Direct decay-energy measurement as a route to the neutrino mass

A high-precision measurement of the$^{131}$Cs→$^{131}$Xe ground-to-ground-state electron-capture Q$_{EC}$-value was performed using the ISOLTRAP mass spectrometer at ISOLDE/CERN. The novel PI-ICR technique allowed to reach a relative mass precision δm/m of 1.4 ⋅ 10$^{− 9}$. A mass resolving power m/Δm exceeding 1 ⋅ 10$^{7}$ was obtained in only 1s trapping time. Allowed electron-capture transitions with sub-keV or lower decay energies are of high interest for the direct determination of the ν$_{e}$ mass. The new measurement improves the uncertainty on the ground-to-ground-state Q$_{EC}$-value by a factor 25 precluding the$^{131}$Cs→$^{131}$Xe pair as a feasible candidate for the direct determination of the ν$_{e}$ mass.