An Improved Search for Neutrinoless Double Beta Decay with KamLAND-Zen

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A neutrino is a subatomic particle that has no electrical charge and a very small mass. Neutrinos are one of the most abundant particles in the universe, but because they have very little interaction with matter, they are difficult to detect. In the last 20 years, remarkable progress has been made in understanding the fundamental nature of the neutrino, including measurements which have indicated that neutrinos must have a non-zero mass. This leads to some very important questions. What is the absolute mass of the neutrino? Why are neutrino masses so small compared to other elementary particles? How did the neutrino obtain a mass? The answers to these questions could be intimately related to the neutrino being its own antiparticle, also known as a Majorana particle, and may explain the prevalence of matter over anti-matter in the Universe.A rare nuclear process, called neutrinoless double beta decay, is a powerful tool that can be used to investigate the Majorana nature of neutrinos. This process, whereby two neutrons simultaneously transform into two protons with the emission of two electrons and no neutrinos, is extremely rare and is difficult to observe due to the vast number of similar processes that could occur inside an experimental detector. A competitive experimental search requires a large quantity of the double beta decay isotope and exquisite suppression of backgrounds. This award will support the Boston University group’s activities in an improved search for neutrinoless double beta decay in the KamLAND-Zen experiment located deep underground in Japan. This award will engage undergraduates from diverse backgrounds through the Research Experience for Undergraduates program at Boston University. At the same time, it will establish a pipeline of students and postdocs who are trained in advanced technical skills that are highly sought-after in academia, industry, and national laboratories.KamLAND-Zen is a kiloton scale liquid scintillator based detector. Recently, the experiment started a new phase of data-taking using roughly 750 kg of 136-Xe instrumented inside the detector. This award will support the following enhancements to KamLAND-Zen: improved background suppression with state-of-the-art deep learning classification algorithms, an independent Bayesian analysis, and the implementation of online event tagging software for the trigger system. The substantial reduction of backgrounds offered by these enhancements will push the neutrinoless double beta decay half-life sensitivity of KamLAND-Zen all the way to 5×10^26 years. This is almost a five-fold improvement over previous KamLAND-Zen measurements. The enhancements developed under this award will set the stage for a major detector upgrade, called KamLAND2-Zen, that will aim for a half-life sensitivity greater than 2×10^27 years.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.