CPviolation effect in long-baseline neutrino oscillation in the four-neutrino model

We investigate $\mathrm{CP}$-violation effects in long-baseline neutrino oscillations in the four-neutrino model with a mass scheme of the two nearly degenerate pairs separated by the order of 1 eV, by using the data from the solar neutrino deficit, the atmospheric neutrino anomaly, and the LSND experiments along with other accelerator and reactor experiments. By using the most general parametrization of the mixing matrix with six angles and six phases, we show that a genuine $\mathrm{CP}$-violation effect could attain as large as 0.3 for $\Delta P({\nu }_{\mu }\to {\nu }_{\tau })\equiv P({\nu }_{\mu }\to {\nu }_{\tau })-P({\overline{\nu }}_{\mu }\to {\overline{\nu }}_{\tau })$ and that the matter effect is negligibly small such as at most 0.01 for $\Delta P({\nu }_{\mu }\to {\nu }_{\tau })$ for $\Delta m^2=\left(1\mathrm{}-5\right)\times {10}^{-3}\hspace{0.5em}{\mathrm{eV}}^2,$ which is the mass-squared difference relevant to the long-baseline oscillation.