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CERN | Geneva | Switzerland

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Making tracks

As the data flood in to the experiments at the LHC, a steady stream of research papers flows out, presenting the latest results and measurements. For anyone not steeped in particle physics, it’s a bewildering brew of cross-sections, multiplicities, rapidities, and the like. The numbers and the plots may hold the key for the physicists, but everyone else is looking for something more tangible, which is probably one reason why ‘event displays’ are in such high demand, by journalists for example. The brightly coloured images of swirling particle tracks at least give the impression that you are seeing the ‘unseeable’. They can also be stunningly beautiful.

In a year that is seeing its fair share of centenaries related to research at CERN, celebrations of the first observation of particle tracks in a device called a ‘cloud chamber’ can now follow those for the discoveries of superconductivity and the atomic nucleus. On 9 June 1911, London’s Royal Society published photographs by Charles Wilson of ‘little wisps and threads of cloud’ – in fact, droplets of condensation formed along the paths of alpha particles (helium nuclei) that had ionised the supersaturated vapour in the chamber. Wilson had initially developed his chamber to study the formation of real clouds, but in an early 20th-century form of technology transfer, he found that the device was sensitive to radiation and he had worked on it to make the tracks visible.

Wilson’s cloud chamber was the direct ancestor of many of the detectors at the heart of today’s huge experiments that record the tracks of the hundreds of particles shooting out from the head-on collisions in the LHC. The underlying principle is the same – making visible the ionised trails left by the particles as they pass through a gas – even if the modern chambers detect the ionisation more directly as electrical impulses rather than by making it visible through trails of droplets. Add a little bit of computer magic to bring colour to the tracks and the results can be just as delightful to a generation used to colour television and computer games as Wilson’s pictures were a century ago. They might not make the strange world of particles more comprehensible but they certainly bring it a step closer to reality.

So next time you see ‘contrails’ in the sky, think of Wilson and, if possible, raise a glass to the centenary of the track in his cloud chamber.

— Christine Sutton

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