Improved limit on the effective electron neutrino mass with the ECHo-1k experiment

The effective electron neutrino mass can be determined by analyzing the endpoint region of the ${}^{163}$Ho electron capture spectrum, provided a measurement with high energy resolution and high statistics using calorimetric techniques. Here, the Electron Capture in ${}^{163}$Ho collaboration, ECHo, presents an analysis of the most precise ${}^{163}$Ho spectrum currently available, obtained with the ECHo-1k experiment and comprising about 200 million events. A very low background rate of $B=9.1(1.3)\times {10}^{-6}$ /eV/pixel/day was achieved allowing for a reliable analysis of the endpoint region. The derived endpoint energy $Q=2862(4)$ eV is in excellent agreement with the one independently determined via Penning-trap mass spectrometry of $Q=2863.2(6)$ eV [1]. The upper limit of the effective electron neutrino mass is improved by almost a factor 2 compared to the lowest current value [2], reaching $m_{{\nu }_{\mathrm{e}}}<15$ eV/c${}^2$ (90% credible interval).