Himal Acharya, M. Aker, D. Batzler, A. Beglarian, J. Beisenkötter, M. Biassoni, B. Bieringer, Y. Biondi, M. Böttcher, B. Bornschein, L. Bornschein, Marco Carminati, Auttakit Chatrabhuti, S. Chilingaryan, D. Díaz Barrero, Bruno Daniel, M. Descher, O. Dragoun, G. Drexlin, F. Edzards, K. Eitel, Enrico Ellinger, R. Engel, S. Enomoto, L. Fallböhmer, A. Felden, Caroline Fengler, C. Fiorini, J. A. Formaggio, Christian Forstner, F. M. Fränkle, Giulio Gagliardi, K. Gauda, Anna Gavin, W. Gil, F. Glück, Robin Größle, Thomas Höhn, Kishan Habib, V. Hannen, L. Haßelmann, K. Helbing, Heino Henke, S. Heyns, R. Hiller, D. Hillesheimer, D. Hinz, A. Jansen, Christoph Köhler, Khanchai Khosonthongkee, J. Kohpeiß, L. Köllenberger, A. Kopmann, Neven Kovač, Luisa La Cascio, L. Laschinger, Thierry Lasserre, J. Lauer, Thanh-Long Le, O. Lebeda, B. Lehnert, A. Lokhov, M. Machatschek, A. Marsteller, E. L. Martín, K. McMichael, C. Melzer, L. E. Mettler, S. Mertens, Shailaja Mohanty, J. Mostafa, Imke Müller, A. Nava, Holger Neumann, S. Niemes, I. Nutini, A. Onillón, D. S. Parno, M. Pavan, Udomsilp Pinsook, J. Plößner, A.W.P. Poon, J. M. L. Poyato, Florian Priester, Jan Ráliš, M. Röllig, S. Ramachandran, R. G. H. Robertson, C. Rodenbeck, Rudolf Sack, Alejandro Sáenz, R. Salomon, J. Schürmann, Peter Schäfer, Ann-Kathrin Schütz, Magnus Schlösser, L. Schlüter, S. Schneidewind, U. Schnurr, Alessandro Schwemmer
Abstract Neutrinos are the most abundant fundamental matter particles in the Universe and play a crucial part in particle physics and cosmology. Neutrino oscillation, discovered about 25 years ago, shows that the three known species mix with each other. Anomalous results from reactor and radioactive-source experiments 1 suggest a possible fourth neutrino state, the sterile neutrino, which does not interact through the weak force. The Karlsruhe Tritium Neutrino (KATRIN) experiment 2 , primarily designed to measure the neutrino mass using tritium β-decay, also searches for sterile neutrinos suggested by these anomalies. A sterile-neutrino signal would appear as a distortion in the β-decay energy spectrum, characterized by a discontinuity in curvature (kink) related to the sterile-neutrino mass. This signature, which depends only on the shape of the spectrum rather than its absolute normalization, offers a robust, complementary approach to reactor experiments. Here we report the analysis of the energy spectrum of 36 million tritium β-decay electrons recorded in 259 measurement days within the last 40 eV below the endpoint. The results exclude a substantial part of the parameter space suggested by the gallium anomaly and challenge the Neutrino-4 claim. Together with other neutrino-disappearance experiments, KATRIN probes sterile-to-active mass splittings from a fraction of an eV 2 to several hundred eV 2 , excluding light sterile neutrinos with mixing angles above a few per cent.