It's not quite as rare as the article makes it out to be. It's not like they by chance happened to detect a single event that only happens every few trillion years. They had a TON (actually three tons) of xenon. Decay events happened every few days, and they collected over 100 of them. Those events have a particular "signature" that allows them to be distinguished from other kinds of events. Even then, they needed to apply statistical methods to compute the odds that the data they were seeing was real and not from random chance, and the result was that the odds were not quite high enough to qualify as a discovery by the usual standards.
Details are in the original paper, which is pretty accessible:
Details are in the original paper, which is pretty accessible:
https://arxiv.org/pdf/1904.11002.pdf