2023

Stukel, M.; Hariasz, L.; Di Stefano, P. C. F.; Rasco, B. C.; Rykaczewski, K. P.; Brewer, N. T.; Stracener, D. W.; Liu, Y.; Gai, Z.; Rouleau, C.; Carter, J.; Kostensalo, Joel; Suhonen, J.; Davis, H.; Lukosi, E. D.; Goetz, K. C.; Grzywacz, R. K.; Mancuso, M.; Petricca, F.; Fijałkowska, A.; Wolińska-Cichocka, M.; Ninkovic, J.; Lechner, P.; Ickert, R. B.; Morgan, L. E.; Renne, P. R.; Yavin, I.

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 (IEC0) over excited-state ones (IEC∗) of IEC0/IEC∗=0.0095stat±0.0022sys±0.0010 (68% C.L.), with the null hypothesis rejected at 4σ. In terms of branching ratio, this signal yields IEC0=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)].