KamLAND-Zen Results: Search for Majorana Neutrinos in Inverted Mass Hierarchy

[Orodruin]

I would like to draw some attention to the KamLAND-Zen collaboration, which recently made their latest results public on the arXiv:

Search for Majorana Neutrinos near the Inverted Mass Hierarchy region with KamLAND-Zen
KamLAND-Zen Collaboration
(Submitted on 10 May 2016)
We present an improved search for neutrinoless double-beta (0νββ) decay of 136Xe in the KamLAND-Zen experiment. Owing to purification of the xenon-loaded liquid scintillator, we achieved a significant reduction of the 110mAg contaminant identified in previous searches. Combining the results from the first and second phase, we obtain a lower limit for the 0νββ decay half-life of T0ν1/2>1.1×1026 yr at 90% C.L., an almost sixfold improvement over previous limits. Using commonly adopted nuclear matrix element calculations, the corresponding upper limits on the effective Majorana neutrino mass are in the range 60-161 meV. For the most optimistic nuclear matrix elements, this limit reaches the bottom of the quasi-degenerate neutrino mass region.

Comments: 6 pages, 3 figures
Subjects: High Energy Physics - Experiment (hep-ex); Nuclear Experiment (nucl-ex); Instrumentation and Detectors (physics.ins-det)
Cite as: arXiv:1605.02889 [hep-ex]
(or arXiv:1605.02889v1 [hep-ex] for this version)

The limits they put on the effective electron neutrino mass improve by almost an order of magnitude (the upper bound is in the range 60-161 meV, depending on the nuclear matrix element used) upon the previous best ones and we are now facing a situation where almost the entire quasi-degenerate region is ruled out and another order of magnitude or so would probe the inverted hierarchy (IH).

One thing worth remembering is that a signal in 0νββ would require neutrinos to be Majorana fermions and a negative result even after probing the IH and seeing IH in other experiments could be due to either neutrinos being Dirac fermions or new physics.

it is also worth noting that this result is a stronger bound than what was expected based on the KamLAND-Zen sensitivity, likely due to favourable statistical fluctuations. It will be quite some time before any other experiment can compete with this.

 

[Garlic]

So finally there are more results on 0vββ experiments, I am really interested.
Could someone explain roughly what quasi-degenerate neutrino mass is?
And why does the experiment use Xe-136, but not another double beta decaying isotope with less half life?
Thank you

 

[Orodruin]

Could someone explain roughly what quasi-degenerate neutrino mass is?

From oscillation experiments, we currently have knowledge on the differences of the mass squares of the neutrino mass eigenstates $\Delta m_{ij}^2 = m_i^2 - m_j^2$. The sign of $\Delta m_{31}^2$ is currently unknown and if it is positive it is referred to as normal ordering and otherwise as inverted ordering. However, oscillation experiments give us no insight to the absolute mass scale of the neutrinos, i.e., the lightest neutrino could still be massless. If it is massless or has a very small mass, the neutrino masses are hierarchical, i.e., their ratios are large. However, if the lightest neutrino mass $m_0$ is such that $m_0^2 \gg |\Delta m_{31}|^2$, all neutrino masses will be of similar size. This would also mean that it would be very difficult to tell the ordering apart in neutrinoless double beta decay experiments as the effective mass measured in these experiments would be very similar.

You can see this in one of the figures of the KamLAND-Zen paper:

The quasi-degenerate regime starts when the lightest neutrino mass is around 0.1 eV.

As for the choice of nucleus, there are many factors other than half-life to consider, e.g., the Q-values and the uncertainties in the nuclear matrix element to mention a few. I am not an expert in the experimental implementation, someone else might be able to be more precise.

 

[Dr.AbeNikIanEdL]

I think I never saw this plot with the grey bands in it (not too surprising as I am not too much into that field). From the paper I take these are predictions based on neutrino oscillation measurements, and they seem to be excluded by the measurement. How is this currently interpreted? It seems to indicate that neutrino oscillation data disfavour neutrinos being majorana particles?

 

[Orodruin]

The grey bands are the upper limits from other 0νββ searches. The reason they are bands and not lines is mainly due to uncertainties in the nuclear matrix elements (NMEs) of the decaying isotopes.

Oscillation experiments are not sensitive to the Dirac/Majorana nature of neutrinos. The only difference in the oscillation formalism is the appearance of two Majorana phases in the lepton mixing matrix, neither of which affects the oscillation probabilities. The red and green bands show the allowed range of the plotted parameters based on oscillation experiments. Like the grey bands, the blue KamLAND-Zen band is the range of upper limits which are put on the effective neutrino mass depending on the NME.

 

[Dr.AbeNikIanEdL]

Ah, I think I misinterpreted what they mean by "dark shaded" in the caption. That way it makes much more sense, thanks for clarifying:)