Figure 1: The flavor content of the neutrino mass eigenstates. The splitting between $ν3$ and $ν2$ (or $ν1$) (labelled here by 1, 2, and 3) is responsible for oscillation at $500km/GeV$ (relevant to neutrinos arriving from the Earth’s atmosphere), whereas the splitting between $ν2$ and $ν1$ leads to an oscillation at the scale of $15,000km/GeV$ (relevant to neutrinos arriving from the sun). $νe$ appearance at the T2K experiment is primarily controlled by the size of $sin2θ13$, the fraction of $νe$ in the $ν3$ mass eigenstate.