Majorana CP violation in approximately μ-τ symmetric models with det (Mv)=0

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(Mv)=0, where Mv is a flavor neutrino mass matrix, and to be consistent with observed results of the neutrino oscillation, the models are subject to an approximate μ-τ symmetry. There are two categories of approximately μ-τ symmetric models classified as (C1) yielding sin2 2θ23 approx. = 1 and sin2 θ13 << 1 and (C2) yielding sin2 2θ23 approx. = 1 and Δ(solar mass)2/|Δmatm2| << 1, were θ23(13) stands for the mixing of massive neutrinos v2 and v3 (v1 and v3) and Δ(solar mass)2 (Δmatm2) 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 m1 approx. = -m2 only realized in (C1) while they are generically small for the inverted mass hierarchy with m1 approx. = m2 in both (C1) and (C2). These results do not depend on a specific choice of phases in Mv but hold true in any models with det (Mv)=0 because of the rephasing invariance. (author)