In Christian art, a halo symbolises holiness. In particle physics, a ring of positive charge around the proton has become the focus of a devilish row.
The dispute concerns an attempt to square a recent suggestion that the radius of the proton is smaller than we thought with the theory of quantum electrodynamics (QED), which has successfully explained quantum phenomena since the 1940s.
A proton's radius cannot be measured directly, but has to be deduced by measuring the energies of different electron "shells" in a hydrogen atom. Through QED, these energies combine with a model of how the proton's charge is distributed to give the proton's radius.
The smaller value for the proton radius came from measurements of an exotic form of hydrogen that contains a heavy type of electron known as a muon. This was expected merely to add precision to previous measurements based on ordinary hydrogen. Instead, the muonic measurements suggested a radius that was a whopping 4 per cent smaller (New Scientist, 10 July, p 10). That could signify a problem either with the muonic measurement or with QED, neither of which seems particularly likely.
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