The size of the proton matters in the field of the ultra-small and it seems that no one can agree on the correct value. The answer was long believed to be well-known, but the puzzle seems to be back to haunt the physics community. The proton seems to have suddenly shrunk in size.
How do we look?
The radius of the proton is found out by shooting high energy electrons at it and then finding how it forms a bound state. It’s very much like forming an atom, except that this atom is much smaller than the normal atoms which make up matter. Energetic electrons fired at protons often get bound to the proton, and form a hydrogen-like object. However, since the electron has a lot more energy than the ordinary hydrogen atom electron, it is attached much closer to the proton than the normal hydrogen electron. As a result, the proton can no longer be treated as a point particle, but its spatial extent become important.
So we can form a bound state and then measure the minute transition between energy levels and these now have an imprint of the proton magnetic moment and the proton radius. And thus, the proton radius can be determined.
For a long time, physicists were safe in their determination of the proton radius and their value was 0.8768 femtometers (a femtometer is a millionth of a billionth of a meter, or a meter divided by 10^15). Case closed, right? Wrong…
A new experimental result threatens to blow this question of the radius wide open again. The muon is a close cousin of the electron. It has a negative charge and behave very much like the electron in a magnetic field, except that it is 200 times heavier than an electron. Recent experiments shoot these heavy electrons – or muons – at protons and these now form a bound state. The higher mass of the muon (by a factor of 200) means that at same energies, the muon is much closer to the proton (by a factor of 800 million). It can ‘see’ the proton much better and measure the radius to greater accuracy.
However, this has produced a shocking reduction in the accepted value – 0.84087 femtometers – a reduction of 4%. That is huge, well above the experimental uncertainties.
So, what’s going on?
Physicists are not very sure what’s going on. Why should the muon behave any differently from the electron? Is the muon, being closer to the proton experiencing some short range force, other than the usual long ranged electromagnetic and the short ranged weak force, that we just don’t know about? Is a new force of nature at work here? Is there new physics, something beyond the Standard Model of particle physics?
The muon measurements were made by a group of scientists at the Max Planck Institute of Quantum Optics, led by Randolf Pohl. Of course, the crudest explanation to all of this is that the experimentalists simply bungled and got the value wrong. No one’s ruling that explanation out right now, but other avenues are also being explored.
Muon scattering experiments like MuSE will only be ready in a few years, so this debate will continue for some time. When size does matter, we just don’t want it to change.