Bfactories

Bfactories, or B factories, are high-luminosity electron-positron colliders built to produce large numbers of B mesons for the study of CP violation and the quark-mixing (CKM) phenomenon. They operate at or near the Υ(4S) resonance, which decays almost exclusively into B meson pairs, providing clean, well-understood initial states.

To enable time-dependent CP measurements, the colliders use asymmetric beam energies, so the produced B mesons are boosted and their decay time differences can be reconstructed. Large, specialized detectors surround the interaction points to identify decay products and measure particle flavors and decay times with high precision.

The first B factories were BaBar at SLAC (USA) and Belle at KEK (Japan), which began data-taking in the late 1990s and yielded decisive measurements of CP violation in the B system. A successor era is led by Belle II at SuperKEKB, an upgrade designed to reach much higher luminosity and accumulate very large B-meson samples.

B factories have made significant contributions to tests of the Standard Model through measurements of the CKM matrix angles and sides, such as sin2beta, and by examining rare decays and CP asymmetries. Their clean environment complements hadron colliders by enabling precision flavor studies with lower backgrounds, though hadron machines such as LHCb also probe B physics in different regimes.

See also: CP violation, CKM matrix, Υ(4S), BaBar, Belle, Belle II, SuperKEKB, LHCb.