Direct measurements of neutrino mass

The turn of the 21st century witnessed a sudden shift in our fundamental understanding of particle physics. While the minimal Standard Model predicts that neutrino masses are exactly zero, the discovery of neutrino oscillations proved the Standard Model wrong. Neutrino oscillation measurements, however, shed light neither on the scale of neutrino masses, nor on the mechanism by which those are generated. The neutrino mass scale is most directly accessed by studying the energy spectrum generated by beta decay or electron capture - a technique dating back to Enrico Fermi's formulation of radioactive decay. In this Article, we review the methods and techniques-both past and present-aimed at measuring neutrino masses kinematically. We focus on recent experimental developments that have emerged in the past decade, overview the spectral refinements that are essential in the treatment of the most sensitive experiments, and give a simple yet effective protocol for estimating the sensitivity. Finally, we provide an outlook of what future experiments might be able to achieve. @2021 Elsevier B. V. All rights reserved.

Automated summary

The turn of the 21st century saw a sudden shift in our understanding of particle physics with the discovery of neutrino oscillations, which contradicted the predictions of the minimal Standard Model. Techniques like studying the energy spectrum generated by beta decay or electron capture are key in directly accessing the scale of neutrino masses. This article reviews past and present methods for measuring neutrino masses kinematically and discusses the outlook for future experiments.