# KamLAND-Zen results

• A
Staff Emeritus
Homework Helper
Gold Member
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.

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.

vanhees71, Garlic, mfb and 1 other person

Garlic
Gold Member
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

Staff Emeritus
Homework Helper
Gold Member
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.

Garlic
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?

Staff Emeritus