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Mikheyev–Smirnov–Wolfenstein effect
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. Works in 1978 and 1979 by American physicist Lincoln Wolfenstein led to understanding that the oscillation parameters of neutrino are changed in matter. In 1985, the Soviet physicists Stanislav Mikheyev and Alexei Smirnov predicted that slow decrease of the matter density can resonantly enhance the neutrino mixing. Later in 1986, Stephen Parke of Fermilab provided the first full analytic treatment of this effect.
==Explanation==
The presence of electrons in matter changes the energy levels of the propagation eigenstates (mass eigenstates) of neutrinos due to charged current coherent forward scattering of the electron neutrinos (i.e., weak interactions). The coherent forward scattering is analogous to the electromagnetic process leading to the refractive index of light in a medium. 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 that the weak interaction couples to (called ''weak isospin'') has an opposite sign. If the electron density of matter changes along the path of neutrinos, the mixing of neutrinos grows to maximum at some value of the density, and then turns back; it leads to resonant conversion of one type of neutrinos to other one.
The effect is important at the very large electron densities of the Sun where electron neutrinos are produced. The high-energy neutrinos seen, for example, in SNO (Sudbury Neutrino Observatory) and in Super-Kamiokande, are produced mainly 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.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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