Antimatter

If we understand these mysterious particles, will we know why we exist?

By Tim Folger|Monday, August 02, 2004
RELATED TAGS: SUBATOMIC PARTICLES

There’s a time machine on the Stanford University campus, and it runs day and night. It won’t hurl anyone into the past or future, but it does something almost as audacious: It reenacts events that occurred just after the Big Bang, when some of the pure energy that filled the cosmos became all the matter that now exists. Inside an 18-foot-high, 1,200-ton particle detector, matter and antimatter moving at nearly the speed of light smash into each other billions of times a second, shattering into subatomic debris that hasn’t existed for about 14 billion years. “We have the gall to believe that we can prepare a situation that is very analogous to what you had at the beginning of time,” says physicist Jonathan Dorfan, director of the Stanford Linear Accelerator Center. “We’re trying to understand what happened during an extraordinarily energetic part of the birth of the universe—and we do a pretty good job.”

Courtesy of Lawrence Berkeley National Laboratory

The BaBar particle detector at the Stanford Linear Accelerator Center is a marvel of electrical engineering. Beneath a massive iron door, opened for maintenance, lies a superconducting magnet and an elaborate array of electronic sensors that track billions of explosive collisions between matter and antimatter particles every second.

For four years, Dorfan and a small army of 600 physicists from three continents have been using two of the world’s biggest and most complex machines—the two-mile-long Stanford Linear Accelerator and the BaBar particle detector—to solve one of the ultimate cosmic mysteries: Why is there any matter in the universe at all?

Strange as it may seem, the laws of physics suggest that immediately after the Big Bang, all the matter suddenly created should have been obliterated by an equal amount of antimatter, the strange and slightly distorted mirror image of normal matter. An electron, for example, which has a negative electric charge, also has a twin, called a positron, with a positive charge. Whenever a normal particle and an antiparticle meet, they annihilate each other, converting all their mass into energy in a pyrotechnic demonstration of Einstein’s famous law, E = mc2.

And therein lies the source of one of the greatest dilemmas of science. Physicists believe that by the time the universe was just 10-33 of a second old (that’s a millionth of a billionth of a billionth of a billionth of a second), the temperature had dropped from unimaginably hot to a mere 18 million billion billion degrees. That was cool enough for the first particles of matter and antimatter to condense from pure energy. But to balance the cosmic energy books—and to avoid violating the most fundamental laws of physics—matter and antimatter should have been created in exactly equal amounts. And then they should have promptly wiped each other out. Yet here we are. Somehow a bit of matter managed to survive.

To understand why, Dorfan and his colleagues spend their waking hours creating antimatter particles and smashing them into regular matter. Time and time again, the researchers have documented that antiparticles and normal particles decay at different rates. However, the difference is far too small to explain the amount of matter that exists in the universe.

So some particle physicists are beginning to bet that the key to solving this conundrum may have been right under their noses all the time. If they’re correct, the potential solution is passing through their bodies, billions of times a second, in the form of elusive, ghostly particles that rival antimatter for sheer weirdness: neutrinos. A surprising theory holds that all the matter in the universe may have started out as neutrinos, which is no small irony, considering that until very recently neutrinos were thought to be entirely without mass and as immaterial as the light illuminating this page.

If you’re the sort who believes in the balance of nature, or in a harmonious, rational cosmos, think again. The universe is profoundly out of whack. Physicists realize things are out of kilter because they can literally count the number of photons—particles of energy—in the universe today and compare that with the total number of matter particles. Photons outnumber matter by a billion to one. If matter and antimatter had completely destroyed each other after the Big Bang, the universe today would contain only photons, because every time antimatter and matter self-destruct, their combined mass is transformed into a small unit of energy, a single photon.

“Since we know how many photons there are compared with ordinary matter, that tells us that most of the matter and antimatter did annihilate, and only a little tiny bit of matter was left over,” says David Hitlin, a physicist at Caltech and the founding director of the BaBar team. But why would nature favor matter over antimatter?

Before physicists knew about the Big Bang, no one spent much time worrying about this cosmic imbalance. Antimatter was simply seen as exotic stuff in an eternal cosmos. The renowned British physicist Paul Dirac first posited the existence of antimatter in 1928, and four years later researchers at Caltech detected the first documented antiparticles—positrons produced by the impact of cosmic rays on the atmosphere. Since then antimatter has been observed many times in cosmic rays, in particle accelerators, and during the radioactive decay of elements, but usually as isolated, short-lived particles like positrons or antiprotons rather than as whole atoms. Antimatter has even been used as a medical diagnostic tool in positron-emission tomography, which uses positrons to find tumors. Although there are no naturally occurring antimatter atoms, in 1995 physicists at the European Center for Nuclear Research (CERN) in Geneva cobbled together a few atoms of antihydrogen by linking a positron to an antiproton and have since made tens of thousands more. The rarity and expense of producing antimatter make it the most costly substance on Earth, $1,750 trillion per ounce by one estimate.

Despite the scarcity of antimatter on Earth, Dirac and other early theorists speculated there might be antiplanets, antistars, even an antiuniverse. But astrophysicists have searched in vain for signs of antimatter galaxies, which would reveal their presence in a very dramatic way: Any matter hitting an antimatter star, for instance, would create a titanic burst of energy. “We know if there were an interface somewhere in the universe between antimatter and matter, then there would be annihilation going on at this interface,” says Hitlin. “Astronomers can look for signs of this, but they don’t see it. The limits are about as far away as we can see—about 14 billion light-years. And we just don’t see anything. You can’t say the antimatter is hiding in a corner. In the good fraction of the universe that we can see, there’s no antimatter.”

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.

