From Big to Small, BABAR Collaborations Work Together
Philippe Grenier, Peter Kim, Silke Nelson, Veronique Ziegler and Chris West are all breathing a sigh of relief after four months of exhausting analysis and peer review. This analysis group within the larger BABAR collaboration is celebrating last week's successful trip to the International Conference on High Energy Physics (ICHEP), where they announced the detection of the ηb particle (pronounced "eta-sub-b"). The paper announcing the ηb bottomonium results was accepted by the journal Physical Review Letters on July 15, 2008. Though most of the analysis was done by the five SLAC physicists, the author list goes on for two pages. For every one of BABAR's several-hundred collaborators, this is a job well done.
From a wide view, this large collaboration functions as a single entity. Over BABAR's eight-year run on the PEP-II ring at SLAC, every member of the collaboration has contributed to the tasks that keep an experiment of this size running. That includes working on the detector, and collecting and filtering data. But while the BABAR experiment requires support from a large number of scientists and engineers, it also relies on smaller groups within the collaboration to perform data analysis and peer review.
The analysis group for ηb includes research physicists Grenier and Kim, post-docs Nelson and Ziegler, and West, a PhD student. The team worked furiously to get their results out in time for the ICHEP. Data collection at the PEP-II ring finished in early April, and the team needed their paper complete by early July. For them, working on the small scale was essential. They met every day during the last few weeks of analysis, discussing what each person had done over the past 24 hours and what would be done until they met again.
Throughout the analysis, other members of the collaboration were available to give the group feedback and ideas. Ziegler, who has been working for BABAR for almost six years, says this is one key advantage to working within such a large collaboration. She says, "There's a lot of expertise here. Whenever you have a problem, there is always someone else who has encountered it before who can give you feedback or ideas."
The group spent those four hectic months working with the BABAR data, looking for signs that the tiny ηb particle had appeared where it was expected. The bottomoniums, like many families of high energy particles, cannot be seen directly. Rather, researchers can only hope to find signatures that the particles leave behind. It's like looking for cars using the tread marks they leave on the road. As the ηb's older sister 3S decays, it breaks apart into ηb and a photon. That photon is the tire track the analysts were seeking. To detect this photon is easy enough; what has stumped scientists is how to pick one photon out from a large group of them. There are many other photons of very similar energies that are also emitted when a 3S particle is created. Figuring out which one carries information about ηb is like looking for Mario Andretti's tread marks after the Indie 500.
The ηb analysts at SLAC try to explain the process that they went through to find this elusive photon. It mostly involves looking at a graph of a long, sloping curve that has a gentle bump in the middle. Through some thorough analysis, the researchers can zoom in to see two smaller hills next to that larger bump. The smaller two almost overlap each other. Each hill represents photons of a certain energy. "You can see," Grenier says, "how if you didn't have precise measurements, these three bumps would seem like one. But with this data and analysis you can see the signature of ηb." The group credits the incredible luminosity of the PEP-II ring with producing data that allowed them to directly observe ηb's signature.
Preparing for publication, the researchers entered the peer review process within BABAR. "First the paper goes to a review committee, where you get feedback and questions that you have to answer," says Ziegler. "Then the paper is posted for the entire collaboration to review. And then you get even more questions and comments."
"The BABAR peer review process is very thorough," says Grenier, "It’s very tough and very long. Sometimes it can take weeks." This intense process ensures that BABAR produces consistently strong results. The collaboration acted quickly to have the ηb results ready for the ICHEP conference, Grenier says. "Our review was done in a matter of days. So that means there were people probably working non-stop." After approval from the collaboration, the group immediately submitted the paper to Physical Review Letters, where it was accepted on July 15.
With BABAR's data collection complete, it may be years or even decades before analysts have folded, dissected and dug through the sea of information provided by the experiment. At this point in time, it may be impossible to say what incredible new discoveries the collaboration will find deep within the data.