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Thursday, May 6, 1999 Published at 13:34 GMT 14:34 UK


Matter or antimatter, that is the question

BaBar: Looking for the key

By BBC News Online Science Editor Dr David Whitehouse

A new experiment is designed to answer the most fundamental question about our Universe - why it is made of matter and not antimatter.

BBC Science Correspondent Pallab Ghosh reports from California
The BaBar experiment at the Stanford Linear Accelerator Center in California will start work on Sunday. Beams of matter and anti-matter will be smashed into each other and the fleeting debris of the collisions examined.

Just days later a similar Japanese experiment, Belle, will also begin. Next year an upgrade to the world's most powerful particle machine, the Tevatron at Fermilab near Chicago, will also begin work on the problem.

All the instruments will investigate the tiny differences between matter and anti-matter.

Great puzzle

One of the great puzzles of the universe is why it is mostly made of one kind of matter instead of equal amounts of matter and anti-matter.

[ image: The experiments should start at the weekend]
The experiments should start at the weekend
Matter and anti-matter are counterparts. Bring them together and they annihilate each other in a burst of energy.

It is believed that the cosmos was formed with equal amounts of matter and anti-matter but today the universe is overwhelmingly made of matter. Anti-matter is rare.

Results reported earlier this year from Fermilab suggested that matter and anti-matter are not after all identical "mirror images" of one another.

This could explain why all the anti-matter that existed at the Big Bang has disappeared.

The phenomenon they think they spotted is technically called direct Charge-Parity (CP) violation. It means that particles behave differently if you swap matter for anti-matter and also swap left and right.

[ image:  ]
Physicists say that this "asymmetry" would have been important during the first moments of the Big Bang and may have resulted in almost all anti-matter being destroyed.

The observation of direct CP violation is an exciting one for physicists as it disagrees with all the currently held theories about the nature of matter. BaBar and Belle will look further into this puzzle.

The race is on

At BaBar intense beams of electrons and their anti-matter equivalent, positrons, will be smashed into each other.

BaBar is the work of more than 600 scientists and engineers
In the energy liberated during the collision, a short-lived particle called a B-meson is created as well as its anti-particle, the B-bar (physicists put a line - or bar - over the symbol to represent an antiparticle).

After about one thousandth of a billionth of a second, these particles decay into different particles. Measuring the details of this decay process will reveal the intracacies of CP violation.

Scientists position large detectors around the point of impact of the electron-positron beam to measure this.

[ image: Fermilab: Also in the race]
Fermilab: Also in the race
The six by six metre detector surrounds the impact point and captures the decay particles. It produces a computer display showing where the B and B-bar particles are produced and charts the millimetre they travel before they turn into other particles.

Looking for CP violation however will not be a swift task. It is estimated that BaBar will have to take data for at least two years before its measurement will be accurate enough.

By that time other teams, such as those at the Japanese Belle detector and the Tevatron at Fermilab near Chicago may have got the answer first.

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