Rare ${}^{40}$K 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 non-zero ratio of intensities of ground-state electron-captures ($I_{{\text{EC}}^0}$) over excited-state ones ($I_{\text{EC*}}$) of $I_{{\text{EC}}^0}/I_{\text{EC*}}=0.0095\stackrel{\text{stat}}{\pm }0.0022\stackrel{\text{sys}}{\pm }0.0010$ (68% C.L.), with the null hypothesis rejected at 4$\ddot{I}ƒ$. In terms of branching ratio, this signal yields $I_{{\text{EC}}^0}=0.098\%\stackrel{\text{stat}}{\pm }0.023\%\stackrel{\text{sys}}{\pm }0.010\%$, roughly half of the commonly used prediction, with consequences for various fields [L. Hariasz et al., companion paper, DOI: 10.1103/PhysRevC.108.014327].

This is a companion paper to Hariasz et al (KDK collaboration) "Evidence for ground-state electron capture of ${}^{40}$K" [arXiv:2211.10343; DOI: 10.1103/PhysRevC.108.014327]. As such, both texts share some figures and portions of text. This version includes the journal DOI