Quark mixing, CKM unitarity: The unitarity problem
H Abele, E Barberio, D Dubbers, F Glück, JC Hardy, WJ Marciano, A Serebrov, N Severijns
European Physical Journal C | Published : 2004
In the Standard Model of elementary particles, quark-mixing is expressed in terms of a 3 x 3 unitary matrix V, the so called Cabibbo-Kobayashi-Maskawa (CKM) matrix. Significant unitarity checks are so far possible for the first row of this matrix. This article reviews the experimental and theoretical information on these matrix elements. On the experimental side, we find a 2.2 σ to 2.7 σ deviation from unitarity, which conflicts with the Standard Model.