# Extended Data Table 1 Systematic uncertainties

1. a, Uncertainties in the half-life calculation are given as percentages of the corresponding variable values. b, Systematic uncertainties incorporated as fit constraints are given in the unit used in the fit. All parameters are shared between the χ2 functions for both volumes, with the exception of the volume and high-energy acceptance multipliers. The volume multipliers are chosen such that the fitted high-energy acceptance ranges between the lower limit derived from the data and unity. The parameter pulls of the fit are given in units of the uncertainty σ. ROI, region of interest; ppt, parts per trillion; $${T}_{1/2}^{{\rm{ECEC}}}$$, 2νECEC half-life.