The Mikheyev-Smirnov-Wolfenstein effect (often referred to as matter effect) is a particle physics process which can act to modify neutrino oscillations in matter. 1978 work by American physicist Lincoln Wolfenstein and 1986 work by Soviet physicists Stanislav Mikheyev and Alexei Smirnov led to an understanding of this effect.
The presence of electrons in matter changes the energy levels of the propagation eigenstates of neutrinos due to charged current coherent forward scattering of the electron neutrinos (i.e., weak interactions). This means that neutrinos in matter have a different effective mass than neutrinos in vacuum, and since neutrino oscillations depend upon the squared mass difference of the neutrinos, neutrino oscillations may be different in matter than they are in vacuum. With antineutrinos, the conceptual point is the same but the effective charge, 'weak isospin' that the weak interaction couples to, of electron antineutrino has opposite sign.
The effect is important at the very large electron densities of the sun where electron neutrinos are produced. The high energy neutrinos seen e.g. in SNO (sudbury Neutrino Observatory) and in Super-Kamiokande are produced as the higher mass eigenstate in matter ν2m, and remain as such as the density of solar material changes. (When neutrinos go through the MSW resonance the neutrinos have the maximal probability to change their nature, but it happens that this probability is negligibly small—this is sometimes called propagation in the adiabatic regime). Thus, the neutrinos of high-energy leaving the sun are in a vacuum propagation eigenstate, ν2, that has a reduced overlap with the electron neutrino νe = ν1 cosθ + ν2 sinθ seen by charged current reactions in the detectors.
For high energy solar neutrinos the MSW effect is important, and leads us to expect that Pee=sin2θ. This was dramatically confirmed in the Sudbury Neutrino Observatory, where the solar neutrino problem was finally solved. There it was shown that only ~34% of the electron neutrinos (measured with one charged current reaction of the electron neutrinos) reach the detector, whereas the sum of rates for all three neutrinos (measured with one neutral current reaction) agrees well with the expectations. This allowed the determination sin2θ ≈ 1/3. Earlier, Kamiokande and Super-Kamiokande measured a mixture of charged current and neutral current reactions, that also support the occurrence of the MSW effect with a similar supporession, but with less confidence.
For the low energy solar neutrinos, on the other hand, the matter effect is negligible and one should apply the vacuum oscillation formula Pee=1-(sin22θ)/2. For the same value of the solar mixing angle, θ, this would correspond to a suppression Pee ≈ 60%. This is consistent with the experimental observations on such neutrinos by the Homestake Experiment, the first experiment to reveal the solar neutrino problem, followed by those of Gallex/GNO and SAGE (collectively, gallium experiments) that measured the lowest energy neutrinos and provided a strong support to Homestake. These results are supported by the results of the reactor experiment KamLAND, that alone is able to provide also a measurement of the parameters of oscillation that is consistent with all other measurements.
The MSW effect can also modify neutrino oscillations in the Earth, and future search for new oscillations and/or leptonic CP violation may make use of this property.
* Neutrino oscillation