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CP Violation in B Decays: Motivation and History

In order to test the Standard Model ("SM", the current theoretical model which describes the electro-weak and strong interactions), it is necessary to measure matter-antimatter differences in the form of the violation of the Charge and Parity (CP) symmetry. Redundant measurements of CP violation will verify the existence of three families of quarks and will probe possible physics beyond what is predicted by current models.

To this aim, the decays of mesons that have one “bottom” quark and one “down” or “up” quark (called B0 and B+ mesons) are studied. The corresponding anti-particles are called anti-B0 and B. These mesons are of interest because with them you can study at the same time three elements of the quark mixing matrix, VCKM (Vub, Vcb, and Vtd) and they present significant CP violation in some of their decays.

Since 1980, for twenty years, the experiments CLEO, using the accelerator CESR at Cornell(United States) and ARGUS, using the accelerator DORIS-II at DESY (Germany) have produced many B0 anti-B0and B+B pairs and have measured the absolute values of elements of the VCKM matrix. They have also discovered mixing between the mesons B0 and anti-B0 .

On the other side, CP violation in B decays can be observed primarily by measuring the time between the production and the decay of the mesons, which these two experiments could not do. For this reason two new experiments were built: Belle at KEK (Japan) and BaBar at SLAC (United States), using the KEK-B and PEP-II accelerators, respectively. These accelerators have electron and positron beams with different energies. As a result, the produced B mesons will travel for a measurable distance before decaying.

Both experiments started in 1999 and in 2001. They have measured by how much the CP symmetry is violated in a specific, particularly sensitive decay, B0J/YKs.

Since then CP violation has been observed in many other channels and the elements of the VCKM matrix have been determined with high accuracy. This confirmed that only three families of quarks exist, and that the SM accurately describes the electroweak interactions at the level of precision achieved. Nonetheless, there is much still to be learn in this field, both in terms of achieving better accuracy in the determination of the parameters of the SM and in terms of looking for deviations from it due to the existence of new, never before observed, particles.

In addition, the extremely high luminosity of the PEP-II machine provides to the BABAR experiment very large samples of tau leptons, charm, and charmonium mesons and in general many non B anti-B events. These samples are efficiently reconstructed by BABAR, which is highly competitive with the other experiment (Belle) devoted to study of the same physics.

—Riccardo Faccini, BaBar Physics Analysis Coordinator, 2005–2006.
—Minor updates by Abi Soffer, Physics Analysis Coordinator, 2011-2012.

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