Direct neutrino-mass measurement with sub-electronvolt sensitivity

Abstract Since the discovery of neutrino oscillations, we know that neutrinos have non-zero mass. However, the absolute neutrino-mass scale remains unknown. Here we report the upper limits on effective electron anti-neutrino mass, m ν, from the second physics run of the Karlsruhe Tritium Neutrino experiment. In this experiment, m ν is probed via a high-precision measurement of the tritium β -decay spectrum close to its endpoint. This method is independent of any cosmological model and does not rely on assumptions whether the neutrino is a Dirac or Majorana particle. By increasing the source activity and reducing the background with respect to the first physics campaign, we reached a sensitivity on m ν of 0. 7 eV c –2 at a 90% confidence level (CL). The best fit to the spectral data yields {}m ^2{} m ν 2 = (0. 26 ± 0. 34) eV 2 c –4, resulting in an upper limit of m ν < 0. 9 eV c –2 at 90% CL. By combining this result with the first neutrino-mass campaign, we find an upper limit of m ν < 0. 8 eV c –2 at 90% CL.