The inverse femtobarn

Tomorrow I’m going to Te Aroha to give a talk to their continuing education group on the Large Hadron Collider. This is something I foolishly agreed to do many months ago (maybe it was even last year) before I realized how much lecturing I had this semester. Still, I’ve given the talk a few times before, so it’s not a major effort (it just means I’ll be late for a lab class later in the afternoon.) Anyway, to prepare for tomorrow, I thought I’d look up the latest goings-on at CERN on their twitter page, and found this amusing press release.

In June, the LHC’s two big experiments ATLAS and CMS broke through a milestone in data collection – hitting 1 inverse femtobarn of data. Now, that caused me  to stop and think for a bit. What is an inverse femtobarn’s worth of data?  The press release goes on to say that it is bascially a lot – equivalent to seventy million million collisions.

Breaking down this unit goes something like this.  A barn is a measure of the cross-sectional area of a collision in particle physics. Basically, if you imagine your two colliding particles as balls, ask yourself what is the cross-sectional area of each ball? Obviously, the bigger the cross-sectional area, the more likely they are to hit if you fire them at each other. In particle physics, we can’t really push the ‘balls’ analogy too far, but still it should give you an idea of what we mean by collision cross-section. A barn equates to ten to the minus 28 metres squared. Pretty small in terms of the everyday world, but actually pretty large in particle physics terms. The terminology is attributed to some of the wartime researchers into atomic weapons, who, talking about the ‘size’ of the uranium atom, remarked that it was ‘as big as a barn’. In a manner that only physicists can, they went on to define the units ‘outhouse’ and ‘shed’ for various fractions of a barn, but only the unit ‘barn’ has stuck. 

The ‘femto’ bit is simply a standard prefix, meaning 10 to the power minus 15. So a femtobarn is an area of 10 to the power minus 43 metres squared. A touch on the tiny side.

Now, to discuss collisions, the particle physicist uses something called luminosity, measured in particles  per unit time per unit area. The ‘barn’ comes in here as the unit area, so we can measure luminosity in particles per second per barn, or even particles per second per femtobarn. If we multiply this by the time that we have run the collider for, we get a measure of the integrated (total) luminosity in particles per femtobarn.  If we were to multiply this by the cross-sectional area of the collision (so many femtobarns), we’d get the total number of collisions.

Confused?  I think I am too. Basically, I think (but I am open to being corrected) the inverse femtobarn is a measure of how well the collider has done, allowing for the fact that some collisions are easier to achieve than others. One inverse femtobarn is pretty good going, corresponding to a shed load of data (or maybe a barn load of data) if you pardon the pun.

One thought on “The inverse femtobarn”

  • Nice article.
    The LHC hits protons with protons. The p-p cross-section at 7 TeV is about 110 millibarns, of which the inelastic scatterings (the ones that are detected in the experiments) account for about 60 millibarns. That is 60 X 10^12 femtobarns, which gives 60 million million collisions per inverse femtobarn.
    Perhaps the 70 comes from a different energy of collision, or averaged over different energies of collision, as in the Tevatron.

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