Neutrino oscillation experiments showed that neutrinos have mass, but the values of the neutrino masses are still unknown. Limits on masses can be set by double beta decay experiments and cosmological observations, at around 100 meV to eV level, but these involve assumptions and are therefore model dependent. As massive neutrinos affect the phase space of beta decays, precision measurement of beta decay energy spectrum shapes, especially near the end-point, is a way to directly obtain information on the neutrino masses.
The KArlsruhe TRItium Neutrino (KATRIN) experiment, located in Karlsruhe, Germany, aims to determine the neutrino mass by measuring the spectrum shape of tritium beta decays, with using 100 GBq gaseous tritium and tens-ppm level resolution spectrometer. Construction of the apparatus started in 2004, and operation with tritium started in 2018. Only with a few weeks of data taking, KATRIN updated the limit on neutrino mass from the previous value of 2.0 eV, set by the Mainz and Troitsk experiments, to 1.1 eV. With a number of improvements and added measurement time after the first result, KATRIN recently achieved a sub-eV sensitivity, and set a limit at 0.8 eV.
In this talk, I will start with a review of techniques of direct neutrino mass measurements, followed by details on the KATRIN experiment including the apparatus, operation, and systematics. I will then show the obtained data, analysis methods, and the results. I will conclude the talk with future prospects of KATRIN and other direct measurement experiments.