The KATRIN experiment also looks for sterile neutrinos

Some oscillation experiments suggest that in addition to the three mass states of neutrinos, of which the active neutrinos ν_e, ν_µ and ν_τ are composed, there could be a fourth mass state m₄.

The corresponding so-called sterile neutrino would not feel the weak interaction of the Standard Model of Particle Physics, but it would manifest itself not only in oscillation experiments, but also by changing the shape of the continuous beta spectrum. From the characteristic kink in this spectrum, it would be possible to determine both the energy of the state m₄ and its admixture to the active neutrino states. However, previous beta spectroscopic measurements did not show any sterile neutrinos.

The international experiment KATRIN, in which scientists from the Institute of Nuclear Physics participate, is focused primarily on the mass of the electron neutrino. The first one-month KATRIN run of the beta tritium spectrum showed an improved upper limit of 1.1 eV for this fundamental quantity [Phys. Rev. Lett. 123, 221802 (2019)]. Recent analysis of the same spectrum has found no sterile neutrinos with energies up to 30 eV whose admixture to active neutrinos would exceed 2% [Phys. Rev. Lett. 126, 091803 (2021)]. The sensitivity of KATRIN to both active and sterile neutrinos will further increase with increasing exposure.