Is the Unitarity of the quark-mixing-CKM-matrix violated in neutron β-decay?

This article is about the interplay between the Standard Model of elementary particle physics and neutron β-decay. Since the Fermi decay constant is known from muon decay, the Standard Model describes neutron β-decay with only two additional parameters. One parameter is the first entry |Vud| of the CKMmatrix. The other one is λ, the ratio of the vector coupling constant and the axial vector constant. In principle, the ratio λ can be determined from QCD lattice gauge theory calculation, but the results of the best calculations vary by up to 30%. In neutron decay, several observables are accessible to experiment, which depend on these parameters, so the problem is overdetermined and, together with other data from particle and nuclear physics, many tests of the Standard Model become possible. |Vud| results significantly from the neutron lifetime τ and the β-asymmetry parameter A0.