Superallowed Fermi decays yield the most precise value of Vud, the up-down quark mixing matrix element of the Cabbibo-Kobayashi-Maskawa (CKM) matrix. This rotation matrix between the weak eigenstates and the mass eigenstates of the quark is unitary within the 3-generation Standard Model and the most accurate test of its unitarity is coming from its top row. Critical to this test are the values of Vud and Vus, the accepted value of the latter having changed significantly over the last few years to improve the agreement with the unitarity condition.
New capabilities in high-precision mass measurements on short-lived isotopes available with the CPT spectrometer at Argonne have allowed the Q-value of a number of these superallowed decays to be measured to high accuracy and yield significant discrepancies with the previously accepted Q value for these decays determined by nuclear reactions. All new measured Q values are higher than previously determined, increasing the ft value, and reducing the extracted Vud value. The status and consistency of these new measurements, together with the resulting changes in the value of Vud and the unitarity condition, will be reviewed. The conclusions that can be drawn at this point about the nuclear structure dependent corrections that are present in this determination of Vud will also be given. Additional measurements on the other superallowed emitters in the data set are in preparation and the expected near-term evolution of the accuracy of this CKM unitarity test will be presented.
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