We analyze the prospective impact of supersymmetric radiative corrections on tests of charged current universality involving light quarks and leptons. Working within the R-parity conserving Minimal Supersymmetric Standard Model, we compute the corresponding one-loop corrections that enter the extraction of the Cabibbo-Kobayashi-Maskawa matrix element $V_{ud}$ from a comparison of the muon-decay Fermi constant with the vector coupling constant determined from nuclear and neutron $��$-decay. We also revisit earlier studies of the corrections to the ratio $R_{e/��}$ of pion leptonic decay rates $��[��^+ \to e^+ ��(��)]$ and $��[��^+ \to ��^+ ��(��)]$. In both cases, we observe that the magnitude of the corrections can be on the order of $10^{-3}$. We show that a comparison of the first row CKM unitarity tests with measurements of $R_{e/��}$ can provide unique probes of the spectrum of first generation squarks and first and second generation sleptons.

38 pages, 17 figures