Beta decay and neutrino mass: KATRIN and beyond

The precise measurement of neutrino masses represents a critical frontier in particle physics, with implications that extend beyond the Standard Model and into cosmology. While cosmological observations and neutrinoless double beta decay experiments provide stringent constraints on neutrino properties, direct neutrino mass measurements are uniquely model-independent and critical for cross-validating of results. The Karlsruhe Tritium Neutrino (KATRIN) experiment is a leading initiative in this domain, employing beta-decay spectroscopy to measure the incoherent sum of neutrino masses ($m_\beta$) with unprecedented sensitivity. KATRIN has progressively improved the upper limit on neutrino mass, achieving $m_\beta < 0.45$ eV at 90 % confidence level with the combined analysis of its first five campaigns. With ongoing data acquisition and improved methodologies, KATRIN aims to reach a final sensitivity of $m_\beta < 0.3$ eV by 2025. Looking ahead, KATRIN is expanding its scope to include searches for sterile neutrinos at the keV scale with the novel TRISTAN-detector. Furthermore, it is actively exploring next-generation technologies, such as atomic tritium sources and differential detection methods, to push sensitivities below the inverted ordering range ($m_\beta < 0.05$ eV). KATRIN++ is envisioned to be a next-generation $m_\beta$ experiment and follows the mission to identify and develop scalable technology by using the existing KATRIN and TLK infrastructures.