Penning-trap mass measurement of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mmultiscripts><mml:mi>He</mml:mi><mml:mprescripts/><mml:none/><mml:mn>3</mml:mn></mml:mmultiscripts></mml:math>

We have determined the atomic mass of <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mmultiscripts><a:mi mathvariant="normal">He</a:mi><a:mprescripts/><a:none/><a:mn>3</a:mn></a:mmultiscripts></a:math> to 12 parts per trillion relative precision from a measurement of the cyclotron frequency ratio of <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"><c:mmultiscripts><c:mi mathvariant="normal">He</c:mi><c:none/><c:mo>+</c:mo><c:mprescripts/><c:none/><c:mn>3</c:mn></c:mmultiscripts></c:math> and <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"><e:mmultiscripts><e:mi mathvariant="normal">C</e:mi><e:none/><e:mrow><e:mn>4</e:mn><e:mo>+</e:mo></e:mrow><e:mprescripts/><e:none/><e:mn>12</e:mn></e:mmultiscripts></e:math> in a Penning trap. From the obtained mass of the <g:math xmlns:g="http://www.w3.org/1998/Math/MathML"><g:mmultiscripts><g:mi mathvariant="normal">He</g:mi><g:none/><g:mo>+</g:mo><g:mprescripts/><g:none/><g:mn>3</g:mn></g:mmultiscripts></g:math> ion, the mass of the neutral atom can be calculated without loss of precision: <i:math xmlns:i="http://www.w3.org/1998/Math/MathML"><i:mrow><i:mi>m</i:mi><i:mo>(</i:mo><i:mmultiscripts><i:mi mathvariant="normal">He</i:mi><i:mprescripts/><i:none/><i:mn>3</i:mn></i:mmultiscripts><i:mo>)</i:mo><i:mo>=</i:mo><i:mn>3.016</i:mn><i:mspace width="0.16em"/><i:mn>029</i:mn><i:mspace width="0.16em"/><i:mn>322</i:mn><i:mspace width="0.16em"/><i:mn>011</i:mn><i:mo>(</i:mo><i:mn>35</i:mn><i:mo>)</i:mo></i:mrow></i:math> u. This result provides independent, high-precision data for the last missing link between the masses in the regime of light ions, thereby resolving discrepancies in the reported masses of light atomic nuclei from various experiments.