International collaboration offers clues to origin of matter in the Universe - demonstrating a fundamental difference between matter and anti-matter

Brussels, 03 Aug 2004

Wherever you are, a quick glance around will be sufficient to confirm that the world in which we live is made up of nuclear matter - protons, neutrons and electrons. For some time, however, scientists have been aware of the existence of antimatter particles, which appear identical to matter particles except that when in close proximity to each other they both annihilate.

Our current understanding suggests that moments after the Big Bang, matter particles and antimatter particles were created in equal amounts. Given their tendency to annihilate with each other we would expect no particles to remain, yet theorists argue that due to subtle differences in the behaviour of matter and antimatter, known as CP violation, for every billion annihilations that occurred, one matter particle survived - a tiny imbalance maybe, but enough to create everything in the visible Universe.

Exactly what causes CP violation is still unknown, but hundreds of scientists around the world are currently trying to address the issue, and a recent breakthrough by one of the largest collaborations may significantly contribute to finding the answer by demonstrating a fundamental difference in the behaviour of matter and antimatter.

The BABAR experiment involves 600 scientists from 75 institutions around the world, including France, Germany, Italy, the Netherlands, Norway, Russia and the UK. Using equipment at the Stanford Linear Accelerator Centre (SLAC) in the US, the BABAR researchers are able to collide electrons and their antimatter counterparts, positrons, to produce millions of particle and anti-particle pairs known as B and anti-B mesons. These pairs are short-lived, and quickly decay into lighter subatomic particles, such as kaons and pions, which can be detected by the scientists.

Marcello Giorgi, from the Italian national institute of nuclear physics and spokesperson for BABAR, explains: '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.'

By analysing the decay patterns of more than 200 million pairs of B and anti-B mesons, the researchers discovered a striking asymmetry. 'We found 910 examples of the B meson decaying to a kaon and a pion, but only 696 examples for the anti-B,' said Mr Giorgi. This is the first time that such an asymmetry has been observed simply by counting the number of decays of B and anti-B mesons to the same final state, and the effect is known as direct CP violation.

'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,' added Christos Touramanis, principal investigator for BABAR at the University of Liverpool. 'With this discovery, the full pattern of matter-antimatter asymmetries is coming together into a coherent picture.'

Professor Ian Halliday, chief executive of the UK Particle Physics and Astronomy Research Council, one of the co-funders of BABAR, concluded: '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.' Professor Halliday added, however: 'There is still much to discover and learn on this fundamental issue.'

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