Physics - Probing Majorana Neutrinos

Laura Baudis Department of Physics, University of Zurich, Switzerland

30 January 2023• Physics 16, 13

The detection of neutrinoless double-beta decay will confirm that the neutrino is its own antiparticle. Data from the KamLAND-Zen experiment contain no strong evidence of such events, constraining neutrino properties.

Figure 1: The 136Xe isotope is known to decay by double-beta decay (left), in which two protons transform into two neutrons, which emit two electrons and two antineutrinos. If neutrinos are their own antiparticles, 136Xe can undergo neutrinoless double-beta decay (right), in which no neutrinos are emitted. The 136Xe isotope is known to decay via double-beta decay (left), in which two protons transform into two neutrons, which emit two electrons and two antineutrinos. If neutrinos are their own antiparticles, 136Xe can undergo neutrinoless double-beta dec… Show more Figure 1: The 136Xe isotope is known to decay by double-beta decay (left), in which two protons are converted into two neutrons and two emit electrons and two antineutrinos. If neutrinos are their own antiparticles, 136Xe can undergo neutrinoless double-beta decay (right), in which no neutrinos are emitted.×

Despite being one of the most abundant particles in the universe, neutrinos are extremely difficult to detect. Almost 100 years after they were predicted, and almost 70 years after their detection, several of the particles’ properties remain unknown, especially their mass and their “nature” – whether they are their own antiparticles. An extremely rare nuclear decay without the emission of neutrinos, called neutrinoless double-beta (0𝜈𝛽𝛽) decay, may shed light on these questions (see Viewpoint: The Hunt for No Neutrinos), but so far this hypothetical process has not been observed. Now, the KamLAND-Zen Collaboration has reported an improved search for 0𝜈𝛽𝛽 decay in a xenon-charged liquid scintillator detector, with an exposure reaching 1 ton-year for the first time [1]. The resulting lower limit for the decay half-life translates into an upper limit on the effective neutrino mass of about 100 meV, which approximates the lower limit estimates that come from other neutrino observations. The implication is that physicists may be approaching this neutrino mystery.

While the standard model predicts that neutrinos are massless, we know from neutrino oscillation experiments that they must be massive: specifically, for neutrinos to oscillate between their three “flavors”, the differences of their squared masses must not be zero. The oscillation data imply that at least one neutrino state must have a mass greater than about 50 meV, but the observations do not tell us about the absolute mass scale, or which of the three states is heaviest (the data leave two possible masses -orders then called “normal” and “inverted”). Nor do they answer the fundamental question of why neutrinos are so much lighter than other elementary particles.

One method of constraining neutrino masses is to study nuclei that decay by double-beta decay ( 2𝜈𝛽𝛽), in which two neutrons transform into two protons, which emit two electrons and two antineutrinos. However, if neutrinos are Majorana particles—that is, if they are their own antiparticle—then a 2𝜈𝛽𝛽 decaying nucleus will sometimes decay without emitting any neutrinos—a process known as 0𝜈𝛽𝛽 decay (Fig. 1). Most attempts to observe this decay involve measuring the total energy of the two electrons and looking for a peak at the Q value of the reaction, which is the difference between the rest mass energy of the initial and final products . Such a peak would imply a surplus of events in which no energy is carried away by neutrinos. Detecting this signature presents a formidable challenge, as the 0𝜈𝛽𝛽 decay is expected to be rare. Experiments must meet a number of requirements: a very large number of double-beta decay nuclei, an extremely low level of background, an excellent energy resolution to filter out a potential signal, and a high efficiency to locate two final-state electrons. To optimize these properties, physicists have used a variety of isotopes and detector concepts, including crystals cooled to cryogenic temperatures, high-pressure gas detectors, and large liquid scintillators [2].

KamLAND-Zen collaboration

Figure 2: Cutaway diagram showing the KamLAND-Zen experiment’s concentric, onion-like structure.

KamLAND-Zen collaboration

Figure 2: Cutaway diagram showing the KamLAND-Zen experiment’s concentric, onion-like structure.×

The KamLAND-Zen experiment at the Kamioka Observatory in Japan searches for 0𝜈𝛽𝛽 decay using a large liquid scintillator charged with the 136Xe isotope, which is known to undergo double-beta decay. To ensure that the level of background events is as low as possible, the detector has an onion-like structure (Fig. 2). A spherical inner balloon contains 13 tons of liquid scintillator in which 745 kg of Xe (comprising about 91% 136Xe) is dissolved. Around this inner core are three concentric shells: the first contains a liquid scintillator, the second contains 1879 large photomultiplier tubes (PMTs), and the third is a water Cherenkov detector. Particles interacting in the liquid scintillator—including particles created by rare decay of 136Xe nuclei—generate light that is detected by the PMTs. From these signals, each event’s energy and position is reconstructed with a relative energy resolution of 4.2% around the Q value (2.48 MeV) and a spatial uncertainty of 8.7 cm.

