Limit on ν ¯e mass from observation of the β decay of molecular tritium

We report the most sensitive direct upper limit set on the mass m_ν of the electron antineutrino. Our measurements of the shape of the {β} decay spectrum of free molecular tritium yield, under the assumption of no new physics other than that of mass, a central value for m_ν² of -147±{}68±{}41 eV², which corresponds to an upper limit of 9.3 eV (95% confidence level) on m_ν. The result is in clear disagreement with a reported value of 26(5) eV.