The most precise and convincing confirmation of the conservation of the vector current (CVC) comes from measurements of superallowed nuclear decay. It also provides the most demanding test available of the unitarity of the Cabibbo-Kobayashi-Maskawa (CKM) matrix, a basic pillar of the Electroweak Standard Model. Current experiments focus on tests of the small correction terms that must be applied to the data, with the goal of improving the precision of these tests. A recent result raises unexpected questions.
