Probing the Standard Model with Superallowed Nuclear Beta Decay

Central to the Electroweak Standard Model is the Cabibbo-Kobayashi-Maskawa (CKM) matrix, a unitary 3 × 3 matrix that relates the weak- to the mass-eigenstates of the three generations of quarks. The Standard Model itself does not prescribe the values of the individual matrix elements—they must each be determined experimentally—but it does insist that the full matrix must satisfy all conditions of unitarity. Thus, any conclusive experimental demonstration that the unitarity conditions are not met would prove the need for “new physics” extending beyond the current Standard Model. Conversely, if the experiment were instead to confirm unitarity, this result would circumscribe the scope of any possible new physics to an extent limited only by the quoted experimental uncertainties on the unitarity sum. Either way, there is compelling motivation here for precision measurements of the CKM matrix elements aimed at the most demanding tests possible of the matrix's unitarity.