Implications of the mikheyev-smirnov-wolfenstein (MSW) mechanism of amplification of neutrino oscillations in matter

Mikheyev and Smirnov have recently proposed a novel and plausible solution of the solar neutrino problem, based on the resonant amplification of the neutrino oscillations in matter. We comment on several aspects of this mechanism. (i) For the values of neutrino masses and mixing angles predicted by the seesaw model of grand unified theories, the MSW effect may take place naturally in the Sun, leading to a considerable reduction of the flux of solar electron neutrinos, with the dominant transition being v_e → v_τ (rather than v_e → v_μ). (ii) Oscillations between the ordinary neutrinos (v_e, v_μ, v_τ) can affect primordial nucleosynthesis, but the effect is small (i.e., the abundance of ⁴He is predicted to change by less than 1.3 × 10^-3). (iii) A comparison of some of the general properties of neutrino oscillations in matter and in vacuum is given.