  The discovery of neutrinoless double-beta decay could soon be within reach.
This hypothetical ultra-rare nuclear decay offers a privileged portal to
physics beyond the Standard Model of particle physics. Its observation would
constitute the discovery of a matter-creating process, corroborating leading
theories of why the universe contains more matter than antimatter, and how
forces unify at high energy scales. It would also prove that neutrinos and
anti-neutrinos are not two distinct particles, but can transform into each
other, with their mass described by a unique mechanism conceived by Majorana.
The recognition that neutrinos are not massless necessitates an explanation and
has boosted interest in neutrinoless double-beta decay. The field stands now at
a turning point. A new round of experiments is currently being prepared for the
next decade to cover an important region of parameter space. In parallel,
advances in nuclear theory are laying the groundwork to connect the nuclear
decay with the underlying new physics. Meanwhile, the particle theory landscape
continues to find new motivations for neutrinos to be their own antiparticle.
This review brings together the experimental, nuclear theory, and particle
theory aspects connected to neutrinoless double-beta decay, to explore the path
toward - and beyond - its discovery.
