Italians Set Sights on CP Violation

NAPLES, ITALY--The titans of the particle physics world, the CERN laboratory near Geneva and Fermilab near Chicago, are racing to confirm a slight difference in the behavior of matter and antimatter, also known as "CP violation." But in Frascati, south of Rome, a more modest outfit at the National Institute for Nuclear Physics (INFN) hopes to rob them of that prize. Last week, this upstart machine, called KLOE, recorded its first real data.

The hunt for a matter-antimatter imbalance has been on since 1964, when physicists performed collision experiments to study the neutral kaon--a short-lived particle that is sometimes matter, sometimes antimatter. They found that for a small fraction of neutral kaons the "mixing" between particle and antiparticle followed a different path, resulting in different decay products. This suggested a breakdown of so-called "charge-parity symmetry" and became known as "indirect" CP violation because the CP violation takes place in the "mixing" and not in the decay itself.

In the late 1980s, researchers at CERN detected the first hints of "direct" CP violation, in which some kaons and their antiparticles decayed in different ways. Those hints were strengthened earlier this year in observations made by the KTeV group at Fermilab (ScienceNOW, 4 March 1999). But the Frascati group is hoping to get a clearer picture of the effect by producing kaons in lower energy collisions, yielding decays that are "cleaner" and easier to analyze.

While the Fermilab and CERN groups produce kaons by colliding protons with a fixed target, the researchers at the INFN use their new electron-positron collider, DAFNE, to speed electrons and their antiparticles, positrons, to an energy of 510 million electron-volts in two 100-meter-long rings and collide them inside the KLOE detector. They annihilate and produce short-lived entities called phi particles, which decay into kaon-antikaon pairs. KLOE locks onto any pairs of neutral kaons, searching for the one-in-a-million decay expected to show direct CP violation.

Paolo Franzini of Rome University, KLOE's spokesperson, says it will take some time to record enough events to get a good fix on CP violation. "For a first measurement, which is of the same accuracy as KTeV, we will need 6 to 9 months of collecting data," he says. To improve on that, "we have to collect at least 500 million events, and so far we have seen five events."