First direct neutrino-mass measurement with sub-eV sensitivity

Abstract

We report the results of the second measurement campaign of the Karlsruhe Tritium Neutrino (KATRIN) experiment. KATRIN probes the effective electron anti-neutrino mass, $m_{\hat{I}}½$, via a high-precision measurement of the tritium ${\hat{I}}^2$-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 $m_{\hat{I}}½$ of $0.7eV/{\mathrm{c}}^2$ 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 $m_{\hat{I}}{½}^2=(0.26\pm 0.34)\mathrm{e}{\mathrm{V}}^4/{\mathrm{c}}^4$, resulting in an upper limit of $m_{\hat{I}}½<0.9eV/{\mathrm{c}}^2$ ($90\%$ CL). By combining this result with the first neutrino mass campaign, we find an upper limit of $m_{\hat{I}}½<0.8eV/{\mathrm{c}}^2$ ($90\%$ CL).

19 pages, 10 figures, 5 tables