It's not yet clear whether the neutrino is a Majorana particle — that is, identical to its antiparticle. The answer could be found in the neutrinoless double-beta decay: a hypothetical radioactive process producing two electrons and two antineutrinos that annihilate each other. Azusa Gando and colleagues report the latest results of the KamLAND-Zen experiment, setting 1.07 × 1026 years as the lower limit on the half-life of this decay. This can be translated into a limit on the effective Majorana neutrino mass. And its value almost reaches a very interesting regime — that of the inverted mass hierarchy, one of the two possible orderings of neutrino masses.
For such precision measurements, one needs an extremely clean background. The KamLAND-Zen experiment uses 13 tons of xenon-loaded liquid scintillator in a nylon balloon inside the KamLAND detector. Gando et al. purified the xenon liquid scintillator in several stages to reduce radioactive contamination, but to push sensitivity even further — to the inverted mass hierarchy regime — they will have to upgrade to a larger balloon.