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 Committee on Data for Science and Technology (CODATA), which calculates values for fundamental constants in physics, who was not involved in the new work.
Like most quantum objects, a proton is fuzzy around the edges. Its size is defined by the extent of its positive charge rather than a crisp physical boundary. This charge 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 of these shells can be combined to deduce the proton's radius, using a theory known as quantum electrodynamics (QED).
Muonic atoms
There is a way to make this measurement even more accurate, though: replace the electron with a muon. This particle is also negatively charged but much larger than the electron, so its energy shells sit closer in and overlap more with the proton radius.
Creating such a "muonic atom" has been on the to-do list since 1969, says Randolf Pohl of the Max Planck Institute of Quantum Optics in Garching, Germany, when it was first proposed as a test for QED. But the starting point for the experiment – the muon's second-to-lowest shell – persists for much less than a microsecond under ordinary conditions, which is not enough time to measure its energy.
Pohl and his colleagues only recently developed a set-up that allows them to prolong that state and measure the proton's radius using muonic atoms.
'Impossible' error
They fed slow-moving muons into a container of diffuse hydrogen gas, at one-thousandth of the pressure of the atmosphere. As the muons latched on to hydrogen nuclei, they started out in high energy shells.
Most of them dropped straight to the lowest energy shell, but 1 in 100 fell only as far as the second-lowest shell. The team then had a microsecond-long window to hit these electrons with a laser pulse tuned to exactly the frequency needed to push them up into the next shell and measure its energy.
To their surprise, when they combined this measurement with the energy of the shell below, their calculations revealed a proton radius of 0.84184 femtometres, less than a trillionth of a millimetre and a whopping 4 per cent smaller than that gleaned using the hydrogen atom.
This is a much bigger discrepancy between the two experimental results than expected. "The relevant theorists tell us that an error of such a magnitude is 'impossible'," says Pohl.
New physics?
Mohr reckons the problem is likely to lie with an error in one of the measurements; either that of the hydrogen atom or the muonic atom, or with an error in the calculations.
Savely Karshenboim, also a CODATA member at the Max Planck Institute of Quantum Optics, is betting on an error in the muonic atom study because it "contradicts another convincing result".
If such errors are ruled out, however, the discrepancy would point to a problem with QED, 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.
Pohl stands by his experimental result, but cautions against leaping to this conclusion. "New physics can of course always be used to explain any discrepancy, but before such a claim can be made, a lot of hard work is ahead."
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