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
鈥渘ormal鈥� matter鈥攖he neutrons, protons and electrons we see now鈥攚as
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鈥攃alled
B factories鈥攚ere built to produce pairs of Bs and anti-Bs.
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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
(快猫短视频, 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. 鈥淲hen it came up,
we said OK, that鈥檚 it, we鈥檝e gotta go with it,鈥� 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. 鈥淚t鈥檚 fair to say
there鈥檚 no inconsistency,鈥� 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. 鈥淭his is certainly a huge step in
our confidence in the model.鈥� 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. 鈥淲e just don鈥檛 know how they鈥檙e related
yet,鈥� he says.