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Open Questions about Neutrinos Today
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[QUOTE="vanhees71, post: 6435946, member: 260864"] I think what's measured as particles are indeed the "other" particles rather than neutrinos as "particles". You cannot measure neutrinos as a "particle", because there's no interaction that projects to the mass eigenstates but only such that project to flavor eigenstates which are not mass eigenstate Take the "neutrino-mass" determination with, e.g., the strategy to measure the endpoint of the electron-energy spectrum very accurately (a la KATRIN) by measuring the electron energy spectrum in tritium decay, i.e., ##\text{t} \rightarrow ^3\text{He}+\text{e}^-+\bar{\nu}_{\text{e}}##. Let's make it idealized considering fully accurate measurements of arbitrary precision. So what's measured? You take an ensemble of tritium nuclei at rest and measure precisely the energy of the electron from the ##\beta## decay of each triton. Of course you don't measure the (anti-)neutrino but only the energy of the electron. Since the decay is not into a mass eigenstate of the anti-neutrino but in the electron-flavor eigenstate (which is established by measuring definitely an electron in the ##\beta## decay). Now the energy of the electron depends on the mass of the neutrino. So when measuring the endpoint of the electrons' energy spectrum you don't get a definite value but a distribution of end-point energies due to the superposition of neutrino-mass eigenstates. So what you'll measure is an average of the neutrino masses weighted with the corresponding PMNS matrix elements (modulus squared). [/QUOTE]
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Open Questions about Neutrinos Today
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