June 4, 2004  




BABAR Tests Matter-Antimatter Theory in New Ways

By Kate Metropolis

A physicist in the twenty-third century telephones her automobile mechanic. “Hello,” she says, “I’ve been having trouble with my new car. It’s a red Alpha Romeo. Any chance you can fix it? Oh, by the way, it’s made out of antimatter.”

Graphic by Alan Chou

The only repair manual the mechanic has is for the matter model. “No problem,” he thinks. “I’ll just follow the manual and substitute antimatter parts,” so he tells his customer to bring in her car. Using a special tool kit to handle the antimatter, the mechanic works on the car, following precisely the instructions in the manual.

A few days later, the customer is back in his shop. She’s not happy. “The car just doesn’t drive the way it should,” she complains.

“Ah,” says the mechanic, “there’s a subtle difference between the way matter and antimatter behave. The rules for the car just don’t carry over perfectly to the anticar.”

In fact, the universe would be vastly different if the laws of physics were precisely the same for all particles and their antimatter counterparts. The Big Bang, the explosion in which the universe was created, produced exactly equal amounts of matter and antimatter, and whenever they get close enough, they annihilate each other. If the annihilation had been complete, all matter, as we know it, would have been converted into light.
This dominance of matter in the universe, which most of us never wonder at, is bizarre. It is as extraordinary as measuring how long it takes a marble to roll down a board and discovering that your result depends on the time of day you do the measurement.

The laws that govern particle behavior might have been as blind to the difference between matter and antimatter as the law of gravity is to the difference between morning and afternoon. But the fact that they do distinguish matter from antimatter, a phenomenon particle physicists call CP violation, means that there’s “a one in a billion imbalance in favor of ordinary matter,” says B
ABAR physicist Christophe Yeche, from Saclay.

The current theory of particle physics, called the standard model, predicts that all the differences in the behavior of matter and antimatter can be expressed in terms of three angles—called alpha, beta, and gamma—that, like the angles of a triangle, add up to 180 degrees. (This is a strange-sounding prediction; it would take an article at least this long to explain it, so I hope you’re willing to take it on faith.)
“Measuring each of these angles,” says B
ABAR physics analysis coordinator Jeff Richman, “is a holy grail of this field.”

The Quest for Alpha

B mesons and anti-B mesons provide the largest window into the phenomenon of CP violation. Their behavioral difference lies in the probability of their decays into particular particles: the B mesons decay into given states a different percentage of the time than the anti-B mesons.

Both B
ABAR and Belle, a collaboration in Japan, use B mesons to explore CP violation. The initial plan of both groups was to measure the angle alpha by looking at the very rare events in which a B meson (or an anti-B meson) decays into a pair of charged pions. However, while the experiments were being built, the CLEO collaboration at Cornell performed measurements suggesting that the pion approach was fatally flawed, at least for the amount of data that BABAR and Belle will acquire, because the decay process was so complicated to interpret. “Nature,” says Yuval Grossman, a theorist at the Technion, the Israel Institute of Technology, “was unkind.”

The hope of measuring alpha still burns bright in B
ABAR, thanks to kindness on the part of nature and hard work by members of the collaboration from LBNL, Liverpool, and Saclay.

The researchers discovered that the decay of a B meson to the particle pair rho+ rho- is more likely than the decay to charged pions and the decay was well suited to measuring CP violation.

However, measuring this decay was challenging, according to Andrei Gritsan, leader of the rho-rho team at LBNL. “We had to distinguish very rare decays, occurring only three times in a hundred thousand, and measure their parameters in the presence of a large background.” B
ABAR‘s first results characterizing the rho+ rho– decay were published in Physical Review, and the collaboration will present even better results this summer, after analyzing additional data.

Then the physicists measured alpha by comparing the time it takes a B meson to decay into rho+ rho- with the time it takes its antiparticle, an anti-B meson, to decay into the same set of particles.

“Our direct measurement is a first,” says B
ABAR physicist Christos Touramanis, head of the Liverpool group.

These results have been submitted to Physical Review. The precision so far is limited, but the results are consistent with standard model predictions.
Or, as Richman puts it, “This is one more banana peel the standard model could have slipped on.”

Grossman, who is not a member of B
ABAR, characterizes the results as “very important. A year ago, we didn’t know you could use rho+ rho- to get a really fundamental parameter.”

“A Physics Program, Not a Single Measurement”

ABAR has also obtained new results on another of the holy-grail angles: beta.

Four years ago, the first major result announced by both B
ABAR and Belle was the measurement of beta in a certain class of B decays, called the “golden modes” because the experimental measurement and its theoretical interpretation are so straightforward. The standard model predicts that measurements of beta in B decays will yield the identical value; yet many physicists expect some types of B decays to disagree with the golden-mode value.

“The goal is to try to conclude whether the data show signs of physics beyond our current understanding,” says Zoltan Ligeti, a theorist at LBNL. “Evidence that the standard model can’t account for data would do that.”

Both collaborations have been pushing hard to analyze different classes of B decays to obtain measurements of beta. B
ABAR results so far are consistent with the standard model. Belle interprets one of its results as a possibly inconsistent with the standard model.

“It’s constructive to think about what Belle’s result might mean if it becomes statistically significant,” Grossman says, “but right now it is not statistically significant.”

ABAR has found additional B decays that can be used to search for contradictions to the standard model, but initial measurements still agree with standard model predictions. “The measurement of the angle beta is not just a single measurement,” Richman emphasized at the BABAR collaboration meeting in April. “It’s a physics program.”

Stay Tuned

ABAR and Belle have both started the marathon of preparing for the International Conference of High Energy Physics, to be held this August in Beijing. New data are pouring in (the PEP-II accelerator is now delivering B mesons at triple the rate it was designed to), and more data promise more accurate measurements.

“We’ve demonstrated that we can do these measurements,” Richman says. “Now PEP-II has given us a lot more data. We could discover that everything still fits together, or that the new results can’t fit into the simple standard model picture. A lot of us will be on the edge of our seats when the new results come out.”
“Understanding the laws of nature is really what we are after,” Grossman says.

“People sometimes think that the most important discovery is a new particle. That is particularly exciting when it teaches us something new about the fundamental laws. But experimental results like the measurements of alpha and beta, which could prove our theories right or wrong, are just as valuable.”


The Stanford Linear Accelerator Center is managed by Stanford University for the US Department of Energy

Last update Thursday June 03, 2004 by Emily Ball