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Counting B Particles

How many B mesons does the B-factory make?

The BaBar detector has measured about 760 million of them so far. We know this because Chris Hearty of the University of British Columbia counts them.

The number matters to nearly all of BaBar's physics results. Most analyses work with a total number of events. The published result tells what fraction of particles decayed in a specific way. Some decays are common, some exceedingly rare.

To come up with his number, Hearty actually counts the overall number of Upsilon 4S events, each one made from a b quark and an anti-b quark. It decays into two B mesons, which in turn fall apart in hundreds of different ways that are examined by the BaBar collaboration. Papers usually quote the number of Upsilon 4S events—about 380 million to date.

The PEP-II rings are usually set to produce collisions at 10.58 GeV (giga electronvolts), just the right energy to make the Upsilon 4S.

"About three percent of what our triggers pick up is Upsilon 4S; 97 percent is other stuff: non-b quark pairs, lepton pairs, two photon events and garbage," said Hearty.

The counting strategy is to run PEP-II off-peak, taking 10 percent of BaBar's data at 10.54 GeV. Collisions at this lower energy still make everything normally seen in the detector, except Upsilon 4S. Hearty counts the total number of a distinctive pair: muon+ muon-, and divides that into the total number of events.

"This tells you that for every muon pair, you have say, five other quark anti-quark events that aren't Upsilon 4S," Hearty said.

This ratio, garnered from off-peak data, is applied to the on-peak events to determine the total number of Upsilon 4S events.

Hearty and his graduate student Grant McGregor are working to reduce the systematic error in the counting technique. When you study a rare decay with only 16 events, a systematic error of 1.1 percent is meaningless in the face of the giant statistical error. But some BaBar analyses now demand a smaller systematic error, which Hearty thinks can be achieved.

"This will be a challenging master's thesis for Grant," he admitted.

—Heather Rock Woods
    SLAC Today, July 24, 2006