New Neutrinoless Double Beta Decay Searches Designed

Some rare decays of radioactive isotypes are atoms such as the decay of calcium-48 to titanium-48, are well understood and derive almost entirely from the well measured properties of the original isotype, the decay product, and the mass of the subatomic particles involved in intermediate steps of the decay (in the case of neutrinoless double beta decay, the absolute Majorana mass of the neutrino).

Proposed experiments with novel isotypes have the potential to greatly improve the threshold at which neutrinoless double beta decay rates can be measured or rule out.

In the Standard Model with Dirac mass neutrinos, neutrinoless double beta decay is categorically forbidden because it violated baryon number and/or lepton number conservation.  In the Standard Model with Majorana neutrinos, one can predict very precisely the expected rate of neutrinoless double beta decay as a function of absolute Majorana neutrino mass in a manner that while not actually model independent, is still quite robust over a broad range of plausible models with Majorana neutrinos.

Current neutrinoless double beta decay detection experiments are not yet sufficiently precise to distinguish between Dirac neutrinos and Majorana neutrinos for masses consistent with the expectations associated with neutrino oscillation measurements and a normal hierarchy of neutrino masses.  

Small neutrino masses imply rare neutrinoless double beta decays, and while experiments can't yet distinguish between Dirac and Majorana neutrinos, we can say with increasing confidence that the sum of the three mass eigenstates for neutrino masses is probably less than 100 meV, which is tiny (about 5.11 million times or more lighter than a single electron, and about 9 billion times lighter than a proton, for one of each of three kinds of neutrino mass combined - the lightest of the three neutrino mass eigenvalues is probably on the order of 1 meV or less).

If the mass of a proton is the U.S. national budget, the lightest neutrino rest mass might be a single dollar or maybe even just spare change.

Theorists love Majorana neutrinos, but my personal conjecture ad belief, for a variety of reasons beyond the scope of this post, is that the likelihood that neutrinos have Majorana mass (i.e. the likelihood that neutrinos are their own anti-particles) is very low.  So, honestly, I'm not terribly excited by this development, because I expect a null result from the new experiments.

For those of you more interested in theory, a recent article has some interesting numerological discussions of the Standard Model mass and mixing angle constants, suggesting that "sum rules" may be at work and can be used to predict the neutrino masses and mixing angles.