Search for Majorana Neutrinos near the Inverted Mass Hierarchy Region with KamLAND-Zen

Abstract: We present an improved search for neutrinoless double-beta ($0\nu\beta\beta$) decay of $^{136}$Xe in the KamLAND-Zen experiment. Owing to purification of the xenon-loaded liquid scintillator, we achieved a significant reduction of the $^{110m}$Ag contaminant identified in previous searches. Combining the results from the first and second phase, we obtain a lower limit for the $0\nu\beta\beta$ decay half-life of $T_{1/2}^{0\nu} > 1.07 \times 10^{26}$ 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 61-165 meV. For the most optimistic nuclear matrix elements, this limit reaches the bottom of the quasi-degenerate neutrino mass region.

• The paper reports an improved lower limit for 0νββ decay in 136Xe with T1/2 > 1.07×10^26 years at 90% confidence using 504 kg-years exposure.
• It employs sophisticated xenon purification and a spherical liquid scintillator detector setup to significantly reduce 110mAg background interference.
• The obtained constraints on the effective Majorana neutrino mass (61–165 meV) drive future detector enhancements aimed at probing the inverted mass hierarchy.

Search for Majorana Neutrinos with KamLAND-Zen: A Review

The pursuit of neutrinoless double-beta decay ($0\nu\beta\beta$) continues to be a focal point in neutrino physics, as its discovery would reveal the Majorana nature of neutrinos and demonstrate lepton number non-conservation. The KamLAND-Zen collaboration has made significant advancements in this domain, particularly with the isotope $^{136}$Xe. The recent study reported an improved lower limit for the $0\nu\beta\beta$ decay half-life of $^{136}$Xe, yielding $T_{1/2}^{0\nu} > 1.07 \times 10^{26}$ years at 90% confidence level, a noteworthy improvement over previous measurements.

Methodology

KamLAND-Zen leverages the KamLAND detector infrastructure, optimizing for low background through sophisticated purification techniques of xenon-loaded liquid scintillator (Xe-LS). This meticulous approach included the extraction, purification, and reintroduction of Xe-LS to diminish contaminants such as $^{110m}$Ag, which previously hindered sensitivity.

The experimental setup features a spherical inner balloon (IB) containing the Xe-LS, surrounded by a larger liquid scintillator as an active veto shield. The structure is designed to maximize the detection of $0\nu\beta\beta$ events by capturing the scintillation photons emitted from decay processes.

Results

With an accumulated exposure of 504 kg-years of $^{136}$Xe, the experiment established an upper limit on the effective Majorana neutrino mass, $\left<m_{\beta\beta}\right>$, within the range of 61 to 165 meV, depending on the nuclear matrix element calculations applied. This constraint approaches the lower edge of the quasi-degenerate neutrino mass region.

The experiment's enhanced sensitivity was predominantly attributed to the reduced presence of $^{110m}$Ag, facilitated by improved purification procedures, and a comprehensive analysis that separated data into two precise periods. Statistical methods rejected the hypothesis that standard radioactive decay accounted for observed background reductions, suggesting alternative decay processes or spatial redistributions of $^{110m}$Ag within the detector.

Implications and Future Directions

The constraints placed on the neutrino mass scale by KamLAND-Zen are the most stringent to date, influencing theories surrounding Majorana neutrinos and potentially providing insights into the neutrino mass hierarchy. Practically, these findings challenge existing models and encourage refinements in nuclear matrix element calculations to further elucidate the fundamental properties of neutrinos.

Future developments in the KamLAND-Zen experiment aim to incorporate increased amounts of enriched xenon and implement detector enhancements to reduce background influences, such as muon spallation effects, while improving energy resolution to suppress $2\nu\beta\beta$ decay tail interference.

These upgrades, alongside innovation in detector design and methodology, aim to lower the sensitivity threshold further, potentially enabling the probing of $\left<m_{\beta\beta}\right>$ below 50 meV, thereby approaching the sensitivity required to explore the inverted mass hierarchy. Such advances signify a pivotal step toward unraveling the elusive properties of neutrinos and their role in the universe.

In conclusion, KamLAND-Zen stands at the frontier of $0\nu\beta\beta$ decay research, offering invaluable insights and setting new benchmarks in the quest to uncover the Majorana nature of neutrinos, with far-reaching implications for fundamental physics.