Lawrence Berkeley National Laboratory

In this work we present a keV-scale sterile-neutrino search with a low-tritium-activity data set of the KATRIN experiment, acquired in a commissioning run in 2018. KATRIN performs a spectroscopic measurement of the tritium β$$\upbeta $$-decay spectrum with the main goal of directly determining the effective electron anti-neutrino mass. During this commissioning phase a lower tritium activity facilitated the measurement of a wider part of the tritium spectrum and thus the search for sterile neutrinos with a mass of up to 1.6keV$$1.6\, \textrm{keV}$$. We do not find a signal and set an exclusion limit on the sterile-to-active mixing amplitude of sin2θ<5×10-4$$\sin ^2\theta < 5\times 10^{-4}$$ (95%$$95\%$$ C.L.) at a mass of 0.3 keV. This result improves current laboratory-based bounds in the sterile-neutrino mass range between 0.1 and 1.0 keV.