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Three double-ring B factories—PEP-II, KEKB, and SuperKEKB—were built with asymmetric beam energies to produce high luminosity at the Υ(4S) resonance producing copious B mesons and associated particle physics data, delivered to their respective particle physics detectors, BaBar, Belle, and Belle II. In PEP-II and KEKB, the primary goal was achieved to make the first measurements of charge parity (CP) violation in decays in the B meson system. PEP-II and KEKB have finished beam operations, but particle data analysis continues. SuperKEKB is extending those measurements. The beam operation of SuperKEKB with Belle II has recently started. These three accelerators have pushed the state of the art of accelerators of two-ring colliders, including small beam emittances, small interaction point beam sizes, large stored beam currents, crab cavities, top-up injection with the detectors on, bunch-by-bunch feedbacks, crab waist operation, high beam–beam parameters, high peak luminosity, and high integrated luminosity.
J.T. Seeman (Thu,) studied this question.