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 ^{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_{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 et al., Nat. Phys. 226, 921 (2024)NPAHAX1745-247310.1038/s41567-024-02461-9]. 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 et al., Phys. Rev. Lett. 135, 141801 (2025)PRLTAO0031-900710.1103/s9vl-7n24], reaching m_{ν_{e}}