Investigating the neutrino mass scale with the ultra-low background KamLAND-Zen detector

The ultra-low background KamLAND-Zen detector, hosted by research institutes inside and outside Japan demonstrates the best sensitivity in the search for neutrinoless double-beta decay, and sets the best limit on the effective Majorana neutrino mass.

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The physicist Ettore Majorana hypothesized that the uncharged neutrino particle could be its own antiparticle, and thus could be annihilated by itself.

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The Majorana neutrino is almost completely ruled out now.

The current best fit to the data, and conceptually simplest approach is for the neutrino to be like all the other leptons (electron, muon and tau) and not be its own antiparticle.

I used to keep track of the best measured values of neutrino masses. Problem 1 is that the mass of the electron neutrino is very badly constrained because it can’t be calculated from neutrino oscillation observations and is too small to be measured directly. Problem 2 is that last time I looked, mass differences squared could be consistent with experiment in two completely different regions. Less of a problem is that everything works with mass difference squared rather than either mass or mass difference.

Also note the paper title. “Search for Majorana Neutrinos near the Inverted Mass Hierarchy Region with KamLAND-Zen”

This is like searching for a black cat swimming underwater at night. Both Majorana neutrinos and the inverted mass region are highly unlikely, on Occam’s razor grounds. “Inverted mass” is the assumption that electron neutrinos are heavier than muon and tau neutrinos, no-one wants that to be true.