The Exclusion Range For Neutrinoless Double Beta Decay Continues To Expand

Figure 4: The effective Majorana mass mββ as a function of the smallest neutrino mass mMIN. We have used the current best-fit values and the 2σ errors of the oscillation parameters. The Majorana phases α21 and α31, and δ, are varied within their allowed intervals [0, 180º].

"While the grey area on top has been sought for, and excluded by direct searches, experiments will need to reach a sensitivity in the range of 0.01 eV in the effective mass parameter m(ββ) (yellow line) to be able to prove that the mass hierarchy of light neutrinos is normal. That is something that should become possible in the next few years."

From this new paper via this blog post.

The black oval added editorially by me in the figure above is the most likely part of the graph to correspond to reality based upon astronomy data regarding the maximum sum of the neutrino masses of the three neutrino masses, neutrino oscillation data, and other considerations, assuming for sake of argument that neutrinos have Majorana mass rather than Dirac mass (all of the other fundamental fermions of the Standard Model have Dirac mass only). For reference, as I noted in my previous post:

Assuming the increasingly experimentally favored normal hierarchy, and given that the mass difference between the heaviest neutrino mass and the middle neutrino mass is about 49.4 meV and the difference between the middle neutrino mass and the lightest neutrino mass is about 8.7 meV, [and considering plausibly motivated expectations discussed previously in that post] one would expect the lightest neutrino mass to be a little less than 1 meV, but probably not much lower than 0.5 meV.

This implies absolute neutrino masses of about 0.9 meV, 9.6 meV, and 59 meV, with a sum of the three neutrino masses equal to about 69.5 meV plus or minus about half an meV. This is very close to the minimum mass possible for the sum of the three neutrino mass eigenstates, given what we know already.

Unless neutrinos are Majorana particles (i.e. particles that are their own antiparticles), neutrinoless double beta decay doesn't happen at all, a conclusion that will take experiments somewhat more than 100 times more sensitive than the state of the art experiments done to date. We should be a factor of ten from that threshold in a few years, however.

If neutrinos are Majorana particles, the rate at which neutrinoless double beta decay occurs is a function of the effective Majorana neutrino mass shown on the Y axis, and the absolute mass of the lightest of the three neutrino masses. This is something that our experiments to date wouldn't have been able to see anyway, without regard to the nature of the neutrino mass hierarchy.