High-precision atomic mass measurements for a CKM unitarity test

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• We overview the current status of the Q-values of the superallowed beta emitters.
• We highlight some cases which deviated significantly from previous data.
• We give some candidates for future measurements.

The Cabibbo–Kobayashi–Maskawa (CKM) quark-mixing matrix describes the transformation of quarks from weak-force eigenstates to mass eigenstates. The most contributing element in this matrix is the up-down matrix element Vud, derived in most precise way from the nuclear beta decays and in particular, from decays having superallowed 0+ → 0+ decay branch. What high-precision mass spectrometry community can offer are decay energies of such decays derived from parent–daughter mass differences, which are ideally, and in almost all cases, determined with Penning trap mass spectrometry directly from parent–daughter cyclotron frequency ratio. Typically frequency (and thus mass) ratios are determined with 10−9 relative precision, which allows decay energies to be determined within 100 eV-level.