By measuring the cyclotron frequency ratio of ${}^3\mathrm{He}^{+}$ to ${\mathrm{HD}}^{+}$ directly, with confirmation from measurements using ${\mathrm{H}}_3{}^{+}$ as an intermediary, we obtain $M\left[{}^3\mathrm{He}^{+}\right]/M\left[{\mathrm{HD}}^{+}\right]=0.998048085122\left(23\right)$ and hence the mass difference $m_p+m_d-m_h=0.00589743219\left(7\right)$ u. This result disagrees by more than four standard deviations with the value inferred from current precise literature values for the atomic masses of the proton, deuteron, and nucleus of ${}^3\mathrm{He}$.

• Received 21 October 2017

DOI:https://doi.org/10.1103/PhysRevA.96.060501