Physicists Clarified The Size Of Proton

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Across the board, the problem of the proton’s anomalous size, which arose 15 years ago, has been solved. Modern particle physics does not allow for anomalies in particle characteristics—their accuracy reaches 12 decimal places, leaving no room for uncertainty. However, measurements of the proton’s radius conducted in 2010 did not match previous values, which remained a mystery until now.

In 2010, an experiment was conducted with an exotic hydrogen atom, in which the electron was replaced by a muon. The results showed that the proton could be approximately 4% smaller than expected. Physicists rushed to find sources of measurement error and put forward theories to explain the deviation. However, a new study in 2019 yielded roughly the same results.

Both experiments measured the proton’s size using indirect measurements in a normal hydrogen atom. A hydrogen atom consists of only a proton and an electron, bound together by electromagnetic force. This force, in turn, depends on the proton’s size, meaning that a reliable (albeit complex) way to determine it is to measure the electron’s transition from one energy level to another.

Both groups used lasers to manipulate electrons in atoms. One team measured two transitions between the S and P orbitals, while the other measured the transition between the S and S levels, a much more complex process. Based on the data obtained, they calculated the proton’s radius—and the results agreed not only with each other but also with the landmark 2010 experiment.

Thus, the proton’s size mystery has been resolved in favor of a smaller radius, which is fully consistent with the Standard Model and eliminates the need for new particles or forces. “We believe this is the final nail in the coffin of this mystery,” stated one of the authors of the Nature paper. This not only resolves a long-standing debate but also opens the way to even more rigorous tests of fundamental physics.