If discovered, neutrinoless double beta decay would prove that neutrinos -- highly abundant elementary particles with extremely small mass -- are their own antiparticles. That information would help researchers determine how heavy neutrinos actually are and how they acquire their mass. Although the EXO-200 experiment did not observe the decay, its complete data set, published on the arXiv repository and accepted for publication inPhysical Review Letters, defined some of the strongest limits yet for the decay's half-life and for the mass neutrinos may have.

EXO-200 operated at the Waste Isolation Pilot Plant (WIPP) in New Mexico from 2011 to 2018. Within its first months of operations, it discovered another rare process: the two-neutrino double beta decay of the same xenon isotope. EXO-200 was an important precursor for next-generation experiments, such as the proposed nEXO, that would have a much better chance of discovering the neutrinoless decay.