HOW big is a proton? The most accurate measurement yet suggests it鈥檚 smaller than we thought. This could be due to an error, or it might just hint at totally new particle physics.
鈥淭he new experiment presents a puzzle with no obvious candidate for an explanation,鈥� says Peter Mohr of the international , which calculates values for fundamental constants in physics, who was not involved in the new work.
The proton鈥檚 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 鈥渟hells鈥�, each with a different distribution in space. One shell鈥檚 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鈥檚 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 鈥渕uonic鈥� atom and found that this yields a proton radius 4 per cent smaller than that gleaned using the hydrogen atom (Nature, ).
鈥淭he relevant theorists tell us that an error of such a magnitude is 鈥榠mpossible鈥�,鈥� says Pohl. Mohr says that the problem is likely to lie with an error in the measurements or a mistake in the calculations.
鈥淭he theorists tell us that an error of such a magnitude in the radius of the proton is 鈥榠mpossible鈥欌��
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.