Proton Radius Puzzle Deepens With New Measurement | Quanta Magazine
The same group that discovered a curious discrepancy in measurements of the size of the proton, giving rise to the “proton radius puzzle,” has now found a matching discrepancy in measurements of a nuclear particle called the deuteron. The new finding, published in the journal Science, increases the slim chance that something is truly amiss, rather than simply mismeasured, in the heart of atoms.
The puzzle is that the proton — the positively charged particle found in atomic nuclei, which is actually a fuzzy ball of quarks and gluons — is measured to be ever so slightly larger when it is orbited by an electron than when it is orbited by a muon, a sibling of the electron that’s 207 times as heavy but otherwise identical. It’s as if the proton tightens its belt in the muon’s presence. And yet, according to the reigning theory of particle physics, the proton should interact with the muon and the electron in exactly the same way. As hundreds of papers have pointed out since the proton radius puzzle was born in 2010, a shrinking of the proton in the presence of a muon would most likely signify the existence of a previously unknown fundamental force — one that acts between protons and muons, but not between protons and electrons.
Still, Pohl is highly skeptical that the puzzle is evidence of new fundamental physics.
His personal guess is that physicists have misgauged the Rydberg constant, a factor that goes into calculating the expected differences between atomic energy levels. While it is considered one of the most accurately measured constants, a small error could account for the proton and deuteron radius puzzles.
To test this possibility, physicists in Toronto are attempting to measure the proton radius in a way that sidesteps the Rydberg constant. Other experiments are under way to test alternative hypotheses, mundane and exciting alike. Pohl’s group is diving into muonic helium, a system in which the effects of a new force, if it exists, should be enhanced, since there are two protons. We’ll keep you posted.
accédé après la frustration du #paywall du Monde
Le mystère du proton qui rétrécit