The T2K experiment has observed electron neutrino appearance in a muon neutrino beam produced 295 km from the Super-Kamiokande detector with a peak energy of 0.6 GeV. A total of 28 electron neutrino events were detected with an energy distribution consistent with an appearance signal, corresponding to a significance of 7.3$��$ when compared to 4.92 $\pm$ 0.55 expected background events. In the PMNS mixing model, the electron neutrino appearance signal depends on several parameters including three mixing angles $��_{12}$, $��_{23}$, $��_{13}$, a mass difference $��m^2_{32}$ and a CP violating phase $��_{\mathrm{CP}}$. In this neutrino oscillation scenario, assuming $|��m^2_{32}| = 2.4 \times 10^{-3}$ $\rm eV^2$, $\sin^2 ��_{23} = 0.5$, and $��m^2_{32} >0$ ($��m^2_{32} <0$), a best-fit value of $\sin^2 2 ��_{13}$ = $0.140^{+0.038}_{-0.032}$ ($0.170^{+0.045}_{-0.037}$) is obtained at $��_{\mathrm{CP}}=0$. When combining the result with the current best knowledge of oscillation parameters including the world average value of $��_{13}$ from reactor experiments, some values of $��_{\mathrm{CP}}$ are disfavored at the 90% CL.

8 pages, 6 figures