Enlarge /. One way to measure the charge radius of a proton is to bounce something off of it (a proton sized bracket is only available via metaphor).
How big is a proton? That doesn't sound like a very complicated question, but it turns out that it has the potential to destroy much of modern physics. That's because different methods of measuring the proton's charge radius produced results that mismatched – and not just a little: the answers were four standard deviations apart. But now a new, and possibly improved, measurement brings them much closer to agreement, even though it is not close enough to consider the problem solved.
We seem to have a problem
There are several ways to measure the charge radius of a proton. One is to ricochet other charged particles off the proton and measure its size by their deflections. On the other hand, it is to be investigated how the charge of the proton influences the behavior of an electron that it orbits in a hydrogen atom that consists of only one proton and one electron. The exact energy difference between different orbitals is the product of the charge radius of the proton. And when an electron passes from one orbital to another, it emits (or absorbs) a photon with an energy equal to that difference. Measure the photon and you can work back on the energy difference and thus the charge radius of the proton.
(Actual wavelength depends on both the radius of charge and a physical constant. So you actually need to measure the wavelengths of two transitions to get values for both the radius of charge and the physical constant. For the purposes of this article, however, we will will only focus on one measurement.)
A rough match between these two methods seemed to leave the physics in good shape. But then the physicists went and did something fun: they replaced the electron with its heavier and somewhat unstable equivalent, the muon. According to our understanding of physics, the muon should behave exactly like the electron except for the mass difference. So if you can measure the muon orbiting a proton in a short amount of time before it decays, you should be able to produce the same value for the proton's radius of charge.
Of course, it yielded a different value. And the difference was big enough that a simple experimental mistake probably wouldn't explain it.
If the measurements were really different, it indicates a serious problem in our understanding of physics. If the muon and proton do not behave equally, then quantum chromodynamics, an important theory in physics, is irretrievably broken in some way. And having a broken theory is something that makes physicists very excited.
The new work is largely an improved version of previous experiments as it measures a specific orbital transition in standard hydrogen, which is made up of an electron and a proton. First, the hydrogen itself was brought to a very low temperature by passing it through an extremely cold metal nozzle on its way to the vacuum container in which the measurements were carried out. This limits the influence of thermal noise on the measurements.
The second improvement is that the researchers worked in the ultraviolet region, where shorter wavelengths helped improve precision. They measured the wavelength of the photons emitted by the hydrogen atoms using what is known as a frequency comb, which creates photons with an evenly spaced series of wavelengths that act a bit like the markings on a ruler. All of this helped measure the orbital transition with an accuracy 20 times more accurate than the team's earlier attempt.
And the result that the researchers get also contradicts previous measurements of normal hydrogen (though not more recent). And it's much, much closer to the measurements taken with muons orbiting protons. From a quantum mechanics perspective, this is good news.
But not good news as the two results are still outside of the other's error bars. Part of the problem is that the added mass of the muon makes the error bars extremely small in these experiments. This makes it very difficult for results obtained with a normal electron to agree with the muon results without completely overlapping them. And the authors acknowledge that the difference is likely to be disregarded only for errors that increase the uncertainty enough to allow overlap, and cite the prospect of "systematic effects in one (or both) of these measurements".
The work is therefore an important milestone in finding ways to improve the accuracy of the results, and the result suggests that quantum chromodynamics is likely to be okay. But it doesn't completely resolve the difference, which means we'll need some more work before we can really breathe easily. This is annoying enough to possibly explain why science put the paper out on Thanksgiving if fewer people were paying attention to it.
Science, 2020. DOI: 10.1126 / science.abc7776 (About DOIs).