Courtesy of the Stanford Linear Accelerator Center

During the construction of BaBar, physicist Gerard Bonneaud mounts a ladder to check external cables while David Hitlin, the founding director of the project, examines an interior chamber. “The art of this is to build an apparatus that allows you to do something that hasn’t been done before but that is not so far beyond the pale that you’ll get into trouble,” says Hitlin.

The heart of the experiment is the enormous detector, which is shielded by three-foot-thick concrete walls that serve as a radiation shield. The system is set up to shut off automatically if the high-energy beams inside the accelerator rings go astray. “If we lost control, the beam could easily burn a hole through the accelerator,” says Hitlin. “Then it’s a loose beam, like a blowtorch. These beams are 10 or 20 times more intense than any that have been used in an accelerator before.”

The detector was designed to track where collisions occurred, what kinds of particles were produced, and how far each of them traveled. It worked flawlessly. In the summer of 2001, the BaBar researchers, after sorting through some 30 million B and B-bar decays, announced that they had found evidence of a difference between the decay rates of matter and antimatter. Now, with three more years of data behind them, the result is unequivocal. The problem is that the measured difference is too small by a factor of a billion to explain the amount of matter in the universe. If the B and anti-B process were the only one in nature able to generate a matter-antimatter imbalance, the universe wouldn’t be completely empty, but it would be very sparsely populated. And the odds against the existence of a planet like Earth would have shot up a billionfold.

“I don’t think it’s too flippant to say that the amount of matter that would be around would be a billionth of what it is now,” says Smith. “A universe with a billionth as much matter would be a very different place.”

Given the magnitude of what the BaBar research team set out to accomplish—understanding the origin of everything that exists—perhaps it’s not surprising that they haven’t yet succeeded. The result is at once a technical tour de force and an unsettling puzzle. It agrees perfectly with what physicists call the standard model, an overarching theory that describes all known phenomena dealing with the parts of atoms and how they behave. The standard model has been tested by experiments countless times, and it has never failed to predict what physicists would see. So the fact that it agrees so precisely with the outcome of the BaBar experiment is an important clue. But it suggests to many physicists that while the theory is evidently an incredibly accurate guide to the universe today, it will need to be modified if we are ever to understand the extreme conditions that existed in the first few instants after the Big Bang. “We now know enough to know that in order to solve this problem we’re going to have to learn something really new,” says Hitlin.

Hitlin isn’t exaggerating. If the BaBar results are proof that conventional wisdom has been pushed to its limits, then the most promising alternative is strange indeed. It may be that matter never would have survived the universe’s primordial fireworks had it not been for the behavior of neutrinos, tiny particles that were once regarded as little more than curiosities. Although scientists believe they outnumber all other particles in the universe, neutrinos are almost undetectable (see “The Unbearably Unstoppable Neutrino,” Discover, August 2001). For nearly 70 years after their discovery in 1930, they were thought to be without any mass. But in 1998 physicists concluded that neutrinos probably do have a small amount of mass. They also found some evidence that the mass of any one neutrino can change, increasing or decreasing on the fly.

If the mass shifting of neutrinos is confirmed, it would bolster the case for an entirely new solution to the matter-antimatter mystery. The theory is called leptogenesis, a name taken from the lepton particle family, to which neutrinos belong. Proponents of the theory suggest that an exceptionally heavy but unstable breed of neutrino existed in the very early universe. Even before BaBar’s results, Gerard ’t Hooft, a physicist at the University of Utrecht in the Netherlands and a 1999 Nobel Prize winner, speculated that during the extreme conditions following the Big Bang, neutrinos could have changed into protons and neutrons.

“At the temperature of the universe now, the chance that something like this would happen is zero, basically,” says Yuval Grossman, a theoretical physicist from the Technion in Israel who is on sabbatical at the Stanford Linear Accelerator. “But at high temperatures it happened all the time in the early universe.”

If heavy neutrinos did sire protons and neutrons during the Big Bang, Grossman and others argue, they could be the source of nature’s bias toward matter. The idea is that when the heavy neutrinos decayed, they would have generated more neutrinos than antineutrinos. This second generation of neutrinos and antineutrinos would then have changed their masses, becoming protons and antiprotons. But with the genesis of more neutrinos than antineutrinos, the process would have yielded more protons than antiprotons, leading to the fateful imbalance between matter and antimatter at the dawn of time. “This is extremely speculative,” says Hitlin. “There’s no experimental evidence for it, but it’s the kind of thing where you might be able to devise an experiment. But as hard as it was for us to do in the B mesons, it will be much more difficult with neutrinos.”

Physicists will probably never directly observe heavy neutrinos. If they exist, they are expected to be 15 orders of magnitude heavier than protons, and the energies needed to produce them are far beyond what any accelerator can reach. But physicists in Japan and Europe are looking for evidence of oscillations in neutrino masses. And an ambitious new experiment is scheduled to begin next year at Fermilab, in which a beam of neutrinos will be shot through 450 miles of Earth’s crust toward a 6,000-ton detector at the bottom of an old iron mine in northern Minnesota. The physics is so new that no one knows what to expect. If the theory of leptogenesis turns out to be right, then everything we see in the universe, from galaxies to DNA, descends from particles that were once thought to barely qualify as matter.

In the meantime, Hitlin and Dorfan, who have each devoted nearly 20 years of their lives to the BaBar antimatter experiment, have no illusions about the difficulties ahead. Sitting in his office, Hitlin conveys a palpable sense of bafflement. “The question we ask ourselves is, ‘Now what?’ It’s still a puzzle,” he says. “It will take a generation to sort this out.”

For his part, Dorfan is confident that the answer to the antimatter mystery is out there. That’s because whatever happened at the beginning of time, it left behind one absurdly obvious clue. “In the end,” he says, “there is the irrefutable evidence that we are here.”

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