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

A high-precision measurement of the 131Cs→131Xe ground-to-ground-state electron-capture QEC-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 ⋅ 107 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 νe mass. The new measurement improves the uncertainty on the ground-to-ground-state QEC-value by a factor 25 precluding the 131Cs→131Xe pair as a feasible candidate for the direct determination of the νe mass.