In their recent study, the KamLAND-Zen team analyzed data collected between February 2019 and May 2021 and found a total of 24 candidate events. With no excess above the expected background, this detection count corresponds to fewer than 6.2 events (at the 90% confidence level) attributable to 0𝜈𝛽𝛽 decay. Combined with the collaboration’s previous result [3] using half the target mass (381 kg of enriched Xe), the new result implies a lower limit on the half-life of 2.3 × 1026 years. If one assumes that the decay occurs mainly through the exchange of light Majorana neutrinos, then the half-life translates into an upper limit on the effective Majorana neutrino mass in the range 36–156 meV. This minimum half-life is just within the 1026-1028 year range associated with the inverted neutrino mass ordering, which means that KamLAND-Zen is beginning to explore this scenario for the first time, partially ruling out theoretical models that suggest a Majorana neutrino mass predicted in this region.

The experiment’s exposure of nearly one ton-year is a first in the field of 0𝜈𝛽𝛽-decay searches. While its energy resolution is 10 times less accurate than that of crystal-type detectors (which reach relative resolutions at the per-mile level), the obtained sensitivity demonstrates the power of a large amount of the decaying isotope combined with a low, though not -nil , background. Given the moderate depth of the Kamioka observatory underground, this background arises in part from long-lived spallation products—with half-lives ranging from several hours to days—generated in Xe by cosmic-ray-induced muons. This background can be ruled out with new event classification methods, which rely on time and distance estimators and on the detection of multiple neutrons released in the spallation process. Researchers are working to improve these methods using faster electronics.

The other limiting background comes from the tail of 136Xe’s 2𝜈𝛽𝛽 decay spectrum, the effect of which can only be reduced by improving the energy resolution. Increasing the resolution by a factor of 2 is a major goal of the future KamLAND2-Zen detector, which will use a liquid scintillator with a higher light yield and high-quantum-efficiency PMTs. With its one ton 136Xe, KamLAND2-Zen should reach a sensitivity of 20 meV after five years of data collection. Thus, while KamLAND-Zen first started to investigate the reverse neutrino mass ordering region, the upgrade can cover the full reverse order scenario, for which the smallest allowed effective mass value is (18.4 ± 1.3) meV [4]. This goal aligns with that of other planned projects, such as CUPID [5]LEGEND-1000 [6]nEXO [7]PandaX-III [8]DARWIN [9]NEXT-HD [10]and SNO+[11]. With half-life sensitivities around 1028 years, these future experiments will have a significant chance of discovering 0𝜈𝛽𝛽 decay and may therefore solve some of the mysteries surrounding neutrinos. Even more important than pinning down the particle’s mass and Majorana nature, such a discovery would establish that a fundamental symmetry of nature – the conservation of lepton number – is violated. Such a violation is considered an important ingredient in models that attempt to explain our Universe’s matter-antimatter asymmetry.

ReferencesS. Abe et al. (KamLAND-Zen collaboration), “Search for the Majorana nature of neutrinos in the inverse mass-ordered region with KamLAND-Zen,” Phys. Rev. Lett. 130, 051801 (2023).M. Agostini et al., “Towards the discovery of matter creation with neutrinoless double-beta decay,” arXiv:2202.01787 [hep-ex].A. Gando et al., “Search for Majorana neutrinos near the inverted mass hierarchy region with KamLAND-Zen,” Phys. Rev. Lett. 117, 082503 (2016).M. Agostini et al., “Testing the inverse neutrino mass ordering with neutrinoless double-𝛽 decay,” Phys. Rev. C 104, L042501 (2021). WR Armstrong et al. (CUPID Interest Group), “CUPID pre-CDR,” arXiv: 1907.09376.N. Abgrall et al. (LEGEND Collaboration), “LEGEND-1000 preconceptual design report,” arXiv:2107.11462.G Adhikari et al. (nEXO Collaboration), “nEXO: neutrinoless double beta decay search for 1028 year half-life sensitivity,” J. Phys. G: Nucl. Share. Phys. 49, 015104 (2021).X. Chen et al., “PandaX-III: Search for neutrinoless double beta decay with high pressure 136Xe gas time projection chambers,” Sci. China: Phys., Mech. Astron. 60, 061011 (2017).F. Agostini et al. (DARWIN Collaboration), “Sensitivity of the DARWIN observatory to the neutrinoless double beta decay of 136Xe,” Eur. Phys. J. C 80, 808 (2020).C. Adams et al. (NEXT Collaboration), “Sensitivity of a ton-scale NEXT detector for neutrinoless double-beta decay searches,” J. High Energ. Phys. 2021, 164 (2021).V. Albanese et al. (SNO+ Collaboration), “The SNO+ Experiment,” J. Instrum. 16, P08059 (2021). About the Author

Laura Baudis is a professor in the Department of Physics of the University of Zurich. Her research focuses on the development of detectors for the direct detection of dark matter particles and for neutrino physics. She is particularly interested in the fundamental nature of dark matter and in the properties of neutrinos. She received her PhD from the University of Heidelberg, Germany. She is a member of the Academy of Sciences and Letters, Mainz, Germany, and she received the 2022 Charpak-Ritz Prize from the French and Swiss Physical Societies.

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