Do neutrinos refract?

Question

The most benign of interactions is refraction. While neutrinos rarely interact with matter in a sense like the photoelectric effect, does that mean that they don't refract either?

Answer 1

Neutrinos are weakly interacting quantum mechanical point particles, with very small mass.

Refraction is a classical mechanics phenomenon, happens to waves traveling in a medium and it is a collective synergy of many photons impinging on the field of the atoms and molecules of the medium. Individual photons are not refracted but are scattered. In synergy with the zillions of photons in the light wave refraction appears as an emergent phenomenon.

Neutrinos have very small probability of scattering with the atoms and molecules but there do exist neutrinos coming from the sun in great numbers and thus the question is not irrelevant. There exist studies that derive a refraction index of the neutrino flux within various models for neutrinos for the sun, as for example here.

It is pointed out that, if neutrinos are to maintain coherence over the required distance for the Mikheyev-Smirnov-Wolfenstein solutions to the solar-neutrino problem, effects arising from neutrino multiple scattering must be considered. We give a simple derivation for the neutrino index of refraction that takes into account this effect. The same method is also shown to be useful for situations with varying matter densities and neutrino mixing. We also examine the question whether the coherence of propagating neutrinos in matter will be affected by switching on an external magnetic field, assuming neutrinos have a large magnetic moment.