91av

MATTER and antimatter are not exact mirror images of each other, say
scientists at the Stanford Linear Accelerator Center in California. The result
will help physicists solve the mystery of why our Universe seems to contain more
matter than antimatter.

Physicists believe that after the big bang, matter and antimatter were
created in equal proportions. But today the Universe seems to contain mainly
matter, so most of the antimatter must have disappeared some time before
“normal” matter—the neutrons, protons and electrons we see now—was
formed. For nature to have favoured matter over antimatter in this way, they
must have slightly different properties.

In 1964, physicists first spotted a difference in the decay rates of a
subatomic particle called a K meson and those of its antimatter partner. To be
certain, researchers wanted to see if other particles exhibited the same
phenomenon, known as charge-parity (CP) violation. The most promising candidate
was a particle called a B meson, and special particle accelerators—called
B factories—were built to produce pairs of Bs and anti-Bs.

Since it started work in June 1999, the B factory at Stanford has produced 32
million such pairs. The huge Babar detector that measures their decay rates
produces a number known as sin2b. If matter and antimatter are exact mirror
images, sin2b should be zero. Charge-parity violation, as predicted by the
standard model of particle physics, should produce a value of 0.7.

In February this year, preliminary results from the Babar team produced a
figure slap in the middle, with large enough uncertainty to agree with either result
(91av, 17 February, p 9).
But the Babar team plugged on, trying
to gather enough data to reduce the uncertainty. To prevent any subconscious
bias from creeping into the calculation, the latest value of sin2b produced by
Babar is kept concealed from the scientists by the computer.

Two weeks ago, the computer disclosed the new result: 0.59. “When it came up,
we said OK, that’s it, we’ve gotta go with it,” says Stewart Smith of Princeton
University, spokesman for the Babar team. This figure has an uncertainty of
0.14, making it consistent with charge-parity violation. “It’s fair to say
there’s no inconsistency,” says Smith.

A rival B factory at the High Energy Accelerator Research Organization (KEK)
in Tsukuba, Japan, also produced an inconclusive result earlier this year. It is
due to report its latest results in the next few weeks.

Ken Peach of the Rutherford Appleton Laboratory in Oxford, who has worked for
many years on CP violation in K mesons, says that the Babar results are an
exciting confirmation of the standard model. “This is certainly a huge step in
our confidence in the model.” He also says it is possible that whatever created
the large imbalance between matter and antimatter in the Universe produced this
smaller effect in the standard model. “We just don’t know how they’re related
yet,” he says.