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It's known that in the kinetic energy spectra of electrons from the negative beta decays that at the end of the spectra some electrons are found to have a maximum energy that is equal to the energy difference between the initial and final decaying nuclear states.

Could the observation of such electrons suggest that neutrinos are massless? And did some experiments really suggest this in the early studies of neutrinos?

I know that neutrinos have mass, I'm just asking if there were beta decay experiments that suggested that neutrinos have no mass.

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The thing is beta decay spectra from a massless and from a very low-mass neutrino look quite similar. Specifically, with a massless neutrino, you get a spectrum extending all the way to the total available energy; with a massive neutrino you get a small gap - in principle, this can be measured, but if the neutrino mass is very small this gap is small as well, and it happens to be at the very end of the spectrum where you get a very small number of events, making it hard to get good statistics.

A precision experiment dedicated to measuring this is KATRIN; at the moment they still are measuring something compatible with zero: $m^2_\nu  = (0.26 ± 0.34)\text{eV}^2 c^{–4}$.

So this data is compatible with both a massless neutrino and one with, for example, a mass of 0.5eV. This kind of experiment, at the moment, can only put upper limits on the mass of the neutrino.

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    $\begingroup$ And that 0.5eV is out of 18.591keV, or 1 part in 36,000... $\endgroup$
    – Jon Custer
    Mar 29, 2023 at 12:42

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