The precise ${}_{}{}^3{}_{}{}^{}\mathrm{H}$-${}_{}{}^3{}_{}{}^{}\mathrm{He}$ atomic mass difference has been measured by high-resolution (${10}^{-8\mathrm{}}$) ion cyclotron resonance in a 4.7-T magnetic field. The result of 18 599 ± 2 eV favors a nonzero electron antineutrino mass.

• Received 30 July 1984

DOI:https://doi.org/10.1103/PhysRevLett.54.285