June 29, 2001

Lab helps SLAC get to the heart of the matter

The search to resolve a long-standing mystery of particle physics has reached a milestone with publication of the first results from the B Factory experiment at Stanford University’s Linear Accelerator Center (SLAC).

Lab scientists played a key role in analyzing the data from the experiment designed to resolve the mystery of why there is more matter than antimatter in the universe. The "BABAR" collaboration of more than 500 physicists at 72 institutions in nine countries published their first results in the March 19 edition of Physical Review Letters.

Preliminary, but unpublished, results were announced last summer at a meeting in Osaka, Japan (see the Sept. 15, 2000 edition of Newsline).

"This is very exciting," said Doug Wright, principal investigator for LLNL’s B Factory team. "We have more than doubled our data set since last summer and substantially improved the analysis. We’ve achieved the world’s best measurements by far."

The B Factory experiment studies the decay of subatomic B mesons, a heavy and rare form of nuclear matter. By measuring the asymmetries in time distributions between the decay of B and anti-B mesons with great precision, scientists will be able to confirm or refute the currently accepted theory about quark interactions and possibly find evidence for new physics not predicted by current theory.

Cosmological models that result in a universe dominated by matter, such as ours, require the existence of a process called charge-parity, or simply "CP," violation. Only one CP violation process has ever been discovered (in the 1960s) but the theoretical mechanism invented to explain it has, until now, remained untested; and the observed effect is too small to account for the dominance of matter in our universe.

"Current theory is either incorrect, or the CP violation required by cosmology remains undetected," Wright said.
B Factory experiments consist of very sophisticated subatomic sleuthing with scientists seeking rare, often difficult to detect B meson decay events generated in the PEP-II collider and measured with the BaBar detector. Only one in 10,000 events "is an interesting B decay."

The experiment can observe CP violation in two modes, CP- and CP+. A clear and consistent signal in both modes would be an impressive discovery. Of the two modes, CP+ is much more difficult to "extract from background" because other physics processes mimic the signal.

Not only has the BABAR collaboration been able to collect more data, they’ve also been able to improve the analysis, thanks in good measure to the work of postdoctoral physicists David Lange and Vuko Brigljevic. "Finding the CP+ events and extracting the asymmetry is very tricky," Wright said. "These guys made it happen."

All three agree that the results will only get better as the luminosity, or collision rate of the PEP-II accelerator beams, is increased. "If the luminosity of the accelerator is turned up, things will be much clearer and we may start to see evidence that will confront theory," Brigilevic said, explaining that more data not only adds to the signal, but also allows the physicists to better understand the detector and improve the analysis.

Wright adds, "We think we’re onto something, not just because of our data, but data coming from our other experiments as well."

Fueling the intensity of the BaBar collaboration’s effort is the work of a rival Japanese-based group, which also published its first results in the March issue of Physical Review Letters.

While the current data is insufficient to draw any definitive conclusions, preliminary results hint at a CP- violation signal. However, these tantalizing hints are not solid enough to impinge on the integrity of the theory that has been "ironclad" for the last 30 years.

"There’s something wrong with this theory, but we don’t know what it is," said Lange. "We may get a hint in our data, which is essentially at low energy, of new physics that should appear at higher energies."

Wright describes the B Factory experiment as a "project that bridges the gap until the next generation of accelerators, such as the Large Hadron Collider and Next Linear Collider, that can explore directly the physics at higher energies."
"Our results are very promising," he said. "If everything goes as well as we hope, we should be able to make a definitive measurement within a few years."