Kobayashi-Maska wa matrix elements V,, and Vcd can be obtained. With an improved theoretical understanding, semileptonic measurements can test the unitarity of the second row of the Cabibbo-Kobayashi-Maskawa matrix at the 1% level. Semileptonic charm decays continue to play an important role in our understanding of the weak and strong interactions. The absence of interference between final state leptons and hadrons greatly simplifies the interpretation of these decays. Semileptonic decays proceed through simple spectator graphs. Since the dynamics of the weak interaction are well understood, both the weak couplings and the hadronic portion of the matrix element may be studied. Tau Charm Factory data will lead to a thousand-fold increase in the number of reconstructed events with greatly reduced backgrounds, and it will measure every Df3 and Df4 semileptonic decay. The complete picture is an important input to theoretical predictions of hadronic form factors. Better theoretical models in turn lead to an improved determination of Cabibbo-Kobayashi-Makawa (CKM) matrix elements which can be used to test the unitarity of the CKM matrix. The second row of the CKM matrix is the only row or column whose elements have all been measured in first order weak processes. The element V& is measured in semileptonic charm decay (11 and its inverse process, neutrino induced charm * This work was supported in part by the U. S. Department of Energy, under contract DE-AC02-76ER01195