Sensitivity of the T2K accelerator-based neutrino experiment with an Extended run to $20\times {10}^{21}$ POT

Recent measurements at the T2K experiment indicate that CP violation in neutrino mixing may be observed in the future by long-baseline neutrino oscillation experiments. We explore the physics program of an extension to the currently approved T2K running of $7.8\times {10}^{21}$ protons-on-target to $20\times {10}^{21}$ protons-on-target,aiming at initial observation of CP violation with 3$\sigma$ or higher significance for the case of maximum CP violation. With accelerator and beam line upgrades, as well as analysis improvements, this program would occur before the next generation of long-baseline neutrino oscillation experiments that are expected to start operation in 2026.