News of the WeekParticle Physics

A Second Hint of Symmetry Violation

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Science  18 Dec 1998:
Vol. 282, Issue 5397, pp. 2169-2171
DOI: 10.1126/science.282.5397.2169

CHICAGO– The first glimpse of stocking may once have been seen as shocking, as the classic musical has it, but the second one was eagerly anticipated. For more than 30 years, that's been true in physics, ever since experimenters studying the decay of particles called kaons in 1964 were shocked by a violation of their beloved laws of nature. Since then, they have realized that the effect, a basic asymmetry in physics called CP violation, might explain why the universe contains more matter than antimatter, and they have been eager to get a second look at it in other particles. Now another case of CP violation may finally be showing an ankle, in particles called B mesons.

The evidence is tantalizing but not conclusive, says Al Goshaw of Duke University and co-spokesperson of the Collider Detector at Fermilab (CDF) collaboration, whose paper on the work is soon to appear in Physical Review Letters. “If it's there, we'll see it” with more data, he adds. And plenty of data are on the way at Fermilab's Tevatron accelerator, when its Main Injector, which will increase the luminosity of the colliding beams of particles, fires up in 2000, producing hundreds of times more B mesons. The results should also be encouraging for physicists at the Stanford Linear Accelerator's so-called B factory, a particle accelerator specifically designed to study CP violation in B mesons, which could begin taking data by April next year.

The asymmetry that these physicists are so eagerly pursuing is a subtle difference in the behavior of particles when their charges are reversed and space is reflected about the three axes. Because that transformation turns matter into antimatter, any difference in behavior, such as reaction rates, under a CP transformation could help explain why there is much more matter than antimatter in the universe. Since the effect was spotted in kaons, it had not turned up in any other particle, and physicists have wondered whether CP violation is a general principle of nature or is somehow restricted to a single system.

Its expected signature in the CDF experiments was a slight difference in the rate at which B mesons and anti-B mesons decay to a particular set of particles. In an analysis led by a group at the Massachusetts Institute of Technology, the collaboration studied the decays of about 200 B and anti-B mesons created in the debris of proton-antiproton collisions and saw a lower decay frequency for B mesons—but with a weak statistical confidence. “We're not claiming a detection,” says Barry Wicklund, a CDF collaborator at Argonne National Laboratory—just a hint of indecorous behavior to come.

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