First direct neutrino-mass measurement with sub-eV sensitivity

We report the results of the second measurement campaign of the Karlsruhe Tritium Neutrino (KATRIN) experiment. KATRIN probes the effective electron anti-neutrino mass, \(m_{\nu}\), via a high-precision measurement of the tritium \(\beta\)-decay spectrum close to its endpoint at \(18.6\,\mathrm{keV}\). 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 \(m_{\nu}\) of \(0.7\,\mathrm{eV/c^2}\) 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 \(m_{\nu}^2 = (0.26\pm0.34)\,\mathrm{eV^4/c^4}\), resulting in an upper limit of \(m_{\nu}