The seemingly unremarkable fact that the universe is full of matter turns out to be something physicists can't quite account for.
According to the big bang theory, equal amounts of matter and antimatter were created at the birth of the universe, but precious little antimatter is to be found in the universe today.
Everything we see, from our bodies to our cars to the stars in distant galaxies, is made of matter. Cosmic rays and high-energy physics labs routinely create antimatter particles, but they soon interact with particles of matter and vanish in bursts of pure energy.
Somehow, within a fraction of a nanosecond after the big bang, matter gained the upper hand. Physicists believe subtle differences in the behavior of matter and antimatter led to a slight excess of matter in the very early universe. While most of the matter and antimatter created in the big bang quickly disappeared in a blaze of mutual annihilation, about one out of every billion particles of matter survived.
"Until the 1960s, the laws of nature were thought to be completely symmetric between matter and antimatter," says Michael Dine, a leading theorist and professor of physics at the University of California, Santa Cruz. "We now know that the symmetry is not quite exact, but our ideas about where the asymmetry comes from remain somewhat speculative."
Two new accelerators, one at the Stanford Linear Accelerator Center (SLAC) in Palo Alto and another in Japan, have begun to yield results that could reveal exactly how the symmetry between matter and antimatter is broken. The challenge for theorists like Dine will be to incorporate the new experimental results into a theoretical framework that satisfactorily accounts for the observed asymmetry.
In a talk entitled "Why the Universe is Made of Matter," Dine discussed various ideas put forth to explain the source of the asymmetry that enabled matter to dominate the universe. The talk was part of a session on matter and antimatter on Friday at the annual meeting of the American Association for the Advancement of Science (AAAS) in San Francisco. The latest results from accelerator experiments designed to measure the effects of the asymmetry were also presented in this session.
Evidence that the laws of nature are not completely symmetric with respect to matter and antimatter first emerged in 1964, when a violation of the so-called charge-parity (CP) symmetry was observed in ephemeral particles known as K mesons, or kaons. Researchers discovered a tiny discrepancy between kaons and anti-kaons in the way they decay.
In 1967, Soviet physicist Andrei Sakharov laid out the basic principles needed to understand this asymmetry and how it led to the dominance of matter in the universe. Sakharov showed that the violation of CP symmetry is just one of three conditions that must be satisfied to explain how an imbalance arose between matter and antimatter. There must also be violation of a conservation law, called the "conservation of baryon number," and the early universe cannot always have been in thermal equilibrium.
The prevailing theory of particle physics, called the Standard Model, readily accommodates the minute CP violation seen in the decay of kaons. But the violation of CP symmetry allowed by the Standard Model is too small to account for the amount of matter observed in the universe.
"Careful study in recent years has shown that you cannot produce nearly enough matter if the Standard Model is the whole story," Dine says. "To explain why we are here, there must be modifications of the laws of nature at very high energy."
One proposed modification of the Standard Model is supersymmetry, a set of ideas that suggest nature should exhibit a new symmetry at extremely high energies. Supersymmetry allows stronger CP violation than the Standard Model and also offers interesting ways to meet Sakharov's other two conditions for generating the asymmetry between matter and antimatter, Dine says.
While the Standard Model provides only one parameter that violates CP symmetry, supersymmetry predicts a whole new class of subatomic particles and new ways for CP violation to come about. If the theory is correct, the new particles predicted by supersymmetry should be detected when more powerful new accelerators begin operating in the next few years.
Meanwhile, efforts continue to measure accurately the symmetry-breaking parameter predicted by the Standard Model. To do this, physicists are turning from kaons to their heavier cousins, the B mesons. At SLAC and at the High Energy Accelerator Research Organization (KEK) in Tsukuba, Japan, new accelerators called "B factories" have been churning out vast numbers of B mesons and anti-B mesons in experiments designed to measure CP violation in their decays.
Some versions of supersymmetry and other proposed modifications of the Standard Model make quite dramatic predictions for the experiments now being conducted at the B factories. At the AAAS meeting, Dine provided a theorist's perspective on the latest results from those experiments.
Dine says he is hopeful that the new results will not fit neatly within the Standard Model.
"The Standard Model has been a source of frustration because it can't fully explain where the asymmetry between matter and antimatter comes from. If these new experiments support the Standard Model, then we will still have a puzzle," he says. - By Tim Stephens
[Contact: Michael Dine, Tim Stephens]