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)]. 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)], reaching 𝑚$_{𝜈e}$ <15  eV/𝑐$^2$ (90% credible interval).