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 $\Delta m_{\mu e}^2=6\times {10}^{-3}{\mathrm{e}\mathrm{V}}^2$.

Figure 1

Distributions of PID parameters for 32 single-ring events based on (a) Cherenkov ring pattern, and (b) Cherenkov opening angle. The distributions for the data (closed circles), oscillated ${\nu }_{\mu }$ MC events with $({sin}^{2}2{\theta }_{\mu \mu },\Delta m_{\mu \mu }^2)=(1.0,2.8\times {10}^{-3}{\mathrm{e}\mathrm{V}}^2)$ (solid histograms) and expected ${\nu }_e$ signal with full mixing (dashed histograms) are shown. Shaded histograms are the NC component of ${\nu }_{\mu }$ MC events.

Figure 2

The confidence interval for ${\nu }_{\mu }\to {\nu }_e$ oscillations as a function of the effective $\Delta m_{\mu e}^2$ at 90% C.L. (solid line). The dashed line indicates 90% C.L. sensitivity of the experiment for the current statistics. The area to the right of each curve is excluded. The dotted line shows the limit at 90% C.L. by CHOOZ assuming ${sin}^{2}2{\theta }_{\mu e}=\frac{1}{2}{sin}^{2}2{\theta }_{13}$.