  We study the earth matter effects in the Tokai-to-Kamioka-and-Korea
experiment (T2KK), which is a proposed extension of the T2K (Tokai-to-Kamioka)
neutrino oscillation experiment between J-PARC at Tokai and Super-Kamiokande
(SK) in Kamioka, where an additional detector is placed in Korea along the same
neutrino beam line.By using recent geophysical measurements, we examine the
earth matter effects on the oscillation probabilities at Kamioka and Korea. The
average matter density along the Tokai-to-Kamioka baseline is found to be 2.6
g/cm^3, and that for the Tokai-to-Korea baseline is 2.85, 2.98, and 3.05 g/cm^3
for the baseline length of L = 1000, 1100, and 1200 km, respectively. The
uncertainty of the average density is about 6%, which is determined by the
uncertainty in the correlation between the accurately measured sound velocity
and the matter density. The effect of the matter density distribution along the
baseline is studied by using the step function approximation and the Fourier
analysis. We find that the nu_mu -> nu_e oscillation probability is dictated
mainly by the average matter density, with small but non-negligible
contribution from the real part of the first Fourier mode. We also find that
the sensitivity of the T2KK experiment on the neutrino mass hierarchy does not
improve significantly by reducing the matter density error from 6% to 3%, since
the measurement is limited by statistics for the minimum scenario of T2KK with
SK at Kamioka anda 100 kt detector in Korea considered in this report. The
sensitivity of the T2KK experiment on the neutrino mass hierarchy improves
significantly by splitting the total beam time into neutrino and anti-neutrino
runs, because the matter effect term contributes to the oscillation amplitudes
with the opposite sign.
