For 10 years the PEPII electron–positron collider at SLAC has spent most of its time making B mesons in pairs so that particle physicists could study subtle correlations in the decays of those very heavy mesons. They were searching primarily for new physics beyond the field’s standard model (see Physics Today, Physics Today 0031-9228 54

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https://doi.org/10.1063/1.1381091
 May 2001, page 17 ). But early this year, faced with PEPII’s imminent final shutdown, the BaBar collaboration, which operated the so-called B-factory’s detector, decided it would be useful to run the collider in its last few months at several collision energies just below the threshold for making B pairs.

The idea was to take care of some important unfinished business that had eluded PEPII, its competitors, and its predecessors in the three decades since the discovery of the b (for “bottom”) quark, which is five times heavier than the...

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