HOW big is a proton? The most accurate measurement yet suggests it’s smaller than we thought. This could be due to an error, or it might just hint at totally new particle physics.
“The new experiment presents a puzzle with no obvious candidate for an explanation,” says Peter Mohr of the international , which calculates values for fundamental constants in physics, who was not involved in the new work.
The proton’s radius cannot be measured directly but can be inferred from the hydrogen atom, which consists of a proton and an electron. The electron can sit in a variety of energy “shells”, each with a different distribution in space. One shell’s distribution requires the electron to dive in and out of the proton, and another sits entirely outside the proton. The energies of both shells can be combined to deduce the proton’s radius.
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There is a way to make this measurement more accurate, however: replace the electron with a muon. This particle is also negatively charged but much more massive than the electron, so its energy shells sit closer in and can overlap more with the proton radius. Now, of the Max Planck Institute of Quantum Optics in Garching, Germany, and colleagues have successfully created such a “muonic” atom and found that this yields a proton radius 4 per cent smaller than that gleaned using the hydrogen atom (Nature, ).
“The relevant theorists tell us that an error of such a magnitude is ‘impossible’,” says Pohl. Mohr says that the problem is likely to lie with an error in the measurements or a mistake in the calculations.
“The theorists tell us that an error of such a magnitude in the radius of the proton is ‘impossible’”
But if such errors are ruled out, the discrepancy would point to a problem with quantum electrodynamics, a theory that underpins much of particle physics. That deficiency opens the door to new physics at work in atoms, such as previously unknown particles or effects.