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News Release

July 19, 2004



Shedding light on luminosity

What on earth is a "femtobarn," and what does it have to do with the amount of data an accelerator produces?



"Barn" is easier to say than "Oppenheimer" and does not, like "Bethe," sound like the second letter of the Greek alphabet. In June 1943, the barn began appearing in internal technical reports at the secret laboratory at Los Alamos.

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.



In this case, what physicists call the integrated luminosity is 2 tomatoes per square foot (or, in physics jargon, 2 "inverse square feet").

So, 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), and you get 110,000,000 events.

Now, the only question left is why physicists make something so simple sound so complicated.


Editor Note:

This release is the sidebar to a main story titled "Antimatter Experiment hits a key milestone." Its accompanying image, by science illustrator Alan Chou, is available at

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