Improved search for $\nu_\mu \to \nu_e$ oscillation in a long-baseline accelerator experiment

We performed an improved search for $\nu_\mu \to \nu_e$ oscillation with the KEK to Kamioka (K2K) long-baseline neutrino oscillation experiment, using the full data sample of $9.2 \times 10^{19}$\xspace protons on target. No evidence for a $\nu_e$ appearance signal was found, and we set bounds on the $\nu_\mu \to \nu_e$ oscillation parameters. At $\Delta m^2$ = $2.8 \times 10^{-3} \mathrm{eV}^2$, the best fit value of the K2K $\nu_\mu$ disappearance analysis, we set an upper limit of $\sin^2 2\theta_{\mu e}$ $<$ 0.13 at 90% confidence level.