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

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