The LSND Experiment and the Zee Model

A recent experiment LSND at Los Alamos has provided[1] an indication of ν µ − ν e oscillations with ∆m 2 of order 1ev 2 or greater and sin 2 2θ eµ between 2 × 10 −3 and 10 2. Such a value of ∆m 2 for ν µ − ν e oscillations was not expected in the standard seesaw model[2] suggested by SO(10) with a large mass hierarchy because it leads to too large a value of m(ν τ) to fit cosmological constraints. Furthermore within that model the LSND result is inconsistent with the indications of oscillations from both atmospheric neutrinos[3] and solar neutrinos.[4] Here we reconsider an alternative model proposed by Zee.[5] This represents the most direct way to produce Majorana neutrino masses in a theory such as SU(5) in which right-handed neutrinos are totally absent. The general features of masses and mixings in this model have been analyzed.[6] The mass matrix has the form m 2 0