Seminars 李政道研究所-粒子核物理研究所联合演讲

Searching for matter creation via neutrinoless double beta decay

by Dr Pin-Jung Chiu (University of Zurich)





Although the progress of particle physics in the past century has resolved plenty of mysteries of the Universe, there are still long-standing questions yet to be understood. These include the nature of antimatter, the absolute mass scale of neutrinos, and the nature of neutrinos, i.e., whether they are Dirac of Majorana particles. 

A hypothesized, extremely rare process, neutrinoless double beta decay (0νββ), was proposed to find answers to these questions. Two-neutrino double beta decay is a Standard-Model-allowed process, where two neutrons decay simultaneously into two protons, two electrons, and two electron-antineutrinos: 2n → 2p + + 2e  + 2¯νe. On the contrary, 0νββ is a process where the decay products do not include (anti)neutrinos. This is possible if the two emitted (anti)neutrinos annihilate as they are their own antiparticles (νe = ¯νe). As a result, the observation of 0νββ would demonstrate that neutrinos have a Majorana mass and would establish lepton-number violation. Furthermore, it would provide hints of the neutrino absolute mass scale and the neutrino mass ordering, as well as information about the matterantimatter imbalance of the Universe.

 LEGEND (Large Enriched Germanium Experiment for Neutrinoless double beta Decay) will search for 0νββ with high-purity germanium detectors enriched in 76Ge and operated in liquid-argon, which serves as a coolant, a passive shield, and an active veto system. The first phase of the experiment (LEGEND200) will deploy around 200 kg of germanium diodes and reach a discovery sensitivity of T 0ν 1/2 > 1027y (3σ) for the half-life of 0νββ with an experimental live-time of five years. The first physics run has started with an initial 142 kg of germanium diodes (101 detectors). The other 50 kg of detectors will be characterized and installed later to reach its full mass. The second phase of the experiment (LEGEND1000) aims to improve the discovery sensitivity by another order of magnitude with 1 tonne of large-mass, high-purity, enriched germanium detectors operated for ten years. By reducing the background levels with several hardware approaches and improved analysis techniques, as well as operating with detectors of the best energy resolution in the field, LEGEND will perform a quasi-background-free search, where an unambiguous signature can be distinguished at the 0νββ decay Q-value of 2039 keV. 

In this presentation, I will explain the importance of 0νββ to the fields of neutrino physics and physics beyond the Standard Model, as well as discussing the current status and the future plans of the LEGEND-200 and the LEGEND-1000 experiments, respectively.


Dr. Pin-Jung Chiu did her four-year bachelor study at National Tsing Hua University in Taiwan. Afterwards, she moved to Switzerland and did her master's study at ETH Zurich. Her thesis was about electrostatic studies for the proto-DUNE experiment at CERN, this is when she started working in the ?eld of neutrino physics. To explore dierent aspects of particle physics, she changed the topic to work on precision measurements during her PhD. She involved in the neutron electric dipole moment (nEDM) search at the Paul Scherrer Institute (PSI), focusing on the study of a crucial systematic eect, which might aect the experimental sensitivity. In addition, she used the same apparatus to search for signatures of a new interaction, which could be mediated by axions or axion-like particles. Currently, she is a postdoc at the University of Zurich, searching for neutrinoless double beta decay in the GERDA and the LEGEND experiments.

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