Sunday July 25, 2004
Rich and I have learned a new unit of measurement - the inverse femtobarn.
What on earth is a "femtobarn," and what does it have to do with the amount of data an accelerator produces?
Rich picked up a copy of the Stanford Report, a daily Stanford newspaper. In the paper was an article entitled Understanding luminosity through 'barn', a unit that helps physicists count particle events.
A barn is the unit used by nuclear physicist for the size of an atomic nucleus of uranium.
The cross-sectional area of a uranium nucleus is about 10 -24 square centimeters, small on the human scale, but large compared with other atomic nuclei. "Femto" means a factor of 10-15: a thousandth of a millionth of a millionth. A femtobarn, then, is 10-39 square centimeters an incomprehensibly small unit of area.
The term "barn" was coined by two physicists, M. G. Holloway and C. P. Parker, at Purdue University in December, 1942.
Scientists on the BaBar project (at SLAC and around the world) use B meson particle data from the PEP-II accelerator:
In early July, the PEP-II accelerator ... reached a new milestone: It is delivering three times as many particle collisions per second as the machine was designed to produce.Here's where the inverse femtobarn comes in
Researchers designed PEP-II to collide electrons and their antimatter counterparts, positrons, at the precise energy that produces an abundance of short-lived pairs of particles and antiparticles called B mesons, which decay spontaneously into other particles of matter and antimatter. Because the B meson is relatively heavy, it can decay into matter and antimatter in more ways than lighter particles can. If there were no difference between matter and antimatter, both the B meson and the anti-B meson would decay at exactly the same rate.
Some decay patterns are very rare. The BaBar collaboration has seen some decays only a few times in 10 million events. Were it not for the multitude of B mesons PEP-II is providing, studies of such unusual particle behavior would be impossible.
...to calculate how many B-meson pairs PEP-II had delivered by July 1, you multiply the cross-section for those events (1.1 million femtobarns) by the integrated luminosity (100 events per femtobarn),(that's 100 inverse femtobarns)
and you get 110,000,000 events.