A barn is a unit of area, measuring and is used in high-energy physics in reference to the cross-sectional area of a nuclear interaction (think of the particles as balls that have some cross-sectional area, but don’t think about it like that too hard). Even this small area is quite large by nuclear physics standards, so you often see picobarns or femtobarns in practice. When discussing the quantity of interactions going on at say the LHC, you care about the number of collisions per some cross-sectional area, so results are reported in inverse femtobarns (). One inverse femtobarn corresponds to some 70 trillion collisions, though most of these are uninteresting. This value is called the luminosity.
When you want a measure of how much data has been collected over time, you use the integrated luminosity (thought of as a measure of how many events have occurred since we turned the machine one). With that in mind, we can understand this graph of the 2011 data from the ATLAS experiment at the LHC (the data for CMS looks largely the same):
In total, 5.25 inverse femtobarns were recorded, some 350 – 400 trillion collisions, though most of these were of course uninteresting. You’ll also note the 7 TeV value, which means each beam had 3.5 TeV of energy per nucleon.
Now, the data for 2012 (so far) for ATLAS looks like:
Almost the same amount of data as in all of 2011, and it’s only June! Note also that the beam energy has been increased to 4 TeV, meaning 8 TeV at the center-of-mass. The trajectory of the data collected so far has been well-beyond that of 2011:
I’ve read that by the end of the year, three to four times as much data will have been recorded as in all of 2011. Looking at the above graph, one is almost inclined to say “That’s it?” Nonetheless, hats off to the people at the LHC.
(All images are from the ATLAS Public Wiki)