Fermilab Physicists Find New Matter-Antimatter Asymmetry
Batavia, Ill.- Scientists working at the Department of Energy’s Fermi National Accelerator Laboratory have announced a significant advance in the understanding of the difference in the way matter and antimatter behave. Moreover, the physicists, from Fermilab’s KTeV experiment, said they were “shocked” at the size of the 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 the group’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 to the minus 4.”
What did Shawhan mean, and why were physicists so surprised by what he said?
Antimatter has a habit of surprising physicists, beginning with its discovery in 1932, when physicist Carl Anderson first observed the puzzling track of an anti-electron, or positron, in a cloud chamber. Today, the prevailing theory of the fundamental structure of matter, the Standard Model, holds that every particle of matter has a corresponding antiparticle of antimatter. (Just as the antiparticle of the electron is the positron, for example, every quark has an antiquark.) Early in the evolution of our universe, matter and antimatter were equally abundant; but today the universe appears to be made entirely of matter. Antimatter shows up only in cosmic ray interactions – and at particle accelerators such as Fermilab’s Tevatron, where antiparticles are produced in high-energy particle collisions.
Among the collision-produced particles are mesons. Unlike long-lived protons and neutrons, which are combinations of three quarks, mesons are short-lived pairings of a quark and an antiquark. Certain mesons, known as neutral kaons, are combinations of a strange quark or antiquark and a down quark or antiquark. In 1964 scientists got another antimatter surprise when a team led by physicists James Cronin and Val Fitch, studying neutral kaons in experiments at DOE’s Brookhaven Laboratory, 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 mixing (or quantum-mechanical fluctuation) of the neutral kaon with its antiparticle. Other manifestations of CP violation have been firmly established at many laboratories around the world in the years since its discovery, but they could all be traced to this original effect. Among 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, physicists at 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 ?’/?, “epsilon prime over epsilon,” a ratio of different modes of decay of neutral kaons into two 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 cospokesman 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 definitively say it was non-zero. Furthermore, a previous Fermilab experiment saw an effect about three times smaller than the CERN experiment, and not far enough away from zero to confirm the CERN effect.”
The gasp at KTeV’s new result came because it established the existence of direct CP violation beyond reasonable doubt (almost 7 standard deviations), and because it was much larger than anyone, including the experimenters, had expected. The finding definitively rules out the Superweak Theory as the sole source of CP violation. And, while the Standard Model predicts a non-zero effect, the size of the KTeV result raises questions about whether it can be accommodated within the Standard Model.
Fitch, now professor of physics at Princeton University, summed up reaction to the announcement: “It is a most astonishing result. It is quite unexpected, and very, very interesting.”
Secretary of Energy Bill Richardson shared the excitement of the Fermilab scientists. “Thirty-five years ago, scientists at a DOE laboratory discovered a difference in the way matter and antimatter behave,” Secretary Richardson said. “Now physicists at DOE’s Fermilab have taken a giant step in our understanding of this asymmetry between matter and antimatter. I am proud of the role of the Department of Energy in advancing the worldwide exploration of the way our universe works at the most fundamental level.”
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, “but we also want to emphasize that CERN’s NA31 deserves a share of the credit.”
To account for the difference from their previous results, the KTeV analysis team has intensely scrutinized the earlier Fermilab measurement, but, says 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 Frascati, in Italy.”
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 thirty-five 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.
The new experiment began construction in 1992 and took its first data 1996. It used a beam of protons from Fermilab’s Tevatron to create two parallel beams of neutral kaons to search for CP violation. An innovative particle detector constructed of cesium iodide crystals gave experimenters unprecedented precision in making experimental observations. Other technological innovations allowed the collaborators to rule out background events and collect data at very high rates.
KTeV cospokesman Bob Hsiung, a Fermilab physicist, said the startling KTeV result “means that experiments in the near future should be able to observe sizable CP-violating effects first in the mixing of B mesons and then in their decays, as well as in rare kaon decays.”
Fermilab is operated by Universities Research Association, Inc., under contract with the U.S. Department of Energy.
For more information, call Fermilab’s Public Affairs Office at (630) 840-3351.