Rare <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mmultiscripts><mml:mrow><mml:mi mathvariant="normal">K</mml:mi></mml:mrow><mml:mprescripts/><mml:none/><mml:mrow><mml:mn>40</mml:mn></mml:mrow></mml:mmultiscripts></mml:mrow></mml:math> Decay with Implications for Fundamental Physics and Geochronology
M. Stukel, L. Hariasz, P. C. F. Di Stefano, B. C. Rasco, Konrad Rykaczewski, N. T. Brewer, D. W. Stracener, Y. Liu, Zheng Gai, Christopher M. Rouleau, J. Carter, Joel Kostensalo, J. Suhonen, H. L. Davis, E. D. Lukosi, K. C. Goetz, R. Grzywacz, M. Mancuso, F. Petricca, A. Fijałkowska, M. Wolińska-Cichocka, J. Ninković, P. Lechner, Ryan B. Ickert, L. E. Morgan, P. R. Renne, Itay Yavin
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±[over stat]0.0022±[over sys]0.0010 (68% C.L.), with the null hypothesis rejected at 4σ. In terms of branching ratio, this signal yields I_{EC^{0}}=0.098%±[over stat]0.023%±[over sys]0.010%, roughly half of the commonly used prediction, with consequences for various fields [27L. Hariasz et al., companion paper, Phys. Rev. C 108, 014327 (2023)PRVCAN2469-998510.1103/PhysRevC.108.014327].