First direct neutrino-mass measurement with sub-eV sensitivity

Abstract

<title>Abstract</title> We report the results of the second measurement campaign of the Karlsruhe Tritium Neutrino (KATRIN) experiment. KATRIN probes the effective electron anti-neutrino mass, m_ν, via a high-precision measurement of the tritium β-decay spectrum close to its endpoint at 18.6 keV. In the second physics run presented here, the source activity was increased by a factor of 3.8 and the background was reduced by 25% with respect to the first campaign. A sensitivity on m_ν of 0.7 eV/c² at 90% confidence level (CL) was reached. This is the first sub-eV sensitivity from a direct neutrino-mass experiment. The best fit to the spectral data yields m_ν²=(0.26±0.34) eV²/c⁴, resulting in an upper limit of m_ν&lt;0.9 eV/c² (90% CL). By combining this result with the first neutrino mass campaign, we find an upper limit of m_ν&lt;0.8 eV/c² (90% CL).