February 19, 2001
New results may help physicists answer a long-standing question about matter
By Tim Stephens
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," said professor of physics Michael Dine, a leading
theorist. "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
such as Dine will be to incorporate the new experimental results into a theoretical
framework that satisfactorily accounts for the observed asymmetry.
In a talk last week titled "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. His talk was part of a session on matter and antimatter
at the annual meeting of the American Association for the Advancement of Science
(AAAS) in San Francisco.
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 said. "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 said.
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 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 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. Dine says he is hopeful that the results of these experiments 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
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