Thursday, June 30, 2005

tomatoes, barns, and champagne

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 (

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.


Professor said...

So, I read this Friday morning and it made my head hurt. Tried reading it today and again, my head hurt.

I guess my question is, why do you hate me so much? What did I ever do to you?

Mark Oreglia said...

Okay, the concepts of cross section and luminosity can make your head hurt,
but they are really simple ... the confusion arises because they are jargon.

What the particle colliders do is create two bunches of protons
(or 1 bunch of protons and 1 bunch of antiprotons) and launch them at each other.
Now, focus on some reaction that is interesting, like p+p -> 2 top quarks + stuff.
How many of these reactions occur after a year of operating the accelerator?
It will depend on 2 things:
1) how dense the bunches of protons are;
2) how probable the particular reaction (top quark production) is.

The probability of achieving the particular reaction is calculable from
the intrinsic laws of physics that describe the reaction; is is called the "cross section",
and is measure in units of area -- but don't be confused by this; the choice if units is
just a convention we created. The density of the protons in the bunches and
the rate at which they collide is measured by the quantity "luminosity", and it has units of 1/(area).
Finally, the number of reactions achieved at the accelerator is given by the luminosity times the cross section.

So that is the significance of achieving a really big luminosity
(for the cognizanti, I really mean "integrated luminosity").
The more "inverse femtobarns" of luminosity you have, the more reactions
of an interesting process you can make. Right now, there is a revolution
in the making -- we believe we are on the verge of discovering the particles
responsible for giving mass to objects and a family of particles that
quite possibly accounts for the lion's share of the mass of the universe.
The problem is, the reactions that create these interesting new particles are thought
to have extremely small cross sections, so you would need lots of luminosity to
make a few of them. The Fermilab people celebrated because they are getting close to producing
a handfull of these hypothetical particles ... if they exist. They also like to drink alcohol whenever
there is the slightest reason.

Interesting note: the conventional units of luminosity (1/area*seconds) is equivalent to miles/gallon.
Yes, it is stupid. When you tell somebody a fact like that at a party, they stop talking to you.
This can come in handy.

CreditGuide said...

From my own experience I can say that some complicated things look very simple when you find the right way to explain them. As with these tomatoes and a barn. That always makes perfect sense.

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