Neutrino 2014 Offers Nothing No Re Neutrinoless Double Beta Decay Or Cosmology

The recap of cosmology based neutrino data and neutrinoless double beta decay experiments is unchanged from news already reported at this blog.

The Cosmology Data

On the cosmology front, the bottom line state of the art figure after considering all of the outstanding data is is Neff of 3.32 +/- 0.27 (with Neff equal to 3.04 in a case with the three Standard Model neutrinos and neutrinos with masses of 10 eV or more not counting in the calculation), and the sum of the masses of each of the neutrino mass eigenstates is less than 0.28 eV (at the 95% confidence level, and less than 0.2 eV at a 68% confidence level).

While the Neff value is not inconsistent with a fourth neutrino species, given the the sum of the three Standard Model neutrino mass eigenstates is not less than about 0.06 eV, this would require that a fourth effective neutrino species have a mass of less than 0.22 eV at the two sigma level and less than 0.14 eV at the one sigma level.

Yet, as noted previously, an additional sterile neutrino with a mass of 0.001 to 0.1 eV has been largely ruled out by two separate sets of reactor experiments, leaving a narrow window of 0.1 eV to 0.22 eV for an additional neutrino, despite reactor anomaly data fitting best to a 1 eV additional neutrino species.

In short, the cosmology data taken as a whole and in light of everything else we know about neutrinos, disfavors the existence of a light reactor anomaly sterile neutrino that oscillates with the ordinary Standard Model neutrinos.  A closing talk at Neutrino 2014 summarized the state of affairs by stating that the dominant 3+1 paradigm is almost ruled out by current experimental data already.  An earlier talk explored these tensions in that data in greater depth.

Neutrinoless Double Beta Decay

Nothing has changed since my last post on neutrinoless double beta decay experiment results in December of 2013.  But, the Neutrino 2014 conference in Boston last week did recap the data from the half dozen or so current experiments and the similar number of experiments that will come on line before long.

EXO and Kamland report consistent results for the rate of 2vBB decays (which the Standard Model permits) at the 1.4 sigma level.

The EXO result is 2.165 +/- 0.0573 * 10^21 years.
The Kamland  result is 2.32 +/- 0.094 * 10^21 years.

These confirmations give us some confidence that the methodology of these experiments are sound, and provides a firm experimental data point from which BSM theories in which it is possible to calculate relative rates of 2vBB decays and 20BB decays can be evaluated.  We now know, as the data points below reveal, that neutrinoless double beta decay, if it happens at all, is at least 10,000 times less common than 2vBB decays.

The GERDA experiment which rules out neutrinoless beta decay for 2.1*10^25 years at the 90% confidence level is the strongest individual exclusion, followed closely by the EXO-200 and Kamland results.  The coefficient in front of 10^25 years at the 90% confidence level for Kamland is 1.9, for CUORICINO is 0.28 and for NEMO-3 is 0.11.  The combined exclusion from all data is a bit stronger, but still on the same order of magnitude.

These correspond to upper limits on Majorana masses of 0.14 eV to 0.34 eV from the most strict results and 0.33 eV to 0.87 eV from the least strict bounds.  Of course, oscillation data put minimum absolute neutrino mass scales for both inverted hierarchies and normal hierarchies well below those levels.

Ruling out a completely Majorana mass for neutrinos if there is an inverted hierarchy is something that is "just around the corner" over the next several years with currently planned experiments.  Ruling out a completely Majorana mass for neutrinos if there is a normal hierarchy is beyond the scope of neutrinoless double beta decay experiments currently on the drawing board and will take quite a while.  Current experiments have detector material masses on the order of 1 ton.  An experiment that would rule out completely Majorana mass for a normal neutrino mass hierarchy would require on the order of 1000 tons of material.

Also, in many BSM theories, including many SUSY theories, there are sources of neutrinoless double beta decay in addition to those associated with Majorana mass neutrinos.  So, as exclusions of neutrinoless double beta decay grow greater over time, the limits on these BSM theories are tightened, even before Majorana mass neutrinos themselves can be ruled out.

Glass half full Majorana neutrino advocates would argue that the results so far say little, because current experiments aren't precise enough to detect neutrinoless double beta decay at the predicted frequencies of their theories, even if it exists.  Glass half empty skeptics would argue that the absence of any positive evidence of lepton number violation is yet another instance of BSM theories failing to deliver any observable evidence in their favor.

More neutrally, in any BSM theory which has both Majorana neutrinos and other sources of neutrinoless double beta decay, the alternative sources can't be more than twice as great as the Majorana sources in an inverted hierarchy and can't be more than about five times as great as the Majorana sources in a normal hierarchy.  This cutoff is sufficiently strong to rule out a fair amount of BSM parameter space.

Also, it isn't entirely clear that lepton number violation from Majorana mass neutrinos alone is actually sufficient to account for the actual lepton number of the universe which is strongly suspected to have far more antileptons than leptons (the lepton number of the universe is largely a function of the ratio of neutrinos to antineutrinos in the universe and would be extremely close to 1:1 if the lepton number of the universe started out at 0 and there were no lepton number violating processes).  If other BSM contributions to lepton number violation and observed neutrinoless double beta decay are not substantial, the question of a non-zero lepton number of the universe (if there is one) remains unsolved.

It is also worth noting that there were no other announcements at Neutrino 2014 of any evidence from any other source of lepton number violating processes.  Neutrinoless double beta decay also remains the most promising place, given current experimental limitations, to look for lepton number violating processes.