Z2 symmetry prediction for the leptonic Dirac CP phase

Model-independent consequences of applying a generalized hidden horizontal Z2 symmetry to the neutrino mass matrix are explored. The Dirac CP phase δD can be expressed in terms of the three mixing angles as 4casacssssxcosδD=(ss2−cs2sx2)(ca2−sa2) where the si, ci are sines and cosines of the atmospheric, solar, and reactor angles. This relation is independent of neutrino masses and whether neutrinos are Dirac- or Majorana-type. Given the present constraints on the angles, δD is constrained to be almost maximal, a result which can be explored in experiments such as NOνA and T2K. The Majorana CP phases do not receive any constraint and are thus model-dependent. Also a distribution of θx with a lower limit is obtained without specifying δD.