Neutrinos are the least understood of all known matter particles. Their most puzzling property is that they have tiny yet finite masses; even though the Higgs mechanism has now been largely confirmed by the LHC as the source of fundamental masses in the Standard Model, the lightness of neutrinos remains a mystery. A crucial probe to determine the scale and nature of neutrino masses is neutrinoless double beta decay. Already hypothesised in 1939, this nuclear process involves the simultaneous decay of two protons into neutrons within a nucleus under the emission of two electrons. If observed, neutrinoless double beta decay would be a definite sign of new physics beyond the Standard Model. Specifically, it would mean that neutrinos are Majorana fermions and it would most likely point to a very high mass scale of new physics where lepton number symmetry is broken. In my talk, I will outline the theory of neutrinoless double beta and associated processes as a probe of physics beyond the Standard Model. I will attempt to motivate the importance and consequences of this process and discuss the intricate problems in dealing with its nuclear nature. Furthermore, I will sketch the underlying physics connections with other searches, such as for lepton flavour violating decays, and explain what we can learn from neutrinoless double beta decay about the matter-antimatter asymmetry of the Universe.
(Host: Sofiane Boucenna)