Media Contact:
Julia Maddock, PPARC Press Office
01793 442094
Julia.maddock@pparc.ac.uk
issued by the Particle Physics and Astronomy Research Council
August 2, 2004, Swindon, England -- Particle physicists from the UK and around the world working on the BABAR experiment at the Stanford Linear Accelerator Center (SLAC) in the USA, announced exciting new results today demonstrating a dramatic difference in the behaviour of matter and antimatter. Their discovery may help to explain why the Universe we live in is dominated by matter, rather than containing equal parts matter and anti-matter.
SLAC's PEP-II accelerator collides electrons and their antimatter counterparts, positrons, to produce an abundance of exotic heavy particle and anti-particle pairs known as B and anti-B mesons. These rare forms of matter and antimatter are short-lived, decaying in turn to other lighter subatomic particles, such as kaons and pions, which can be seen in the BABAR experiment.
"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." said Marcello Giorgi, of SLAC, Pisa University and INFN, Spokesman of BABAR.
By sifting through the decays of more than 200 million pairs of B and anti-B mesons, experimenters have discovered striking matter-antimatter asymmetry. "We found 910 examples of the B meson decaying to a kaon and a pion, but only 696 examples for the anti-B", explained Giorgi. "The new measurement is very much a result of the outstanding performance of SLAC's PEP-II accelerator and the efficiency of the BABAR detector. The accelerator is now operating at 3 times its design performance and BABAR is able to record about 98% of collisions."
While BABAR and other experiments have observed matter-antimatter asymmetries before, this is the first time that a difference has been found by simple counting of the number of decays of B and anti-B mesons to the same final state. This effect is known as direct CP violation and is found to be 13%; a similar effect occurs for decays of Kaons and antiKaons but only at the level of 4 parts in a million!
"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. With this discovery the full pattern of matter-antimatter asymmetries is coming together into a coherent picture. I am very excited and pleased as one of my postgraduate students, Carlos Chavez who is currently at SLAC, was directly involved." said Christos Touramanis of the University of Liverpool.
Dan Bowerman, a member of the BABAR team from Imperial College adds "When the universe began with the big bang, matter and antimatter were created in equal amounts. However, all observations indicate that we live in a universe made only of matter. So we have to ask, what happened to the antimatter? The work at BABAR is bringing us closer to answering this question."
Subtle differences between the behaviour of matter and antimatter must be responsible for the matter-antimatter imbalance that developed in our universe. But our current knowledge of these differences is incomplete and insufficient to account for the observed matter domination. CP violation is one of the three conditions outlined by Russian physicist Andrei Sakharov to account for the observed imbalance of matter and antimatter in the universe.
Professor Ian Halliday, Chief Executive of the Particle Physics and Astronomy Research Council which funds UK participation in BABAR said: "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. There is still much to discover and learn on this fundamental issue."
The results were submitted to the journal Physical Review Letters for online publication Friday July 30th
The Stanford Linear Accelerator Center is a United States Department of Energy laboratory operated by Stanford University.
Some 600 scientists and engineers from 75 institutions in Canada, China, France, Germany, Italy, the Netherlands, Norway, Russia, the United Kingdom, and the United States are working on BABAR.
UK institutions involved are: Universities of Birmingham, Bristol, Brunel, Edinburgh and Liverpool, Imperial College, Queen Mary University of London and Universities of Royal Holloway, Manchester and Warwick and the CCLRC Rutherford Appleton Laboratory.
UK participation in BABAR UK involvement with BABAR is funded by the Particle Physics and Astronomy Research Council (PPARC). UK groups have developed and built, and now operate, vital parts of the BABAR detector such as the forward electromagnetic calorimeter (photon detector in simple terms) and the trigger and online systems (advanced computing hardware and software that is used to select and record the collision events that are of interest for physics). They have also been involved in analysing the data received and producing large simulations of the expected data which are necessary to interpret the results.
PPARC The Particle Physics and Astronomy Research Council (PPARC) is the UK's strategic science investment agency. It funds research, education and public understanding in four broad areas of science - particle physics, astronomy, cosmology and space science.
PPARC is government funded and provides research grants and studentships to scientists in British universities, gives researchers access to world-class facilities and funds the UK membership of international bodies such as the European Organisation for Nuclear Research, CERN, the European Space Agency and the European Southern Observatory. It also contributes money for the UK telescopes overseas on La Palma, Hawaii, Australia and in Chile, the UK Astronomy Technology Centre at the Royal Observatory, Edinburgh and the MERLIN/VLBI National Facility.
PPARC's Public Understanding of Science and Technology Awards Scheme provides funding to both small local projects and national initiatives aimed at improving public understanding of its areas of science.