Study of the decaysD→KππeνandD→K*πeν

We have searched for the exclusive semileptonic decay modes $D^{+}\to \overline{K}\pi \pi e^{+}{\nu }_e$ and $D^{+}\to {\overline{K}}^{*}\pi e^{+}{\nu }_e$ in the data from the Fermilab photoproduction experiment E691. With good sensitivity, we observe no signals in the channels $D^{+}\to K^{-}{\pi }^{+}{\pi }^0{e}^{+}{\nu }_e$ and $D^{+}\to {\overline{K}}^0{\pi }^{+}{\pi }^{-}{e}^{+}{\nu }_e$, and set upper limits of $B(D^{+}\to \mathrm{all} {\overline{K}}^{*}\pi e^{+}{\nu }_e)<\mathrm{}1.2\%$ and $B(D^{+}\to \mathrm{all} \overline{K}\pi \pi e^{+}{\nu }_e)<\mathrm{}0.9\%$, at the 90% confidence level. (For these limits we use isospin invariance to extract limits which include all charge states.) These modes are expected to be the most common semileptonic decays next to the dominant $\overline{K}{e}^{+}{\nu }_e$ and ${\overline{K}}^{*}{e}^{+}{\nu }_e$, but the observed limits represent a small fraction of the inclusive semileptonic branching ratio of (17.0±1.9±1.7)%.