Theorists originally assumed that matter and antimatter were precise mirror images, with identical properties and behavior. In the argot of physics, matter and antimatter were supposed to obey a rule called charge-parity symmetry, which is just a way of saying that the laws of physics should be fair and balanced. But it turns out that the universe is not evenhanded. It was a major surprise—one worth a Nobel Prize—when in 1964 two Princeton physicists, Val Fitch and James Cronin, found that a particle called a kaon violated the assumptions that all particles are equal under nature’s laws. Kaons are short-lived and quickly decay into a spray of other particles. If the universe didn’t play favorites, kaons and antikaons should have decayed at exactly the same rate, into exactly the same types of particles. But they didn’t. The difference was small—happening only once in every 500 decays—and obscure even by the esoteric standards of particle physics. But it was there. Nature did discriminate between antimatter and matter. Physicists were completely baffled. “We are hopeful,” Cronin said at the time, “that at some epoch, perhaps distant, this cryptic message from nature will be deciphered.”
Courtesy of the Stanford Linear Accelerator Center
Beams of antielectrons, or positrons, hurtle through the upper pipe of the Stanford Linear Accelerator at nearly the speed of light, while electrons move at the same speed in the lower pipe. Powerful magnets (the blue and yellow structures straddling the pipes) focus the beams.
In a sense, Cronin and Fitch had found a piece of an answer to a question that had not yet been asked, because their discovery came one year before physicists realized that the universe wasn’t eternal, that it began with the Big Bang. The dissident Soviet physicist Andrei Sakharov was the first to understand that the Big Bang actually created a crisis for physicists: How could they explain the absence of antimatter and the presence of matter in a cosmos where both should have almost instantaneously vanished?
Sakharov proposed a number of conditions that would have been necessary in the early universe to ensure the survival of matter. One of the conditions was that antimatter and matter must differ in some fundamental way. In pointing this out, Sakharov was far ahead of his time. While most physicists were still coming to terms with the idea of the Big Bang, he had already identified a major weakness of the theory and hinted at a solution.
“It was a pretty amazing insight,” says Stewart Smith, a Princeton physicist who is a member of the BaBar team. “It was very early in the game—1967—and he was doing a lot of other things then, like running from the KGB.”
Sakharov’s question remained unanswered for more than three decades—until physicists began analyzing the debris from the collision of matter and antimatter in the BaBar particle detector.
David Hitlin is obsessed with Babar, the elephant featured in the children’s book series originated in the 1930s by the French author Jean de Brunhoff. He has collected a handful of first editions of Babar books, as well as countless related artifacts, including a Babar clock that hangs on his office wall. His prized possession is an original copy of de Brunhoff’s marriage contract. “This was written out in longhand, 15 pages,” he says, as he delicately removes the document from a drawer in his office. “Somehow a copy ended up at the flea market, and I randomly found it.”
Hitlin’s Babar fixation began in 1987 after he, Jonathan Dorfan, and Pierre Oddone, a physicist at the University of California at Berkeley, decided to try to build a particle accelerator unlike any other. They needed a unique machine because they wanted to study the B meson. The B meson is paired with an anti-B meson, which physicists indicate in their equations as a B with a bar drawn over it, and they pronounce it “B-bar.”
The B meson is a heavy relative of the kaon studied by Fitch and Cronin. Like the kaon, it lives only fleetingly in particle accelerators. But its greater mass made it a tempting object of study. Heavy particles can spontaneously decay into matter and antimatter fragments in a greater number of ways than lighter particles, increasing the odds of finding something unexpected. Hitlin, Dorfan, and Oddone wanted to analyze differences in the rate of decay of Bs and anti-Bs that might show how matter managed to survive the conflagration that followed the Big Bang. But first they had to figure out how to accurately measure the short-lived paths of the B meson particles, which decay after traveling on average about one-thousandth of an inch (by contrast, the kaons studied by Fitch and Cronin traveled more than 100 feet).
In most big accelerators, like the one at Fermilab near Chicago or at CERN, two beams of particles at equal energies race through lengths of long, circular pipes in opposite directions before colliding. Because the two colliding beams have equal energies, the B mesons don’t travel far after they smash together. It’s like two Volkswagens colliding head-on and coming to a dead stop. In a traditional accelerator, the B mesons would behave like those Volkswagens, and physicists wouldn’t have a chance to measure any of their properties before they decayed.
Oddone envisioned an accelerator that uses two particle beams of unequal energies—specifically, a beam of electrons that moves with three times more energy than a beam of positrons coming from the opposite direction. This collision is more like an 18-wheeler slamming into a Volkswagen. When the two beams smash together, the resulting debris—including some B and anti-B mesons—continues hurtling in the direction of the electron beam at about half the speed of light. And that allows the research team to take advantage of something that Einstein elucidated in 1905. When objects travel at nearly the speed of light, time slows down for them. This means the B mesons live longer—about a trillionth of a second—before decaying, traveling more than 12 times as far as they would in other accelerators.
The final design called for two separate accelerator rings, each a bit over a mile in circumference, built one on top of the other. The rings are attached to the end of the two-mile-long Stanford Linear Accelerator. The accelerator pumps electrons down a four-inch-wide, two-mile-long copper pipe. After traveling about a mile, some of the electrons are shunted off into a separate pipe, where they smash into a three-foot-wide block of tungsten. Energy from the collision creates positrons, which are funneled back into the linear accelerator, and more electrons. At the end of the linear accelerator, magnets first steer the positrons and electrons into separate rings and then bring them together to collide inside the BaBar detector.
“No one believed we could build this machine when we first proposed it,” says Dorfan. “We had a devil of a time convincing people.” Dorfan, Hitlin, and Oddone had to write thousands of pages describing their scheme before Congress finally approved it in 1993. The accelerator took five years to build. In 1999 it finally started flinging positrons and electrons together, and today it produces Bs and B-bars at the rate of nearly 1 million a day. The accelerator’s day-and-night constant production is so relentless that physicists call it the B factory.