CP violation in B meson decays

The Universe exhibits a remarkable asymmetry without which we could not exist: it contains large quantities of matter but almost no antimatter. Yet in the interactions of elementary particles at high energy as much antimatter as matter is produced. While it is clear that this asymmetry occurred in the very early evolution of the Universe the mechanism is not understood. However one key element is the phenomenon of CP violation, which results in a small difference in the way in which particles and their antiparticles decay. CP violation was discovered in 1963 in the decays of K mesons and has been investigated in detail since then in these decays. During the 1990s new projects were developed to extend these measurements to the decay of B mesons. Two major projects, Belle at KEK in Japan and BABAR at SLAC in the USA, began operation in 1999 and reported first results in 2000. This paper reviews the phenomenon of CP violation in the decays of elementary particles and demonstrates the basis for the measurements of CP violation parameters in B meson decays. A description is given of the BABAR detector together with an outline of the procedure for analysing the data. First results are consistent with predictions of the Standard Model of elementary particles but during the coming years additional data will provide more stringent tests that could reveal new phenomena.