Measurements of neutrino speed

Measurements of neutrino speed have been conducted as tests of special relativity and for the determination of the mass of neutrinos. Astronomical searches investigate whether light and neutrinos emitted simultaneously from a distant source are arriving simultaneously on Earth. Terrestrial searches include time of flight measurements using synchronized clocks, and direct comparison of neutrino speed with the speed of other particles.

Since it is established that neutrinos possess mass, the speed of neutrinos of kinetic energies ranging from MeV to GeV should be slightly lower than the speed of light in accordance with special relativity. Existing measurements provided upper limits for deviations of approximately 10⁻⁹, or a few parts per billion. Within the margin of error this is consistent with no deviation at all.

It was assumed for a long time in the framework of the standard model of particle physics, that neutrinos are massless. Thus they should travel at exactly the speed of light according to special relativity. However, since the discovery of neutrino oscillations it is assumed that they possess some small amount of mass. Thus they should travel slightly slower than the speed of light, otherwise their relativistic energy would become infinitely large. This energy is given by the formula:

with v being the neutrino speed and c the speed of light. The neutrino mass m is currently estimated as being 2 eV/c², and is possibly even lower than 0.2 eV/c². According to the latter mass value and the formula for relativistic energy, relative speed differences between light and neutrinos are smaller at high energies, and should arise as indicated in the figure on the right.

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