In 2010, physicists discovered a discrepancy in the size of the proton. They took a hydrogen atom and switched the electron for a muon (its heavier cousin), and discovered that for some unknown reason the radius of the proton shrunk. This has been called the proton radius puzzle.

The team has repeated the experiment for an atom of deuterium, an isotope of hydrogen, and once the electron was switched for the muon, the scientists observed once again a size discrepancy in the nucleus. This finding was reported in Science.

The muon is an elementary particle about 200 times heavier than the electron. Being so heavy when put into an atom moves it much closer to the nucleus than the electron, and its movement can be used to study the property of particles in the atomic nucleus.

However, the surprising fact is that it shouldn’t change the nucleus. Electrons and muons have the same electric charge, which is what keeps them around the nucleus, but that doesn't affect the size of the proton.

There are a few explanations flying around to explain this discrepancy. One idea is that we are seeing evidence of an unknown force, although that is very unlikely.

The team thinks the discrepancy might not be in the new measurements, but in the old ones. The previous measurements of the proton and the deuteron (the nucleus of deuterium) using hydrogen spectroscopy might not have reached the same degree of precision.

“Naturally, it can’t be that the deuteron – any more than the proton – has two different sizes,” said co-author Aldo Antognini from the Paul Scherrer Institute (PSI) in Switzerland in a statement.

“The mystery could be solved very easily if we assume a minimal experimental problem with the hydrogen spectroscopy.”

Since the proton results, labs around the world have been upgrading their measuring apparatus, obtaining a more precise measurement. There has not been any confirmation that the discrepancy is indeed due to a less precise measurement, but the team is happy that their discovery has led to this global upgrade.

“If our value had agreed with the previous ones, there would not have been this darned mystery of the proton radius; but there also never would have been this worldwide surge of activity that has led to several highly accurate measurement setups,” lead author Randolf Pohl, also from the PSI, added.

Hopefully, new measurements will soon be able to clarify if this is a quirk of the experiments, or some unknown physics at work.