Status and prospects of direct neutrino mass determination

The mass scale of neutrinos remains one of the fundamental open questions in modern physics, with far-reaching implications from particle physics to cosmology. Precision measurements of the kinematics of weak interactions, in particular tritium beta decay and electron capture in ¹⁶³Ho, offer the only model-independent (direct) approach to address this question in a laboratory experiment.
The currently most mature technique relies on the spectroscopy of tritium beta decay near its kinematic endpoint at 18.6 keV. The KArlsruhe TRItium Neutrino experiment (KATRIN) is targeted at improving the sensitivity of this method by an order of magnitude to 200 meV/c² (90% C.L.). To this end, KATRIN utilises an ultra-luminous windowless gaseous tritium source and a high-resolution electrostatic spectrometer. In its recent „First Light“ campaign, KATRIN has successfully inaugurated its full 70-m electron beam line and is now carrying out final commissioning works leading up to the start of tritium measurements.
At the same time, novel approaches studying the electron capture spectrum in ¹⁶³Ho with cryogenic microcalorimeters (notably, ECHo, HOLMES, and NuMECS), or developing innovative techniques for tritium beta spectroscopy using radio-frequency detection of single electrons (Project 8), are gaining momentum. Experiments exploiting these new techniques are currently in the conceptual design and prototype characterisation phases, with the aim of developing these complementary methods further towards a sub-eV sensitivity on the neutrino mass.