Dramatic difference discovered in behaviour of matter and antimatter
Today, August 2nd 2004, particle physicists from the UK and around the
world working on the BABAR experiment at the Stanford Linear Accelerator
Center (SLAC) in the USA, announced exciting new results demonstrating a
dramatic difference in the behaviour of matter and antimatter. Their
discovery may help to explain why the Universe we live in is dominated
by matter, rather than containing equal parts matter and anti-matter.
SLAC’s PEP-II accelerator collides electrons and their antimatter
counterparts, positrons, to produce an abundance of exotic heavy
particle and anti-particle pairs known as B and anti-B mesons. These
rare forms of matter and antimatter are short-lived, decaying in turn to
other lighter subatomic particles, such as kaons and pions, which can be
seen in the BABAR experiment.
“If there were no difference between matter and antimatter, both the
B meson and the anti-B meson would exhibit exactly the same pattern of
decays. However, our new measurement shows an example of a large
difference in decay rates instead.” said Marcello Giorgi, of SLAC,
Pisa University and INFN, Spokesman of BABAR.
By sifting through the decays of more than 200 million pairs of B and
anti-B mesons, experimenters have discovered striking matter-antimatter
asymmetry. “We found 910 examples of the B meson decaying to a kaon
and a pion, but only 696 examples for the anti-B”, explained Giorgi.
“The new measurement is very much a result of the outstanding
performance of SLAC’s PEP-II accelerator and the efficiency of the
BABAR detector. The accelerator is now operating at 3 times its design
performance and BABAR is able to record about 98% of collisions.”
While BABAR and other experiments have observed matter-antimatter
asymmetries before, this is the first time that a difference has been
found by simple counting of the number of decays of B and anti-B mesons
to the same final state. This effect is known as direct CP violation and
is found to be 13%; a similar effect occurs for decays of Kaons and
antiKaons but only at the level of 4 parts in a million!
“This is a strong, convincing signal of direct CP violation in B
decays, a type of matter-antimatter asymmetry which was expected to
exist but has not been observed before. With this discovery the full
pattern of matter-antimatter asymmetries is coming together into a
coherent picture. I am very excited and pleased as one of my
postgraduate students, Carlos Chavez who is currently at SLAC, was
directly involved.” said Christos Touramanis of the University of
Liverpool.
Dan Bowerman, a member of the BABAR team from Imperial College adds
“When the universe began with the big bang, matter and antimatter
were created in equal amounts. However, all observations indicate that
we live in a universe made only of matter. So we have to ask, what
happened to the antimatter? The work at BABAR is bringing us closer to
answering this question.”
Subtle differences between the behaviour of matter and antimatter must
be responsible for the matter-antimatter imbalance that developed in our
universe. But our current knowledge of these differences is incomplete
and insufficient to account for the observed matter domination. CP
violation is one of the three conditions outlined by Russian physicist
Andrei Sakharov to account for the observed imbalance of matter and
antimatter in the universe.
Professor Ian Halliday, Chief Executive of the Particle Physics and
Astronomy Research Council which funds UK participation in BABAR said:
“We still don’t understand fully how the matter dominated
Universe we live in has evolved. However this new result, and recent
related measurements in BABAR and other experiments around the world,
have greatly advanced our understanding in this area. There is still
much to discover and learn on this fundamental issue.”
Notes for Editors
For more information on CP violation go to:
http://www2.slac.stanford.edu/tip/special/cp.htm
The results were submitted to the journal Physical Review Letters for
online publication Friday July 30th
