A CP-symmetry transformation swaps a particle with the mirror image of its antiparticle. The LHCb collaboration has observed a breakdown of this symmetry in the decays of the D0 meson (illustrated by the big sphere on the right) and its antimatter counterpart, the anti-D0 (big sphere on the left), into other particles (smaller spheres). The extent of the breakdown was deduced from the difference in the number of decays in each case (vertical bars, for illustration only) (Image: CERN)
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