  The T2K experiment observes indications of $\nu_\mu\rightarrow \nu_e$
appearance in data accumulated with $1.43\times10^{20}$ protons on target. Six
events pass all selection criteria at the far detector. In a three-flavor
neutrino oscillation scenario with $|\Delta m_{23}^2|=2.4\times10^{-3}$ eV$^2$,
$\sin^2 2\theta_{23}=1$ and $\sin^2 2\theta_{13}=0$, the expected number of
such events is 1.5$\pm$0.3(syst.). Under this hypothesis, the probability to
observe six or more candidate events is 7$\times10^{-3}$, equivalent to
2.5$\sigma$ significance. At 90% C.L., the data are consistent with
0.03(0.04)$<\sin^2 2\theta_{13}<$ 0.28(0.34) for $\delta_{\rm CP}=0$ and a
normal (inverted) hierarchy.
