Low Mass Right-handed Neutrinos in the Left-right Symmetric Theory

Abstract It is pointed out that the minimal left-right symmetric theory of the electroweak interactionsbased on SU(2) L × SU(2) R × U(1) B- Lmay accommodate right-handed neutrinos in the low-mass region of order 100 MeV or so, quite a few ordersof magnitudebelow m (W R ). When an attempt is made to construct such a scheme, consistent with the present upper limits on the left-handed neutrino masses, one faces the problem of the evasion of the cosmological bound of 50 eV on ν μ L . We show that this constraint may be avoided in a somewhat simplified scheme based on reasonable gross scales which characterize the Dirac and Majoranaparts of the neutrino mass matrix. In a detailed study of the neutrino mass matrixthis is shown not to be possible when the neutrino Dirac masses are assumed to be of the orderof magnitudeof the corresponding charged lepton masses. A phenomenologically consistent scheme with m ( ν R ) ∼ O(100 MeV) based on Dirac masses of order 1 10 m e is described.