Proton puzzle: Trouble at the heart of the atom: Measuring the muons' orbital energy levels meant first guessing the gaps between the two levels of interest, so a laser could be tuned to the right frequency to bump a muon from one level to another. The team did this by reversing the QED equations and plugging in the accepted value for the proton's radius to give an estimated starting point.
In the first couple of attempts to run the full experiment, in 2003 and 2007, that approach didn't work: the muons did not respond. It was only in 2009, when the team had a new laser that could reach higher frequencies, that they found the muons' sweet spot and persuaded them to dance. Feeding the experimentally determined energy levels back into the QED calculation produced the shocker. The error on the proton's radius had shrunk by a factor of 10, as expected - but the radius had shrunk too. At 0.8418 femtometres, it was about 4 per cent lower than the previous average...
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