After 37 years of searching for further examples of CP violation, physicists now know that there are at least two kinds of subatomic particles that exhibit this puzzling phenomenon.

Stewart Smith, Princeton University

At the time of the Big Bang, the event which most researchers believe marked the beginning of the Universe, there should have been equal amounts of matter and its opposite, antimatter.

Shortly afterwards, the two should have combined and destroyed each other.

But an intriguing phenomenon known as charge-parity (CP) violation shows that there are differences between matter and antimatter that may account for the continued existence of the Universe.

Mesons and anti-mesons

Russian theorist Andrei Sakharov suggested in 1967 that CP violation could explain the existence of our matter-dominated Universe.

Slac's accelerator produces B mesons and their antiparticles [Graphic: Slac]

But until recently, there were only two pieces of evidence for CP violation: firstly, the fact that the Universe exists at all, and secondly that there are minute differences between the decay rates of tiny particles called neutral K mesons and their corresponding antiparticles.

Earlier in 2001, researchers at Cern in Switzerland and Fermilab in the United States published accurate measurements of these differences.

Now, a team collaborating at Stanford Linear Accelerator Center (Slac) has found CP violation at work in a heavier particle/antiparticle pair: the B meson and anti-B meson.

37-year search

"After 37 years of searching for further examples of CP violation, physicists now know that there are at least two kinds of subatomic particles that exhibit this puzzling phenomenon," collaboration spokesman Stewart Smith of Princeton University said.

"We are poised for further discoveries that should open up new directions for particle physics," he added.

More than 600 scientists and engineers, many based in the UK, designed, built and ran a 1,200 tonne detector called Babar at Stanford and used it to make the discovery.

The Slac team have submitted their findings for publication in the journal Physical Review Letters.

Follow up

Professor Tatsuya Nakada of the European particle physics institute Cern said that his institute would be following up the work when it switches on its Large Hadron Collider in 2006.

The Stanford results back up the Standard Model used by physicists to explain the Universe, and Cern hopes to extend it.

"The Standard Model cannot produce enough CP violation to explain the total dominance of matter in the Universe," he told BBC News Online.

"The team at Slac looked at one mode of decay of the B meson.

"We will be able to look at different modes of decay of B mesons and more than one type of B meson," he said.