Mikheyev–Smirnov–Wolfenstein effect

The Mikheyev–Smirnov–Wolfenstein effect is the alteration of neutrino flavor oscillations by coherent forward scattering in matter. A density-dependent resonance can strongly enhance conversion between flavors.

Neutrino oscillation: The periodic change of neutrino flavor as neutrinos propagate, caused by mixing among states with different masses. The effect modifies the oscillations that occur even in vacuum.
Solar neutrino problem: The historical discrepancy between measured solar electron-neutrino fluxes and predictions from solar models. Matter-enhanced conversion resolved the long-standing deficit of detected solar electron neutrinos.
Bruno Pontecorvo: An Italian physicist who developed foundational ideas about neutrino flavor mixing and oscillations. His oscillation framework supplied the vacuum physics later modified by matter.
Neutrino: A neutral, weakly interacting elementary particle that comes in three known flavors. The effect concerns how neutrinos change flavor while traversing matter.
Vacuum neutrino oscillation: Neutrino flavor oscillation during propagation through space without significant matter effects. It lacks the density-dependent potential that reshapes mixing in matter.
Neutrino mixing: The relationship between neutrino flavor states and neutrino mass states, described by a unitary mixing matrix. Matter changes the effective mixing angle that governs flavor conversion.
Solar neutrinos: Neutrinos produced by nuclear reactions in the Sun and detected on Earth. Their changing flavor spectrum provides a major test of matter-enhanced oscillations.
Stanislav Mikheyev: A Soviet physicist who, with Alexei Smirnov, identified resonant neutrino conversion in matter. His work helped establish the resonance’s role in solar neutrino conversion.
Neutrino flavor: The electron, muon, or tau identity assigned to a neutrino through its weak interactions. Flavor is the observable identity altered by oscillations and matter.
Neutrino absorption: A process in which a neutrino is captured in an interaction and removed from the incident beam. The MSW effect changes propagation coherently rather than absorbing neutrinos.

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