With physicists pretty much done in their search for the elusive Higgs boson and now getting ready to discover even more exotic particles, if they exist to be found, you'd think that they'd have something as banal as the proton all wrapped up by now.
But you'd be wrong, one of the most interesting developments in recent years has been a debate on the size of the proton which was believed to have long been determined.
A new type of experiment
provided a wildly different result, one that could be not be explained by simple errors.
Unlike electrons, for example, which in practical terms don't occupy any volume and are essentially a geometric point, a proton is made up of three quarks, plus gluons flying between them keeping it all together.
It has a volume and up until recently, scientists were satisfied that a proton, specifically its charged radius, was about 0.877 femtometers (10-15
These measurements were backed up by theoretical calculations and there was no reason to doubt them, until now.
An international team has used a laboratory in Switzerland, the only one capable of producing muons for the experiment, to provide more accurate measurements of the proton.
To do this they switched the electron in a hydrogen atom, typically made up of one proton and one electron, with a muon.
The muon is a lepton like the electron to which it is very similar, it has the same electric charge and the same spin, but is roughly 200 times heavier. It is also an unstable particle, decaying in about 2 x 10-6
Since it has the same charge as an electron, it can be put in an orbit around a proton, thus forming muonic Hydrogen. Because it's so much heavier, various measurements of the proton it orbits are much more accurate.
However, when scientists tried to measure the proton in muonic hydrogen, they found its charged radius to be 0.84 femtometers. It may not look like much, but it's seven standard deviations from the expected radius and can't be explained by any error or misjudgment in the experiment.
It's also unlikely that previous experiments, which used regular hydrogen atoms, were wrong, though physicists have started redoing some of them.
What this leaves is new physics, the thing that excites physicists the most. No one knows why the proton seems to be smaller when paired with a muon, but whatever the reason, the current accepted models can't explain it.
Some believe this could be proof of new particles and phenomena beyond the standard model, just when it was complete with the discovery of the Higgs boson. There are even some arguing that this could be proof of a new force carrier and therefore a new fundamental force, a groundbreaking discovery if proven correct.