From the Director of Particle Physics and Astrophysics

BaBar Continues to Mine Gold

November 16, 2012

Recently, members of the BaBar collaboration met at SLAC in one of their quarterly gatherings to organize, plan, discuss, critique and finalize physics results and publications. I had the privilege of welcoming many regular visitors to SLAC and to reflect on the outstanding achievements of this remarkable scientific enterprise. There were also quite a number of new faces in the audience, many of whom joined the collaboration recently, including new graduate students eager to mine gold in the BaBar data.

The meeting featured an announcement that the 500th BaBar paper had been accepted for publication, an outstanding accomplishment by any measure. The collaboration is on track to produce 30 papers this year alone. This scientific outpouring is all the more remarkable given that BaBar stopped taking data more than four years ago!

BaBar and the SLAC PEP-II collider, as well as its sister experiment Belle at the KEKB collider in Japan, had their origins in the late 1980s with the realization that electron-positron collisions at these “B factories” could be used to produce large numbers of B mesons and anti-B mesons in a way that would offer a chance to observe and explore differences in the behavior of matter and antimatter. Most of the forces between particles actually do not distinguish between matter and antimatter, but B mesons were predicted to have unique properties with large measurable differences, “CP violation,” in the decays of B mesons and anti-B mesons. Last produced in the early stages of the Big Bang, copious numbers of Bs and anti-Bs decaying at different rates – asymmetrically – could help explain why the universe is now dominated by matter with essentially no antimatter.

BaBar alone attracted some 600 scientists from 10 countries around the world to pursue this idea. In nearly a decade of operation at SLAC’s PEP-II collider from 1999 to 2008, the experiment accumulated one of the world’s great data sets. By 2001 it had discovered asymmetries in the behavior of matter and antimatter, although not at a level able to explain why matter now dominates the cosmos. The advances the experiment made in determining the properties of b quarks are now a cornerstone of our understanding of the Standard Model.

Since April 2008, when Babar ceased operation, the collaboration has continued to mine this uniquely versatile data sample, with its rich array of yet-to-be-explored physics. This has produced definitive results on matter-antimatter asymmetries, precision measurements of the properties of b and c quarks and tau leptons and searches for evidence of physics beyond the Standard Model in rare or forbidden processes.

Just in this last year, BaBar results have attracted considerable attention in both the scientific community and the wider public. Particularly noteworthy is a recent unique demonstration that, under the right circumstances, time has a preferred direction in the evolution of matter and antimatter. Normally, in the microscopic world of particles and forces, there is no such distinction and a film of their evolution looks the same viewed forward or backwards. However, cleverly exploiting the unique way in which B mesons are produced at the B Factory allowed BaBar collaborators to tease out remarkably convincing evidence for differences in the time evolution, “T violation,” of neutral B and anti-B mesons. Time really does have a preferred direction. Fortunately, T violation and the previously observed CP violation exactly balance and therefore preserve a fundamental foundation of modern particle theories. The story first caught the attention of the science writer at the Economist, and subsequently attracted wide public attention. Other results from BaBar this year provide hints for a breakdown of the Standard Model in B decays involving tau-leptons and limit models with additional Higgs bosons.

BaBar’s remarkable data sample will likely not be surpassed this decade. To ensure that the experiment continues to produce physics results, BaBar has broken new ground again by developing a long-term data analysis system. The system will give scientists the software tools they need to analyze BaBar data independent of the underlying computing hardware. Given the rapid evolution of technology that one can anticipate over the coming decade, making physics analysis immune to hardware changes is critical to mining more gold from the BaBar data.

Congratulations to BaBar on its 500th publication, and continued success in unraveling the mysteries of nature.