  In this paper, we consider a pair of entangled charged lepton-neutrino
embedded in a constant magnetic field, which can be produced in the purely
leptonic decay of charged pseudoscalar mesons. Particles with nonzero magnetic
dipole moment will precess in the present of external magnetic field and induce
a nontrivial noncyclic geometric phase. The dependence of this geometric phase
on neutrino mass can be used to determine the Dirac/Majorana nature of massive
neutrinos.
