Xing, ZZ (reprint author), Chinese Acad Sci, Inst High Energy Phys, Beijing 100049, Peoples R China.
A simple extension of the standard model is to introduce n heavy right-handed Majorana, neutrinos and preserve its SU(2)(L) X U(1)(Y) gauge symmetry. Diagonalizing the (3 + n) x (3 + n) neutrino mass matrix, we obtain an exact analytical expression for the effective mass matrix of nu(e), nu(mu) and nu(tau). It turns out that the 3 x 3 neutrino mixing matrix V, which appears in the leptonic charged-current weak interactions, must not be exactly unitary. The unitarity violation of V is negligibly tiny, however, if the canonical seesaw mechanism works to reproduce the correct mass scale of light Majorana neutrinos. A similar conclusion can be drawn in the. realistic Type-II seesaw models.