  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$\sigma$. 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].
