Cosmic tune-up: Cosmic rays help prime BABAR systems

The BaBar detectors normally catch B particle decays, as in the image shown left, taken during the last run. During the cosmic ray runs this winter, the detector saw tracks made by cosmic rays like the event shown right, recorded in December 2004. (Image Courtesy of BaBar) Click here for a high resolution photograph.

Cosmic rays harmlessly stream through everything on Earth--our bodies, the scintillator counters in the Visitor's Center and the BaBar detectors.

Normally, BaBar filters out cosmic rays to reduce background noise. However, the Collaboration uses cosmic rays to check out the detectors before starting a new run--and even in the middle of a long run. Experimenters turned to cosmic rays for extra tune-ups and tests of new equipment during the unexpected downtime that began mid-October, just a week before BaBar was to start taking data after a scheduled downtime.

"Our detectors are sensitive to things going through all the time," said Run Coordinator Tom Meyer (Iowa State University). "We can really exercise the entire detector and data acquisition systems and check them out using the cosmic rays so we can be all ready when the beams arrive."

Earthly cosmic rays come mostly from protons in outer space. When the protons hit the air in our upper atmosphere, the interactions produce a shower of particles, many of which decay to muons that live long enough to reach the Earth's surface.

During the recent cosmic ray experiments, naturally occurring cosmic rays activated detector triggers some 200 times a second. The triggers are what determine whether activity seen in the detectors is experimentally interesting. The triggers are usually set to throw away cosmic ray tracks, as well as events that take place away from the interaction point where the electron and positron beams meet.

BaBar's detectors are like layers of a cylindrical onion, with the beams meeting at the center. The vertex detector is the first layer, surrounded by the drift chamber, the DIRC detector, the calorimeter and finally the Instrumented Flux Return (IFR), which identifies and measures muons (usually the ones generated by particle collisions). The IFR detector gets tested with cosmic rays every two weeks during a regular run.

The BaBar detectors normally catch B particle decays, as in the image shown left, taken during the last run. During the cosmic ray runs this winter, the detector saw tracks made by cosmic rays like the event shown right, recorded in December 2004. (Image Courtesy of BaBar)

In 2004, BaBar underwent upgrades during its scheduled downtime from July through early October. Two of six sections of the IFR were replaced with a completely new technology.

"Checking that out with cosmic rays has been very valuable," said Meyer. The cosmic ray tests allowed researchers to see that the new systems worked, and to wear them in, like a new pair of jeans.

Another upgrade involved reprogramming drift chamber electronics to more efficiently handle, in the short term, the flood of data pouring though the detectors.

"Everything that folks have done in simulation looks like it will work," said Technical Coordinator Bill Wisniewski (BaBar). "But does it? Originally, we would have taken data and fixed any problems on the fly. We now have the cosmic ray run to reassure ourselves. The drift chamber electronics upgrades look good."

For the long term, the collaboration is developing new readout electronics boards for the drift chamber, to be installed during a downtime in 2005. Researchers were able to take a prototype out of the lab and test it during the last week of cosmic ray running, which ended February 4.

"We were seeing how it performs in battlefield conditions," Meyer said. "It's looking great. It gives the team confidence to go ahead with full production."

Wisniewski added, "The drift chamber electronics developers are doing a fantastic job. It's been a welcome luxury to be able to check prototype electronics on the drift chamber before the beams come on."

Faced with extra downtime, the BaBarians have been making small adjustments and improvements in addition to the scheduled upgrades.

"We're walking a fine line between improving the detector and trying to maintain readiness," said Meyer. "Cosmic rays are a good way to verify we really have improved things."

Meanwhile, the Lab is moving forward with the validation process to turn on the linear accelerator and PEP-II to deliver beams to BaBar again. With a reconfigured schedule, collaboration Spokesperson David MacFarlane still expects BaBar can double its data set by the 2006 summer conference season.