Eur. Phys. J. C (2004) 33: 1-8
https://doi.org/10.1140/epjc/s2003-01574-8

experimental physics

Quark mixing, CKM unitarity

The unitarity problem

¹ Physikalisches Institut der Universität Heidelberg, Philosophenweg 12, 69120, Heidelberg, Germany
² Physics Department, Southern Methodist University, Dallas, TX, USA
³ Research Institute for Nuclear and Particle Physics, POB 49, 1525, Budapest, Hungary
⁴ Cyclotron Institute, Texas A&M University, College Station, TX 77843, USA
⁵ Brookhaven National Laboratory Upton, 11973, New York, USA
⁶ Petersburg Nuclear Physics Institute, 188350, Gatchina, Leningrad District, Russia
⁷ Instituut voor Kern- en Stralingsfysica, Katholieke Universiteit Leuven, Celestijnenlaan 200 D, 3001, Leuven, Belgium

Abstract

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 to 2.7 deviation from unitarity, which conflicts with the Standard Model.