Just a quick update, with a more detailed one to come early next week.
Last Friday we attended the celebration of a Tevatron milestone, complete with champagne and speeches: they had achieved One Inverse Femtobarn.
That's what we thought. We asked a few physicists there "so, what is luminosity? And what's an inverse femtobarn?" We got a lot of "uhh, well... um... kind of complicated... let's just say it's a lot of collisions..." etc. In other words, they didn't want to get into a 30 minute lecture on something too complicated for us to understand.
As you hopefully know by now (if not, rest assured in the next couple of posts I'll be giving a layman's primer on how a supercollider works), Fermilab smashes protons together and looks at what happens from the collision. As I've said earlier, scientists hate exceptions and love trends, so the more collisions the better to base their conclusions on. And, of course, they count (or rather, a network of computers counts) the number of collisions they achieve.
In this "run," which is called "Run II" and started in June 2001, they have now achieved a huge amount of collisions. Millions. But, rather than just listing the number of collisions, they use a very complicated method of measuring, which I will attempt to summarize here, quoting liberally from an article in the Stanford newspaper (http://news-service.stanford.edu/news/2004/july21/femtobarn-721.html)
At dinner one night in December 1942, physicists M. G. Holloway and C. P. Parker were lamenting the lack of a catchy unit name for discussing the size of an atomic nucleus of uranium. They considered naming a unit of this area "the Oppenheimer" or "the Bethe," after physicists leading a project involving uranium cross sections.
Since Holloway and Parker were on the campus of Purdue University in Indiana, the barn, a dominant feature of Midwestern U.S. farmlands, naturally came to mind.
This is appropriate for Fermilab, since there are lots of barns on the property. Some of them, in fact, were imported from other farm locations. And don't forget the buffalo.
So, we know where the "barn" of "femtobarn" comes from. Now for the "femto."
Start with a half. We all know what that means. Then, a quarter, an eighth, a 100th, etc. Remember scientific notation? 10 to the negative 2 means you move the decimal to the left by 2 places, resulting in 0.1, or one tenth. "Femto" means a factor of 10 to the negative 15th: or 0.00000000000001, or a thousandth of a millionth of a millionth. A femtobarn, then, is a thousandth of a millionth of a millionth of the nucleus of an atom of uranium. Said another way, that's 10 to the negative 39 square centimeters --- an incomprehensibly small unit of area.
Still with me? Here comes one of those great scientific analogies that can make everything make sense.
Imagine you throw enough tomatoes at a barn to get an average of two tomato hits per square foot. If the barn door is 10 feet by 15 feet, then the cross section for tomato-barn door interactions is 150 square feet, and the number of tomatoes that splat on the door is given by:
150 square feet x 2 tomatoes per square foot = 300 tomato interactions.
So, that's what Fermilab does, except it replaces square feet with femtobarns and tomatoes with protons.
I don't really get it either. Let's just say it's a lot of collisions. (I invite Dr. Oreglia, our science advisor, to comment).
More later, including our third attempt at getting our wireless video camera aloft tomorrow and hopefully meeting the new incoming director, Pierre Odonne.