As sometimes happens in the world of scientific research, two groups working independently come up with the same findings at the same time, such as when a National Institutes of Health-funded group and a private firm called Celera both announced their completion of the human genome sequence.
In July 2001, Virginia Tech physicists, in collaboration with others on the Belle Experiment at the Japanese National Laboratory for High Energy Physics (KEK), obtained a measurement that shows, to a very high degree, an asymmetry in the behavior of matter and antimatter and that the difference is consistent with the prediction of the Standard Model Theory of Particle Physics. Researchers at the Stanford Linear Accelerator Center obtained a similar measurement at the same time as the KEK team did.
The Standard Model, formulated in the 1970s, predicts an asymmetry, or difference, in the decay rates of certain esoteric subatomic particles called beauty or B mesons (matter) and anti-beauty mesons (antimatter). A beauty meson is an unstable particle that has almost six times the mass of the common proton. The matter-antimatter asymmetry in the Standard Model also predicts the leftover matter from the Big Bang, matter that forms humans, galaxies, and everything else.
The new KEK measurement was obtained from data recorded by the Belle detector over the past two years and was announced at the meeting of the Lepton-Photon Symposium in Rome July 23 by co-spokesperson Stephen Olsen of the University of Hawaii. Stanford had announced its results at a July 5 news conference. Both groups published in the same issue of Physical Review Letters.
The Belle measurement shows a higher variance from zero than the Stanford measurement and is therefore of more interest to theoretical physicists because it indicates a higher degree of asymmetry, said Leo Piilonen, associate professor of physics at Virginia Tech. Laynam Chang, Tech’s physics department head and a theoretical physicist, said the larger number is of more interest because it shows why the universe has more matter than antimatter.
The Standard Model predicted both the leftover matter in the universe and the asymmetry in the B-meson decays. The Virginia Tech studies corroborate the asymmetry and provide one way of explaining why the universe seems to be made entirely of matter, when you would expect from the Big Bang theory that there should be equal amounts of matter and antimatter. The presence of more matter explains why we are here, Piilonen said. “Because of this asymmetry, all but one part in a billion of the matter and antimatter particles from the Big Bang combined into light. The rest was left over as the matter of which we are made.”
Virginia Tech’s involvement began in 1992. Research by professors Al Abashian and Kazuo Gotow was critical in the design and initial work of the detector. In December 1993, KEK called for proposals for experiments to be built at its new colliding beam accelerator called KEKB—a factory of B mesons. The proposal for the Belle Experiment from Virginia Tech and colleagues in April 1994 was approved almost immediately.
Virginia Tech developed and constructed resistive plate counters for muon detection, which were shipped to Japan for the accelerator. The researchers took the first data in June 1999 and reported the first results in July 2000.
“The two competing measurements are still consistent within statistical uncertainty with each other and with the Standard Model prediction,” Piilonen said. Additional data in coming years will reduce this uncertainty and make possible a more precise comparison with the Standard Model prediction.
To measure the asymmetry, the researchers looked at a B-meson that disintegrates in one-trillionth of a second into two particles, a J/y and a K-short meson. The J/y then decays instantaneously into two particles, a positive muon and a negative muon.
The K-short meson decays in roughly one ten billionth of a second into a positive pion and a negative pion. “We detected the two muons and two pions,” Piilonen said.
The anti-B meson can decay to exactly the same final state, he said. The researchers then compared how often the B-meson and the anti-B meson decay to this and related final states and got the difference, or the matter/antimatter asymmetry. Not all B mesons decay to this particular final state, however; only 1,137 candidate events were sifted from the 31-million B decays recorded by the Belle detector over the past two years.
“This asymmetry is parameterized by a quantity called sin2f1 with a value measured by Belle of 0.99 ± 0.14 ± 0.06,” Piilonen said. The first number is the Belle measurement of the asymmetry, the second number is the statistical uncertainty, and the third is the systematic uncertainty. The Stanford number was sin2f1 = 0.59 ± 0.14 ± 0.05. Both measurements are inconsistent with a value of zero (indicating no matter-antimatter asymmetry), given their uncertainties.
The group will continue to accumulate data for another five to 10 years and try to reduce the size of the statistical error. They also will use other final states that require more sophisticated reconstruction techniques.
Contact Dr. Piilonen at piilonen@vt.edu. Learn more about the physics department at Virginia Tech at http://www.phys.vt.edu/. And about the The experimental high energy physics group at http://www.phys.vt.edu/research/e-particle.html.
— Written by Sally Harris
