KamLAND-Zen

KamLAND-Zen. It is a pioneering particle physics experiment located deep underground at the Kamioka Observatory in Japan, designed primarily to search for neutrinoless double beta decay. The experiment is a major upgrade of the existing KamLAND detector, incorporating a large, transparent balloon filled with xenon-loaded liquid scintillator at its core to act as both source and detector. By observing this extremely rare nuclear process, the collaboration aims to determine if the neutrino is its own antiparticle (a Majorana fermion), which would have profound implications for particle physics and cosmology. The project represents a significant international effort involving institutions like KEK and Tohoku University.

Overview

The experiment builds upon the formidable infrastructure and expertise developed for the original KamLAND detector, which famously investigated reactor antineutrino oscillations. Situated in the Kamioka Mine to shield it from cosmic rays, the central innovation involves suspending a large, nylon-based inner balloon within the main detector vessel. This balloon is filled with a liquid scintillator doped with enriched isotopes of xenon, specifically Xe-136, which is the candidate for the decay process. The surrounding outer detector, filled with ultra-pure liquid scintillator, acts as an active veto against residual background radiation from the environment and detector materials. This configuration allows for extremely sensitive searches for rare events in the sub-MeV energy region.

Experimental design

The core design philosophy centers on creating a ultra-low-background environment to isolate the signature of neutrinoless double beta decay. The target xenon is dissolved in a proprietary liquid scintillator mixture, allowing the energy from any decay event to be converted into light. This light is detected by an array of over 1,800 high-sensitivity photomultiplier tubes mounted on the stainless steel containment tank. The entire apparatus is immersed within a large water Cherenkov detector that serves as a further passive shield. Key to the design is the careful selection and screening of all construction materials, such as stainless steel and polyethylene, to minimize intrinsic radioactivity from isotopes like uranium and thorium.

Physics goals and results

The primary physics goal is the observation of neutrinoless double beta decay in Xe-136, which would violate lepton number conservation and prove the Majorana nature of neutrinos. Such a discovery would directly measure the effective Majorana mass of the neutrino, informing models of grand unified theory and potentially explaining the matter-antimatter asymmetry in the universe via leptogenesis. The collaboration has set world-leading limits on the decay half-life, exceeding 10^26 years, and constrained the neutrino mass scale. The detector also performs precision spectroscopy of geoneutrinos from the Earth's interior and continues to study reactor antineutrino phenomena from Japanese nuclear power plants like Kashiwazaki-Kariwa.

Technical components

Major technical subsystems include the 3.08-meter diameter inner balloon, constructed from transparent nylon and EVOH film, which holds approximately 1,000 kilograms of xenon-loaded scintillator. The light detection system relies on specially developed 17-inch and 20-inch photomultiplier tubes from Hamamatsu Photonics, with enhanced quantum efficiency. A complex purification system continuously removes radioactive impurities like radon and krypton from the scintillator fluids. The data acquisition system, triggered by coincident PMT signals, records event timing and topology with high precision. Calibration is performed using deployed sources such as americium-241 and neutron generators.

Collaboration and history

The KamLAND-Zen collaboration is a multinational team of scientists from over 40 institutions, primarily in Japan and the United States, including major contributions from KEK, Tohoku University, University of California, Berkeley, and Stanford University. The project was conceived following the success of KamLAND, with the "Zen" (zero neutrino) phase beginning installation in 2011. The experiment has undergone several upgrades, notably KamLAND-Zen 800 with a larger xenon load. Leadership has included spokespersons like Atsuto Suzuki and project managers from Kamioka Observatory. It remains a cornerstone experiment in the global program to understand neutrino properties, operating in synergy with other projects like EXO and GERDA. Category:Particle physics experiments Category:Neutrino experiments Category:Experiments in Japan