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The Proton Radius Puzzle

Two experiments have come up with two wildly different values for the proton's radius. What's going on?

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You would be forgiven for assuming that we understand the proton. It is, after all, the main constituent of matter in the observable universe, the fuel of stellar furnaces. Studies of the proton—its positive charge suitably bound up with a negatively charged electron to make a hydrogen atom—initiated the quantum-mechanical revolution a century ago. Today researchers trigger torrents of ultrahigh-energy proton collisions to conjure particle exotica such as the Higgs boson.

Yet recent studies of the proton have surprised us. The two of us (Bernauer and Pohl), along with our colleagues, have made the most precise measurements of the radius of the proton to date, using two complementary experiments. When we began the exercise, we suspected that our results would help add levels of precision to the known size of the proton. We were wrong. Our measurements of the proton's radius differ by a huge gulf. The difference is more than five times the uncertainty in either measurement, implying that the probability that this is all due to chance is less than one in a million.

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