Direct neutrino-mass measurement with sub-electronvolt sensitivity

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 $m_v$ of ${\text{0.7eV}c^{-2}}$ at a 90% confidence level (CL). The best fit to the spectral data yields $m\frac{2}{v}=\text{(0.26 ± 0.34)eV}^2c^{-4}$, resulting in an upper limit of $m_v{\text{