Neutrino-less double beta decay is a yet undiscovered nuclear decay in which two neutrons inside a nucleus simultaneously (up to quantum uncertainty) turn into neutrons emitting two final state electrons but no neutrinos. This process, unlike any other reaction measured to date, violates lepton number, L, by two units (L=+2 in the final state, L=0 in the initial state) and, via mechanisms physicists have been developing for decades, could represent a portal through which to understand the dominance of matter (over antimatter) in the universe. While lepton number-violating processes are also searched for at accelerators, neutrino-less double beta decay is, generally speaking, the most sensitive way to test this fundamental symmetry of nature thanks to the possibility of observing a very large number of nuclei at once and for long times, surpassing any realistic exposure man-made particle beams can provide (current detectors use ~kilo-moles of isotope and run for 5-10 years).
If discovered, neutrino-less double beta decay would necessarily make neutrinos massive Majorana fermions, i.e. they would be their own anti-particles. No other fundamental fermion of the Standard Model (SM) of particle physics, by virtue of them carrying electric charge, can display this property. Many extensions to the SM predict Majorana fermions, and neutrinos could be the tool at our disposal to prove their existence. Irrespective of its subtending mechanism, neutrino-less double beta decay informs on the nature of neutrinos. In the particular ‘see-saw-I’ models, the existence of very heavy, right-handed Majorana neutrinos provide an explanation for the vanishing masses of the three active neutrinos (e,muon,tau) we are familiar with. In this case, light neutrino masses are directly related to the 0??? decay half life.
The experimental signature of neutrino-less double beta decay is a mono-energetic peak at the end of a continuous 2-neutrino energy spectrum. Key experimental requirements are good energy resolution to separate 2-neutrino decay modes, very low environmental radioactivity (forcing experiments to run underground to have enough shielding from cosmic rays), and technology-dependent particle-identification and design characteristics.
- An article for the general scientific audience introducing Majorana particles (link)
- Articles introducing neutrino-less double beta decay to the general scientific audience, with specific reference to the EXO-200 and competing programs (link1, link2)
- A recent, concise review of the searches for neutrino-less double beta decay (link)
- A recent exhaustive review of neutrino-less double beta decay (link)
nEXO is a 5 tonne, 90%-enriched liquid xenon (LXe) time projection chamber (TPC) designed to be sensitive to 0??? decay half lives of Xe-136 of up to ~1E28 years. It is scheduled to start operations in 2027 at SNOLAB, Ontario, Canada.
EXO-200 (2010-2018) operated at the Waste Isolation Pilot Plant (WIPP) in New Mexico. The first kilo-mole-scale double beta decay experiment to take data, EXO-200 is used 200 kg of 80%-enriched liquid Xe-136 in a double TPC layout. It has demonstrated the competitiveness of shielded, monolithic LXe TPCs for double beta decay searches and laid the foundations for the 25x more massive nEXO experiment. EXO-200 has set a lower bound on the half life of Xe-136 neutrino-less double beta decay of 3.5E25 years, one of the best in the field.
For a list of papers follow the “Publications” tab
NOTE: links are to the published version of papers. With little exception, an open access version of every publication is available at arXiv.org.