  The masses of the lightest atomic nuclei and the electron mass are
interlinked and are crucial in a wide range of research fields, with their
values affecting observables in atomic, molecular and neutrino physics as well
as metrology. The most precise values for these fundamental parameters come
from Penning-trap mass spectrometry, which achieves relative mass uncertainties
in the range of $10^{-11}$. However, redundancy checks using data from
different experiments reveal significant inconsistencies in the masses of the
proton ($m_p$), the deuteron ($m_d$) and helion ($m_\text{he}$), amounting to
$5$ standard deviations for the term $\Delta=m_p+m_d-m_{\text{he}}$, which
suggests that the uncertainty of these values may have been underestimated.
Here we present results from absolute mass measurements of the deuteron and the
${HD}^+$ molecular ion against $^{12}C$ as a mass reference. Our value for the
deuteron $m_d=2.013\,553\,212\,535 (17)$u supersedes the precision of the
literature value by a factor of $2.4$ and deviates from this by $4.8$ standard
deviations. With a relative uncertainty of $8$ parts per trillion (ppt) this is
the most precise mass value measured directly in atomic mass units.
Furthermore, the measurement of the ${HD}^+$ molecular ion,
$m({HD}^+)=3.021\,378\,241\,561\,(61)$u, not only allows for a rigorous
consistency check of our measurements of the masses of the deuteron (this work)
and proton, but also establishes an additional link for the masses of tritium
and helium-3 to the atomic mass unit. Combined with a recent measurement of the
deuteron-to-proton mass ratio the uncertainty of the reference value of $m_p$
can be reduced by a factor of three. This is a post-peer-review, pre-copyedit
version of an article published in Nature. The final authenticated version is
available online at https://doi.org/10.1038/s41586-020-2628-7
