barn

When you want a unit, you size it to the thing you are measuring; you would not weigh diamonds in tonnes. Cross sections — the effective target areas that say how likely particles are to interact — are absurdly tiny by everyday standards, so physicists invented a unit scaled to them. With a touch of wartime humour, they called it the barn, because hitting a nucleus this big was, for them, like hitting the broad side of a barn.

One barn is an area of ten-to-the-minus-twenty-eight square metres, which is roughly the cross-sectional area of a uranium nucleus. That sounds laughably small, and it is — but it is a comfortable size for nuclear cross sections, so it caught on. As with other units, prefixes scale it: a millibarn is a thousandth of a barn, a microbarn a millionth, and the very rare processes at high-energy colliders are measured in femtobarns (a quadrillionth of a barn) or even smaller. The smaller the number of barns, the rarer the interaction.

The barn matters because it is the working unit in which every interaction probability gets quoted, from reactor physics to the discovery of new particles. Experimenters also speak of integrated luminosity in inverse barns (such as inverse femtobarns), which counts how much beam exposure they have collected; multiply that by a cross section in barns and the barns cancel, leaving a plain number of expected events. A nice piece of honesty: the name is a joke, but it is now an official, precisely defined unit — proof that physicists are happy to keep a good pun once it sticks.