When the mysterious "Project M" was conceived in 1956, the idea of a two-mile linear accelerator was just a dream. But once the Atomic Energy Commission and Stanford University signed the Stanford Linear Accelerator Center (SLAC) into existence five years later, construction began on a unique facility that would continue conducting high-energy physics research for decades, generating three Nobel prizes along the way and continuing to break new ground over forty years later.
BaBar detector at Stanford Linear Accelerator Center. (Photo Courtesy of Stanford Linear Accelerator Center)
In August, physicists at SLAC announced new results demonstrating a dramatic difference in the behavior of matter and antimatter. The BaBar experiment, conducted at SLAC's "B-Factory" facility, collided 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 anti-matter are short-lived, decaying in turn to other lighter subatomic particles, such as kaons and pions, which are observed 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," said BaBar spokesman Marcello Giorgi, a physicist at the Instituto Nazionale di Fisica Nucleare (INFN) and the University of Pisa. "However, our new measurement shows an example of a large difference in decay rates instead."
In fact, BaBar experimenters have found striking matter-antimatter asymmetry, with over 23% more of recorded B mesons decaying than anti-B mesons. While BaBar and other experiments have found behavioral asymmetries before, this is the first difference obtained by simply counting up the number of matter and antimatter decays, a phenomenon known as direct charge parity violation.
Understanding differences in the behavior of matter and antimatter is important to scientists because it is key to building a coherent picture of the earliest moments of the universe.
"When the universe began with the Big Bang, matter and antimatter were present in equal amounts," said Giorgi. "But all observations indicate that we live in a universe made only of matter. So what happened to the antimatter? With experiments such as this, we are beginning to come up with possible explanations."
"The new result[s] from BaBar...continue to improve our understanding and may ultimately tell us why the visible universe is only matter."
-Raymond Orbach, Director, Office of Science, Department of Energy
Subtle differences between the behavior of matter and antimatter must be responsible for the imbalance that developed in our universe, but our current knowledge of these differences is incomplete and insufficient to account for the observed matter domination. Charge parity violation such as the one observed at SLAC is one of the three theoretical conditions outlined to account for this observed imbalance.
The B-Factory facility was opened in 1999 as a collaborative project between SLAC, the Lawrence Berkeley National Laboratory, and the Lawrence Livermore National Laboratory. CCST played an important role in bringing the $177 million B-Factory to California.
"Very early on, [CCST] gave the project high priority and brought it prominently to the attention of the Governor's Office and the California Congressional Delegation, whose strong support was essential to our success," said Burton Richter, SLAC director emeritus and CCST fellow.
"This is another great scientific achievement for SLAC," said Raymond Orbach, director of the Department of Energy's Office of Science and a former CCST fellow. "The new result from BaBar, and related measurements at other accelerators around the world, continue to improve our understanding and may ultimately tell us why the visible universe is only matter."
