From theory to certainty: BaBar announces new result on charge parity violation

July 23, 2002

At the International Conference on High Energy Physics in Amsterdam on July 25, the BaBar collaboration working at the Department of Energy's Stanford Linear Accelerator Center (SLAC) announced their new measurement of a parameter known as sine two beta, that expresses the degree of asymmetry between matter and antimatter. This result is the product of three years of intense research and is a classic example of how understanding of our universe advances through high energy physics research.

"This research at SLAC is a wonderful demonstration of the scientific method," said Raymond Orbach, director of the Department of Energy's Office of Science. "What was a theoretical prediction until recently, has been measured with exquisite accuracy by the BaBar experiment, providing a solid platform of understanding to allow physicists to formulate the next question. This is how science advances."

Something is missing in physicists' understanding of how our universe evolved into its current state. At the Big Bang, equal quantities of matter and antimatter should have been created, and subsequently annihilated each other leaving nothing but energy. However the matter universe is here as undeniable proof of the victory of matter over antimatter in this initial cosmic encounter. To establish experimentally the effect that allowed matter to dominate has been a central theme in high-energy physics research. The BaBar experiment at SLAC and the Belle collaboration at the KEK laboratory in Japan have been working relentlessly to tie down an exact measurement of this effect called Charge Parity (CP) violation.

Both experiments follow the infinitesimally short lives - a trillionth of a second - of particles called B mesons and those of their antimatter counterparts, called anti-B mesons or "B-bars." Any difference in behavior of these otherwise exact opposites indicates a difference between matter and antimatter and confirms the existence of CP violation. The first results, announced in the summer of 2001 (SLAC Press Release July 26, 2001, BaBar Physicists Find a Striking Difference Between Matter and Antimatter) gave clear evidence for CP violation in B mesons.

"This was a major discovery but much more data was necessary to turn sine two beta into a fundamental constant of particle physics," said BaBar science team leader Stewart Smith of Princeton University. "The new result from BaBar for sine two beta is 0.74 ± 0.07. This result comes after three years of intense research and analysis of 88 million events. It is the fruit of tremendous effort from the 500 collaborators on the BaBar experiment and the excellent professional abilities of the SLAC accelerator team. The accelerator performed remarkably, allowing us to do outstanding physics, and the conditions at SLAC were ideal for conducting world-class science."

The new measurement from BaBar fits the theoretical expectations based on the "Standard Model" that explains subatomic particles and their interactions. "Working on BaBar has been tremendously exciting," said Hassan Jawahery of University of Maryland, the experiment's physics coordinator. "Only three years ago CP violation in B mesons was just a plausible scenario, but now the precision of our results has anchored CP violation as one of the foundations of the standard model. This is the beauty of doing science. We have moved from obscurity to clarity in a very short time and have created knowledge that will remain forever."

The millions of B and anti-B meson events that the physicists needed to hone the accuracy of their measurement were produced in collisions between beams of electrons and their antimatter counterparts, called positrons, in storage rings called PEP-II.

"PEP-II and BaBar completed the longest known run of data collection at a colliding beam facility ever--18 months of exceptional performance," SLAC director Jonathan Dorfan said. "PEP-II delivered its aggressive 100 million events goal, set when the accelerator was first switched on in July 1999. This is a remarkable achievement for such a 'young' machine. BaBar logged over 95% of the PEP-II data - an unprecedented achievement by a detector of that complexity. Sustaining that level of performance for 18 months was heroic."

The precision of BaBar's new result derives from the huge amount of data flooding out of the detector. The experiment uses the largest database in the world (SLAC Press Release April 12, 2002, World's Largest Database Reaches 500,000 Gigabytes).

"At SLAC, a Department of Energy Laboratory, we have created a truly international scientific collaboration," commented Smith. "The success of BaBar is based on the power and efficiency of collaborative science. More than 500 scientists and engineers from 75 institutions in Canada, China, France, Germany, Italy, Norway, Russia, the United Kingdom, and the United States are working on BaBar. Great advances recently in our ability to distribute data around the collaboration have had enormous impact. Our data is now analyzed not only in the United States, but at linked major computer centers in France, Italy and the United Kingdom as well."

The accuracy of the CP violation measurements coming from BaBar and Belle has established the magnitude of the effect beyond doubt. However, this knowledge shows that the degree of CP violation now confirmed is not enough on its own to account for the matter-antimatter imbalance in the universe.

"Something else happened in addition to CP violation to create the excess of matter that became stars, planets, and living creatures," said Jawahery. "In the future, the BaBar experiment allows us to examine rarer processes and more subtle effects that will give us an even clearer understanding and may point us towards the processes which caused our universe to evolve into its current state. It is a very exciting prospect."
Relevant Web URLs:
BaBar Collaboration:
BaBar database:
Photos of B Factory and BaBar detector:

DOE/SLAC National Accelerator Laboratory

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