Rare 40K decay with implications for fundamental physics and geochronology

Abstract

Potassium-40 is a widespread, naturally occurring isotope whose radioactivity impacts subatomic rare-event searches, nuclear structure theory, and estimated geological ages. A predicted electron-capture decay directly to the ground state of argon-40 has never been observed. The KDK (potassium decay) collaboration reports strong evidence of this rare decay mode. A blinded analysis reveals a nonzero ratio of intensities of ground-state electron-captures ( I EC 0 ) over excited-state ones ( I EC ∗ ) of I EC 0 / I EC ∗ = 0.0095 stat ± 0.0022 sys ± 0.0010 (68% C.L.), with the null hypothesis rejected at 4 σ . In terms of branching ratio, this signal yields I EC 0 = 0.098 % stat ± 0.023 % sys ± 0.010 % , roughly half of the commonly used prediction, with consequences for various fields [L. Hariasz et al. , companion paper, Phys. Rev. C 108 , 014327 (2023) ].