|
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.”
|
