Symmetry violation in a new setting

25 February 1999

The CDF experiment at Fermilab’s Tevatron proton­antiproton collider has produced heroic new evidence for the violation of “CP” symmetry, a hypothetical operation which takes a particle into a mirror image of its antiparticle.

Although a tiny effect in particle physics, CP violation could be the cause of the matter­antimatter asymmetry of the universe ­a Big Bang which presumably produced equal amounts of matter and antimatter resulted in a universe populated entirely by matter.

So far, the only quantitative evidence for this subtle violation has been in the decays of neutral kaons, at a few per mil. CDF now sees tentative evidence for CP violation in the decays of neutral B mesons, in which the strange quark of the neutral kaon is replaced by a heavy b-quark. CDF looks at B decays producing a J/psi and a short lived neutral kaon.

The vital parameter is measured to be 0.79±0.44, a non-zero value indicating CP violation. While CP violation is not completely understood and therefore cannot be predicted from scratch, the measured value is in line with expectations based on the interconsistency of many Standard Model measurements. Several months ago, CDF published an analysis based on a subset of its B decay sample. The new result uses its full available sample.

With a new generation of electron­positron colliders hoping to open up this area of B physics, we plan to publish more on these initial B­CP violation pointers soon.

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