"Meson-antimeson mixing as a probe of new physics"
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Mesons are bound states of a quark and an antiquark. The four neutral
  mesons B_d, B_s, D, and K have the unique property that they mix with
  their antiparticles. This means that a meson-antimeson system
  forms a quantum-mechanical two-state system, exhibiting the familiar
  oscillations between the two states. These neutral meson sytems are
  gold mines for the study of the CP symmetry, which links the
  interactions of particles to those of antiparticles. Since 1964 it is
  known that CP is violated and today CP asymmetries are
  experimentally studied with high precision. These investigations might
  reveal new laws of nature well ahead of collider experiments which aim
  at the production of new particles in high-energy collisions.