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Origins of the Universe

Press release issued: 12 July 2001

Physicists find a new clue to the origin of the Universe

Scientists at Bristol University are part of an international collaboration of physicists who have discovered a second fundamental difference between the behaviour of matter and antimatter during experiments at the Stanford Linear Accelerator Center (SLAC) in California, USA.

They observed this intriguing phenomenon, known as charge-parity (CP) violation, in disintegrations of heavy, short-lived subatomic particles called B mesons. The collaboration has submitted its result for publication in Physical Review Letters, a leading scientific journal.

CP violation could provide a clue as to why our universe, completely made of matter, exists. At the Big Bang, the beginning of our universe, matter and anti-matter were created in equal amounts. Soon afterwards they should have combined to form light particles.

The mysterious phenomenon of CP violation was first discovered in a 1964 experiment led by James Cronin and Val Fitch at the Brookhaven National Laboratory, USA. Their group observed this behaviour in decays of subatomic particles called K mesons, which are about one tenth as heavy as B mesons and live much longer. The two physicists shared a Nobel Prize for the discovery.

Several observations of CP violation have occurred since in experiments with K mesons. But until this new discovery, no other subatomic particles had clearly exhibited this exceedingly rare phenomenon. The discovery of this second striking example of CP violation should aid theorists trying to understand what causes it.

Princeton University physicist, Stewart Smith, spokesman of the collaboration, said: '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, thought to be responsible for the great preponderance of matter in the Universe.

'We are poised for further discoveries that should open up new directions for particle physics.'

The international collaboration includes more than 600 scientists and engineers from 73 institutions in the UK, Canada, China, France, Germany, Italy, Norway, Russia and the United States. They built and have been operating the sophisticated 1,200-ton detector, named BABAR, which was used to make the discovery.

The detector records subtle distinctions between decays of B mesons and those of their antimatter counterparts, called anti-B mesons. Both are more than five times heavier than protons and survive just over a trillionth of a second. Physicists used the detector to observe an unmistakable difference, or asymmetry, between the rates at which B and anti-B mesons decay into a special set of specific final states.

From these measurements, they calculated a parameter called sin 2b (sine two beta), which expresses the degree of asymmetry between matter and antimatter. A non-zero value of this parameter is clear evidence for CP violation among B mesons. The BABAR collaboration measured a value of sin 2b = 0.59 ± 0.14, which is substantially different from zero. The chances that the actual asymmetry is zero are now less than three in 100,000.

A group of 12 scientists from the University of Bristol have played an important part in both the construction of the detector and the analysis of the results.

'Bristol has a rich tradition in particle physics. Nobel Prize winner Professor Paul Dirac was born and educated in Bristol. Professor Cecil Frank Powell received the Nobel Prize in 1950 for his work at Bristol on the discovery of the pion. It is very exciting to be at the forefront of particle physics again,' said Dr Nicolo de Groot, Lecturer in Physics and head of the BABAR group at Bristol University.

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Copyright: 2001 The University of Bristol, UK
Updated: Thursday, 12-Jul-2001 15:02:15 BST

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