New Matter-Antimatter Asymmetry Established

by Judy Jackson

Nature, ever chary of her secrets, is seldom more reticent than on the subject of antimatter. We creatures of matter inhabit a world apparently fashioned only of matter, yet we do not know why. Using particle accelerators, we can create the antimatter counterparts of matter’s leptons and quarks, but when we study these particles of antimatter, we find that instead of behaving like mirror reflections of their material cousins, they can follow rules of their own.

Today, although antimatter remains one of the great mysteries of physics, it is slightly less mysterious than it was a month ago. Years of careful experiments by physicists at CERN and Fermilab have at last produced a significant advance in the understanding of the difference between the behavior of matter and antimatter. Moreover, physicists from Fermilab’s KTeV experiment said they were "shocked" at the size of a long-sought result they reported to a standing-room-only audience at a seminar at Fermilab on February 24. Indeed, there was an audible gasp from the audience of physicists when University of Chicago graduate student Peter Shawhan gave KTeV’s observed value for a phenomenon called "direct CP violation."

"Our result," Shawhan said, "is that epsilon prime over epsilon equals 28 plus or minus 4.1 times 10-4."

KTeV physicists may have been surprised, but CERN physicists in experiment NA31 were elated.

"The CERN physics community offers its congratulations to their colleagues from the KTeV experiment at Fermilab for their exciting new data on the observation of direct CP violation in neutral kaon decays," wrote CERN Director-General Luciano Maiani. "The KTeV result of (2.8 ± 0.4) x 10-3 is particularly pleasing as it is an important step forward in the understanding of CP violation and it confirms with greater precision the earlier result by the NA31 experiment at CERN. The NA31 result was published in May 1988 in Physics Letters B in a paper entitled ‘First Evidence for Direct CP Violation’ with a result of (3.3 ± 1.1) x 10-3 for the relative decay amplitudes of CP violating to CP conserving decays."

Antimatter has been surprising physicists since its discovery by physicist Carl Anderson, in the track of an antielectron, or positron, in a cloud chamber in 1932. The prevailing theory of the fundamental structure of matter, the Standard Model, holds that every particle of matter has a corresponding antiparticle of antimatter. It is believed that early in the evolution of the universe, matter and antimatter were equally abundant, but today antimatter has only been observed in cosmic-ray interactions–and in high-energy particle collisions at accelerators such as Fermilab’s Tevatron.

Among the particles born in high-energy collisions are mesons. Mesons are short-lived pairings of a quark and an antiquark. Certain mesons, called neutral kaons, are combinations of a strange quark or antiquark and a down quark or antiquark.

In 1964, a group of physicists led by James Cronin and Val Fitch were studying neutral kaons in experiments at the Department of Energy’s Brookhaven National Lab when they discovered a slight but definite asymmetry in the behavior of the neutral kaon and its antiparticle–an asymmetry called charge-parity, or CP, violation. Until that discovery, physicists had believed that particles and antiparticles behaved symmetrically, like mirror reflections of each other.

"We were attempting to make a much better test of CP invariance," said Fitch, who with Cronin was awarded the Nobel Prize for the discovery, "and it turned out not to be invariant."

This original CP-violating effect can be described as an asymmetry in the quantum-mechanical fluctuation, or "mixing" of the neutral kaon with its antiparticle. Other manifestations of CP violation have been clearly established at many laboratories in the years since its discovery, but they could all be traced to this original effect. Among the theories proposed to explain CP violation is the Superweak Theory, which posits only mixing effects, with no CP violation in the decays of neutral kaons into other particles.

Yet ever since 1964, scientists at physics laboratories around the world have been attempting to observe an asymmetry in the decay, rather than the mixing, of the neutral kaon. To do so, they have attempted to measure the ratio e'/e, "epsilon prime over epsilon," a double ratio of different modes of decay of neutral kaons and their antiparticles into pi mesons, or pions. If they found a value different from zero, it would signal a new–direct–form of CP violation.

"The Standard Model, if it correctly accommodates CP violation, predicts a non-zero, but small, effect," said University of Chicago physicist and KTeV spokesman Bruce Winstein. "But experiments up until now had not firmly established such an effect. An experiment at CERN, NA31, led by Heinrich Wahl, reported a significant effect, 23 x 10-4, with a precision of 3.5 standard deviations, but that was not yet enough to say definitively that the effect was non-zero. And a previous Fermilab experiment, E731, reported a value of 7.4 x 10-4, with a standard deviation of 1.25–an effect three times smaller than the CERN experiment, and not far enough away from zero to confirm the CERN effect. Most theorists who calculated e'/e in the Standard Model found very small values, closer to the Fermilab result or even smaller. As a result of this situation, both groups designed and constructed experiments aimed at much more precise determinations."

KTeV’s new result establishes the existence of direct CP violation beyond reasonable doubt (almost 7 standard deviations). The finding definitively rules out the Superweak Theory as the sole source of CP violation. The value for e'/e was much larger than many experts had expected.

"One way of explaining the largeness of the effect, within the Standard Model, is if the strange quark’s mass is smaller than is often assumed," said Andreas Kronfeld, a Fermilab theorist. For example if the strange quark has a mass of 70-80 MeV, as obtained by Fermilab theorists Kronfeld and Paul Mackenzie and collaborators, the Standard Model prediction for e'/e agrees well with KTeV’s result.

Fitch, now professor of physics at Princeton University, summed up reaction to the announce-ment: "It is a most astonishing result. It is quite unexpected, and very, very interesting."

KTeV’s Winstein pointed out that the new result, which is based on analysis of only about 20 percent of the collaboration’s total data from a 1996-1997 physics run, is much more consistent with the earlier CERN result than with previous results from Fermilab.

"We are excited to have established direct CP violation," Winstein said. "We want to emphasize that CERN’s NA31 experiment deserves a share of the credit."

To try to understand the difference from their previous results, the KTeV physics analysis team has intensely scrutinized the earlier Fermilab measurement, but, said Winstein, "we have found nothing that could account for the difference, other than an unlikely but still possible fluctuation. We are eagerly awaiting the next results from our colleagues at CERN in experiment NA48. That experiment has significant strengths that complement KTeV’s, and we expect them to report soon on data they have already taken. And the physics community awaits the results of a completely different approach taken by the KLOE experiment at Italy’s Frascati laboratory."

Cronin, co-discoverer of CP violation in 1964 and professor of physics at the University of Chicago, confirmed the significance of the KTeV announcement. "It’s been 35 years since CP violation was discovered," Cronin said. "This is the first time that we have finally learned something new. It doubles our knowledge of CP violation–now there are two parameters instead of only one. Until now, we could explain everything in terms of slight kaon mixtures, but not any more. It’s just sensational!"

The KTeV experiment (for Kaons at the Tevatron) is an 85-member collaboration of experimental groups from the University of Arizona, the University of California at Los Angeles, the University of California at San Diego, the University of Chicago, the University of Colorado, Elmhurst College, Fermilab, Osaka University, Rice University, Rutgers University, the University of Virginia, and the University of Wisconsin.

KTeV began construction in 1992 and first took data late in 1996. It used a beam of 800-GeV protons from Fermilab’s Tevatron to create two parallel beams of neutral kaons to search for CP violation. An innovative particle detector constructed of crystals of cesium iodide gave experimenters unprecedented precision in making experimental observations, while other technological innovations allowed the collaborators to rule out background events and collect data at very high rates.

Now Nature has given up one long-held antimatter secret, but physicists around the world are preparing new experiments to elicit more revelations in the persistent mystery of antimatter.