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

## TL;DR

This paper reports a high-precision measurement of the $^{131}$Cs electron-capture decay energy, significantly refining the $Q_{EC}$ value and impacting the feasibility of direct neutrino mass determination.

## Contribution

The study introduces a novel high-precision mass measurement technique that drastically improves the $Q_{EC}$ value accuracy for $^{131}$Cs, affecting neutrino mass research.

## Key findings

- Achieved a relative mass precision of 1.4×10^{-9} using PI-ICR.
- Obtained a mass resolving power exceeding 10^7 in 1 second.
- Reduced the $Q_{EC}$ uncertainty by a factor of 25.

## Abstract

A high-precision measurement of the $^{131}$Cs$ \rightarrow ^{131}$Xe ground-to-ground-state electron-capture $Q_{\textrm{EC}}$-value was performed using the ISOLTRAP mass spectrometer at ISOLDE/CERN. The novel PI-ICR technique allowed to reach a relative mass precision $\delta m/m$ of $1.4\cdot10^{-9}$. A mass resolving power $m/\Delta m$ exceeding $1\cdot10^7$ 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 $Q_{\textrm{EC}}$-value by a factor 25 precluding the $^{131}$Cs$ \rightarrow ^{131}$Xe pair as a feasible candidate for the direct determination of the $\nu_e$ mass.