Physics & Math / Subatomic Particles

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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