Neutrinoless Double-Beta Decay: To the Ton Scale and Beyond*

Why is the universe dominated by matter, and not antimatter? Neutrinos, with their changing flavors and tiny masses, could provide an answer. If the neutrino is a Majorana particle, meaning that it is its own antiparticle, it would reveal the origin of the neutrino's mass, demonstrate that lepton number is not a conserved symmetry of nature, and provide a path to leptogenesis in the early universe. To discover whether this is the case, we must search for neutrinoless double-beta decay, a theorized process that would occur in some nuclei. By searching for this extremely rare decay, we can explore new physics at energy scales that only existed in the seconds following the Big Bang. 

Detecting this extremely rare process, however, requires us to build very large detectors with very low background rates. The previous generation of experiments has made outstanding progress using a wide range of techniques, probing half lives of up to 10²⁶ years; LEGEND-200, which is now taking data, plans to reach the 10²⁷ year level. Several collaborations are moving forward with plans for ton-scale experiments that would reach effective Majorana neutrino masses of 18 meV, corresponding to fully exploring the Inverted Ordering region in a light Majorana neutrino exchange model. At the same time, research and development activities are underway for future beyond-the-ton-scale experiments that would have even higher sensitivities. Many of these would serve as neutrino observatories, pursuing a broad program of low-energy neutrino physics measurements. I'll discuss recent results from current generation neutrinoless double-beta decay searches, the status of ton-scale searches, and the ongoing R&D laying the groundwork for future beyond-the-ton-scale searches.