Search for Electron Neutrino Appearance in a 250 km Long-Baseline Experiment

We present a search for electron neutrino appearance from accelerator-produced muon neutrinos in the K2K long-baseline neutrino experiment. One candidate event is found in the data corresponding to an exposure of $4.8\times {10}^{19}$ protons on target. The expected background in the absence of neutrino oscillations is estimated to be $2.4\pm 0.6$ events and is dominated by misidentification of events from neutral current ${\pi }^0$ production. We exclude the ${\nu }_{\mu }$ to ${\nu }_e$ oscillations at 90% C.L. for the effective mixing angle in the 2-flavor approximation of ${sin}^{2}2{\theta }_{\mu e}(\simeq \frac{1}{2}{sin}^{2}2{\theta }_{13})>0.15$ at $\Delta m_{\mu e}^2=2.8\times {10}^{-3}{\mathrm{e}\mathrm{V}}^2$, the best-fit value of the ${\nu }_{\mu }$ disappearance analysis in K2K. The most stringent limit of ${sin}^{2}2{\theta }_{\mu e}<0.09$ is obtained at $\Delta m_{\mu e}^2=6\times {10}^{-3}{\mathrm{e}\mathrm{V}}^2$.