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Monday, 14 May, 2001, 16:19 GMT 17:19 UK
The secret of matter discovered
Cern
Cern: One of the labs to piece the information together
By BBC News Online science editor Dr David Whitehouse

Scientists have unravelled one of nature's best-kept secrets and, in so doing, helped to explain why we are all here.

It all comes down to something called direct Charge Parity (CP) violation and the subtle effect is crucial to nature's preference for matter over antimatter.

When the Universe was born, there should have been equal amounts of matter and its counterpart, antimatter. And shortly after the Big Bang, all the matter in the Universe should have disappeared as matter and antimatter collided, destroying each other.

But, it seems that there is a tiny difference between the two - and this left just a bit of matter behind, enough to make the galaxies, their stars, you and me. Thanks to the latest experiments, researchers now have a much clearer idea as to how these fundamental events occurred.

Real effect

To find the effect, scientists looked at the behaviour of a particular sub-atomic particle, called a neutral K meson, created for a fleeting moment in a giant atom smasher.

Following 10 years of detector development, data collection and analysis, the new result is based on the observation of 20 million of CP-violating neutral K meson decays.

The tiny difference in the decay rates of neutral K mesons and their antiparticles has been determined with a precision of one part in a million. Scientists say that the study of direct CP violation is an example of the rigour involved in the establishment of scientific facts.

The first experiments took place at the Cern laboratory in Switzerland and at Fermi National Accelerator Laboratory (Fermilab) in the US. But the initial results, published in 1993, were not precise enough to confirm that direct CP-violation was a real effect. More accurate measurements were clearly needed.

Both teams have now measured the effect with several times greater precision than their predecessors and both results conclude that direct CP violation exists.

Secrets of symmetry

The essence of Charge Parity (CP) is the concept of symmetry. Both C and P are symmetries that are conserved in most particle interactions.

C represents swapping the electric charges of all the sub-atomic particles in an interaction; in other words, swapping particles and antiparticles. P is called parity and it corresponds to looking in a mirror that reverses all three spatial co-ordinates.

Physicists once thought that both C and P were conserved in particle interactions, but in 1956 T Lee and C Yang demonstrated that P could be violated in certain interactions. However, the combination of C and P was still thought to be conserved - but this has proved not to be the case.

The predicted CP-violation was first observed at the US Brookhaven laboratory by scientists in 1964 when their Nobel Prize-winning experiment showed that particles called long-lived neutral kaons occasionally decayed into two pions, a CP-violating process.

Now they have a precise measurement of CP violation and the difference between matter and antimatter, scientists are a step closer to understanding how it was all made in the first place.

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