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

F. Adam1,2, F. Ahrens3,*, L. E. Ardila Perez4, M. Balzer4, A. Barth3, D. Behrend-Uriarte3, S. Berndt5,6, K. Blaum7, F. W. H. Böhm3 et al. (ECHo Collaboration)

F. W. H. Böhm3, M. Braß8, L. Calza3, K. Chrysalidis9, M. Door7,†, H. Dorrer5, Ch. E. Düllmann5,10,11, K. Eberhardt5,10, S. Eliseev7, C. Enss3,4, P. Filianin7, A. Fleischmann3, R. Gartmann4, L. Gastaldo3,‡, M. Griedel3,§, A. Göggelmann12, R. Hammann3,∥, R. Hasse6, M. W. Haverkort8, S. Heinze8, D. Hengstler3, R. Jeske3, J. Jochum12, K. Johnston9, N. Karcher4, S. Kempf2,4, T. Kieck11,10, U. Köster13, N. Kovac3,¶, N. Kneip6, K. Kromer7, F. Mantegazzini3,*, B. A. Marsh9,†, M. Merstorf8, T. Muscheid4, M. Neidig2, Y. N. Novikov7, R. Pandey3, A. Reifenberger3, D. Richter3, A. Rischka7, S. Rothe9, O. Sander4, R. X. Schüssler7, S. Scholl12, Ch. Schweiger7, C. Velte3, M. Weber14, M. Wegner4,2, K. Wendt6, and T. Wickenhäuser3 (ECHo Collaboration)

  • *Present address: Fondazione Bruno Kessler (FBK), I-38123 Trento, Italy and INFN- TIFPA, I-38123 Trento, Italy.
  • †Present address: Laboratoire Kastler Brossel, Sorbonne University, CNRS, ENS-PSL University, Collège de France, Paris, France.
  • ‡Contact author: Gastaldo@kip.uni-heidelberg.de
  • §Present address: Institute for Quantum Material and Technologies, Karlsruhe Institute of Technology, Germany.
  • ∥Present address: Max Planck Institute for Nuclear Physics, Heidelberg, Germany.
  • Present address: Institute for Astroparticle Physics, Karlsruhe Institute of Technology, Germany.
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Phys. Rev. Lett. 136, 121801 – Published 25 March, 2026

DOI: https://doi.org/10.1103/lqkb-hylx

Abstract

The effective electron neutrino mass can be determined by analyzing the end-point region of the Ho163 electron capture spectrum, provided a measurement with high-energy resolution and high statistics using calorimetric techniques. Here, the Electron Capture in Ho163 Collaboration (ECHo) presents an analysis of the most precise Ho163 spectrum currently available, obtained with the ECHo-1k experiment and comprising about 200 million events. A very low background rate of bconst=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 mνe<15  eV/c2 (90% credible interval).

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Phys. Rev. Lett. 136, 121801– Published 25 March, 2026
  • Received 3 September 2025
  • Revised 15 December 2025
  • Accepted 4 February 2026
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