Probing beyond the standard model physics with double-beta decays

Nuclear double-beta decays are a unique probe to search for new physics beyond the standard model. Hypothesized particles, non-standard interactions, or the violation of fundamental symmetries would affect the decay kinematics, creating detectable and characteristic experimental signatures. In particular, the energy distribution of the electrons emitted in the decay gives an insight into the decay mechanism and has been studied in several isotopes and experiments. No deviations from the prediction of the standard model have been reported yet. However, several new experiments are underway or in preparation and will soon increase the sensitivity of these beyond-the-standard-model physics searches, exploring uncharted parts of the parameter space. This review brings together phenomenological and experimental aspects related to new-physics searches in double-beta decay experiments, focusing on the testable models, the most-sensitive detection techniques, and the discovery opportunities of this field.