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Stanford Report, April 30, 2003

Experiment identifies new subatomic particle, adding to understanding of matter


Physicist Antimo Palano, representing the BaBar experiment, presented the evidence for the identification of a new subatomic particle named Ds (2317) to a packed auditorium on April 28 at the Department of Energy's Stanford Linear Accelerator Center (SLAC). Initial studies indicate that the particle is an unusual configuration of a "charm" quark and a "strange" anti-quark. A scientific paper was sent for publication in Physical Review Letters on April 11.

"Congratulations to BaBar!" said SLAC Director Jonathan Dorfan, referring to the multinational collaboration to study the nature of matter and anti-matter. "The discovery of this new particle underlines the vigor of our high-energy physics research program and is a concrete justification for the Department of Energy's support of fundamental physics research. The existence of the particle is not a surprise, but its mass is lower than expected. This result will send theorists back to their drawing boards."

"The BaBar experiment continues to produce important new knowledge adding to our fundamental understanding of the structure of matter," said Raymond L. Orbach, director of the Office of Science at the U.S. Department of Energy. "This impressive accomplishment reflects the success of a strong team of professionals at SLAC -- accelerator physicists, computer specialists, engineers, administrators and many others -- whose efforts were essential for the result. I would like to congratulate them all on their combined achievement."

Six types of quarks are present in nature. The "up" and "down" quarks are the lightest and are found within the nuclei of atoms of ordinary matter. Heavier quarks were present in the early universe and are created today in particle accelerators and in collisions of cosmic rays with atoms in the Earth's atmosphere. SLAC has a long tradition of discovery in the physics of charm quarks, beginning with the Nobel Prize-winning discovery of the first such particle, a charm-anticharm system, in 1974. Ds (2317), the new particle, combines a charm quark with another heavy quark -- an anti-strange -- and has unexpected properties that will provide insight into the force that binds the quarks together. This force, unlike most others in nature, becomes stronger as the distance between the two quarks increases.

"This is an important achievement for BaBar," said collaboration leader Marcello Giorgi of Istituto Nazionale di Fisica Nucleare (INFN) and the University of Pisa. "We have discovered a new charm particle in an experiment designed to probe the difference between matter and antimatter using 'bottom' quarks. Sometimes the most exciting discoveries come from unexpected directions. There has been a buzz of excitement in the experiment in the past few weeks as we performed all possible checks to verify the reality of this new intriguing particle. By carefully studying the experimental data taken since 1999, we have been able to publish a precise measurement of its mass -- 2317 mega electron volts. SLAC's accelerator people have done a fantastic job in providing us with all this data."

Said Bob Cahn, a BaBar collaborator from Lawrence Berkeley National Laboratory: "The unexpected mass will make us look again at the forces between quarks and will stimulate new interest in charm-quark systems."

More than 500 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.