Improved Limit on the Effective Electron Neutrino Mass with the ECHo-1k Experiment

The effective electron neutrino mass can be determined by analyzing the end-point region of the Ho 163 electron capture spectrum, provided a measurement with high-energy resolution and high statistics using calorimetric techniques. Here, the Electron Capture in Ho 163 Collaboration (ECHo) presents an analysis of the most precise Ho 163 spectrum currently available, obtained with the ECHo-1k experiment and comprising about 200 million events. A very low background rate of b const = 9.1 ( 1.3 ) × 10 − 6 eV/pixel/day was achieved allowing for a reliable analysis of the end-point region. The derived end-point 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 [Ch. Schweiger , ]. The upper limit of the effective electron neutrino mass is improved by almost a factor of 2 compared to the lowest current value [B. K. Alpert , ], reaching m ν e < 15 eV / c 2 (90% credible interval).