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In this review titled "Pieces of the Flavour Puzzle" the author Ferruccio Feruglio wrote in the introduction that

"The origin of the parameters in the flavour sector of the Standard Model (SM), minimally extended to include massive neutrinos, is one of the most enigmatic questions in particle physics. Out of the 22 (20 if B-L is conserved) independent low-energy parameters, which with some abuse of language can be called Yukawa couplings, 18 have been measured. Of the remaining four parameters, the absolute scale of neutrino masses is constrained in a limited range, the leptonic Dirac CP-violating phase starts to be constrained by global fits while the two possible Majorana phases are still unknown."

I know what are neutrino mass-squared differences. I also know that an upper limit on the sum of the light neutrino masses come from Planck observations. But what is meant by the absolute scale of the neutrino mass?

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    $\begingroup$ Presumably he means the actual masses rather than their differences. $\endgroup$ – knzhou Sep 24 '18 at 9:10
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He just means the approximate values ("scale", as in order of magnitude) of the neutrino masses. As you mention, the mass-squared differences are actually known to significantly more precision than the masses themselves are. He's using the world "absolute" (referring to the actual mass values) to contrast with "relative" (as in the mass-squared differences).

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  • $\begingroup$ I searched Google and found that this is a widely used term. But nowhere they explain what it means. @tparker $\endgroup$ – SRS Sep 26 '18 at 12:49
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    $\begingroup$ @SRS They don't explain what it means because it is a pretty self-evident term that does not require much explanation. $\endgroup$ – Emilio Pisanty Sep 27 '18 at 23:58
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Note that he focuses on which SM parameters cannot be measured (yet).

With this in mind, I think he refers to the fact that only neutrino mass differences can be measured. Therefore, if we knew one of the masses' absolute value we would know the rest of them as well. It doesn't matter which of the three that would be, thus the reference to their "scale".

Edit:

This link of the T2K experiment actually seems to confirm that "neutrino mass scale" is a synonym for "the mass of any of the neutrinos".

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