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.
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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 (91av, 10 July, p 10). That could signify a problem either with the muonic measurement or with QED, neither of which seems particularly likely.
Now Alvaro De Rújula of the Autonomous University of Madrid, Spain, has another solution: changing our model of how the proton’s positive charge is distributed.
About 75 per cent of this charge is concentrated in a central core, the edge of which is considered the edge of the proton proper. Although the other quarter of the proton’s charge lies outside this (see diagram), the charge distribution in the “halo” is still key to finding the proton radius. So De Rújula decided to explore whether varying the charge distribution in the halo could bring the old and new calculations for the proton’s radius into agreement – and remove the conflict with QED.
He found that it can, if the halo band extends 4.7 times as far as previously thought. He concludes that this is the proton’s true structure (Physics Letters B, ).
The proposal has been contentious since De Rújula first posted it to the arxiv preprint server on 23 August. Chief among the sceptics are and Ian Cloët of the University of Washington in Seattle, who posted a rebuttal just two days later. “De Rújula’s explanation is simply off the wall,” says Miller. “It is as if the amount of water in a thimble were spread out into the volume of a swimming pool”.
This is an exaggeration, counters De Rújula, “unless the thimble covers a whale’s face”.
Miller concedes that a thimble and a pint glass is a fairer analogy. Even so, he and Cloët have calculated that a proton with a charge that extends as far as De Rújula suggests is not compatible with experiments looking at the extent to which electrons are deflected towards protons at different distances ().
De Rújula says the matter could be resolved with new electron-proton collision experiments or fresh analysis of existing data. He is convinced that, somehow, “QED will be vindicated”.
