July 16, 2004  


Got Physics? BABAR Does!

By Kate Metropolis

If Charles Dickens were writing this story, he could have begun “It was the best of times, it was the best of times.”

They may be stressed, but they’re excited. These three BaBar postdocs (clockwise from top: Marcella Bona (INFN, Turin), Chiara Simani (LLNL, Livermore) and Mathew Graham (University of Wisconsin, Madison) typify the intensity in BaBar these days. Not only are they working on their own compelling analyses, they are also members of groups that review other analyses, and they comb the papers and presentations of others for weak spots and oversights. What they’re not doing is sleeping. (Photo by Diana Rogers)

Within a week at the beginning of July, SLAC two ‘centuries’ to celebrate: a month ahead of schedule the PEP-II accelerator met the goal of delivering 100 inverse femtobarns to the BaBar detector in the current run cycle, and the BaBar collaboration submitted its 100th scientific paper.

The most successful organisms are those with parts that work together well. The PEP-II accelerator and its attendant physicists, engineers, technicians, and operators, together with the BaBar detector and collaboration, is a very successful organism. It is orchestral in its complexity, with a huge diversity of parts and expertise.

The organism’s nutrients, in this case, B-mesons containing b or anti-b quarks, are produced by the PEP-II accelerator. As these particles  transform into other particles, the BaBar detector gobbles up the data. Several sophisticated computing systems digest the raw bits into information that physicists can use to decipher the rules of elementary particle behavior.

You can watch this organism evolve from year to year, sometimes even month to month. Evolution is driven by improvements made by grad students and postdocs and professors, technicians and engineers, code-writers, accelerator operators, computing experts, run coordinators, and the people who take shifts with the vigilance of intensive care nurses.

In the accelerator, the positron current is now 2.5 amperes, the highest in the world by 50 percent. Trickle injection, introduced during the current run, increased the number of events produced in a given time interval by 40 to 50 percent. A wealth of improvements over the years doubled, then tripled, the accelerator’s design luminosity.

BaBar, too, has become more efficient. The detector is now able to record data more than 99 percent of the time, as systems have been made more reliable. Physicists have improved their efficiency as well. In 2000 - 2002, the collaboration submitted a total of 34 papers to physics journals. In 2003 they produced an additional 47. On July 6 they submitted their 19th paper of 2004.

“The performance of the accelerator and detector is a marvelous success,” says BaBar spokesperson Marcello Giorgi, “and we have a strong and active team.”

The BaBarian equivalent of bacteria or viruses is mistakes, which can be caused by garbled or incomplete data, or by unconscious bias or oversight in analyzing them. The collaboration has a sophisticated immune system to protect itself. There are 20 people in the data quality group. Each analysis is independently checked dozens of times. In addition, the investigators are blind to the answers they’re getting until they decide they have finished, similar to the way in which the effectiveness of a new medical treatment is evaluated, so researchers don’t stop when they’ve reached a result that they expect or hope for.

Competition may be nearly as powerful a force in this kind of research as in a natural ecosystem. BaBar plans to present more than 50 new results at the International Conference on High Energy Physics in Beijing in August. Belle, an international collaboration investigating the same kinds of particle decays at the Japanese accelerator laboratory, will also be presenting new results.

A visitor to the Research Office Building day or night or on weekends will find members of BaBar pushing themselves to the limits. Wisconsin postdoc Matthew Graham is working on the process that caused excitement when Belle announced a result that has just a one percent chance of agreeing with the standard model. BaBar’s result at that time had a 60 percent chance of being in agreement with the standard model. The discrepancy could be explained by statistical chance, or it could turn out that this measurement makes this part of the standard model toast.

Both collaborations have significantly more data for this round of analyses, so it wouldn’t be a surprise to see the central values of this (and many other) analysis shift. “People who think error bars are for sissies will be disappointed,” says BaBar physics coordinator Jeff Richman.

Ultimately, as Giorgi points out, both collaborations “are on the same adventure. We hope to open a window on new physics.”



The Stanford Linear Accelerator Center is managed by Stanford University for the US Department of Energy

Last update Wednesday July 14, 2004 by Emily Ball