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. (Interestingly, this new physics could also explain a long-standing discrepancy in the measurement of the muon’s anomalous magnetic moment.)
This “would, of course, be fantastic,” said Randolf Pohl of the Max Planck Institute of Quantum Optics in Garching, Germany, who led both the 2010 experiment and the new study. “But the most realistic thing is that it’s not new physics.”
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