  We discuss effects of Majorana CP violation in a model-independent way for a
given phase structure of flavor neutrino masses. To be more predictive, we
confine ourselves to models with $\det(M_\nu)=0$, where $M_\nu$ is a flavor
neutrino mass matrix, and to be consistent with observed results of the
neutrino oscillation, the models are subject to an approximate $\mu$-$\tau$
symmetry. There are two categories of approximately $\mu$-$\tau$ symmetric
models classified as (C1) yielding $\sin^22\theta_{23} \approx 1$ and
$\sin^2\theta_{13} \ll 1$ and (C2) yielding $\sin^22\theta_{23} \approx 1$ and
$\Delta m_\odot^2/|\Delta m_{atm}^2|\ll 1$, where $\theta_{23(13)}$ stands for
the mixing of massive neutrinos $\nu_2$ and $\nu_3$ ($\nu_1$ and $\nu_3$) and
$\Delta m_ \odot ^2$ ($\Delta m_{atm}^2$) stands for the mass squared
difference for atmospheric (solar) neutrinos. The Majorana phase can be large
for the normal mass hierarchy and for the inverted mass hierarchy with
$m_1\approx -m_2$ only realized in (C1) while they are generically small for
the inverted mass hierarchy with $m_1\approx m_2$ in both (C1) and (C2). These
results do not depend on a specific choice of phases in $M_\nu$ but hold true
in any models with $\det(M_\nu)=0$ because of the rephasing invariance.

High Energy Physics - Phenomenology