Mystery of the Shrunken Proton

ananyo writes "The proton, a fundamental constituent of the atomic nucleus, seems to be smaller than was previously thought. And despite three years of careful analysis and reanalysis of numerous experiments, nobody can figure out why. An new experiment published in Science only deepens the mystery. The proton's problems started in 2010, when research using hydrogen made with muons seemed to show that the particle was 4% smaller than originally thought. The measurement, published in Nature, differed from those obtained by two other methods by 4%, or 0.03 femtometers. That's a tiny amount but is still significantly larger than the error bars on either of the other measurements. The latest experiment also used muonic hydrogen, but probed a different set of energy levels in the atom. It yielded the same result as the Nature paper — a proton radius of 0.84 fm — but is still in disagreement with the earlier two measurements. So what's the problem? There could be a problem with the models used to estimate the proton size from the measurements, but so far, none has been identified. The unlikely but tantalizing alternative is that this is a hint of new physics."

easy

[AbrasiveCat]

The universe it growing (including our meter sticks) and the proton is staying the same size.

Re:easy

[michelcolman]

But what are our meter sticks made of? Why would they grow with the universe if they are made of particles that stay the same size?

Re:easy

[WillgasM]

Wait. I thought the whole point of this experiment was that they extrapolate the size of the proton by actually measuring the size of the orbital. So it's not necessarily that the proton has gotten smaller, just that muons orbit closer than electrons. If anything, they've taken some of the empty space out of the atom.

Maybe a muon does make a proton shrink

[Anonymous Coward]

My first idea would be that the muon does indeed shrink the proton. After all, the proton is not some solid body, but consists of interacting charged quarks. The muon has a higher probability to be inside the proton (that's exactly why it is useful for measuring its size), and thus lowers the charge density there (it adds some negative charge density to the proton's positive charge density). The electrostatic repulsion inside the positively charged proton should certainly affect its size; decreasing that repulsion due to the partial screening by the muon should therefore allow the proton to shrink a bit. Not much, but maybe enough to explain the difference.

Re:It's not smaller, everything else is bigger!

[WillgasM]

You are exactly the opposite of right. They don't actually measure the proton, they measure an orbital and do some math to determine the size of a proton. They would expect a muon to orbit at the same distance since it has the same charge as an electron, but they're getting a smaller sized orbital and therefore determining that the proton has shrunk. In reality, protons are the same size, and we're stumped as to why muons are behaving differently than electrons. If anything, muonic hydrogen has less empty space than regular hydrogen. Nothing expanded. The overall size of the atom shrunk even though the components stayed the same size.