🧩 The mystery of the proton radius finally solved

For more than a decade, physicists have faced a puzzling situation: the size of the proton, the nucleus of the hydrogen atom, is not the same depending on the measurement technique used. This problem has been named the proton radius puzzle.

Experiments using electrons gave a radius slightly larger than those using muons, heavier particles. Such a discrepancy could signal a flaw in the Standard Model, the theory that describes elementary particles. A more reliable and precise measurement was therefore needed to settle the matter.


That is how a team from Colorado State University took up the challenge with a laser spectroscopy technique of unparalleled precision. Their result places the proton radius at about 0.84 femtometer, a value very close to the predictions of the Standard Model. This measurement appears to end the controversy.

Associate Professor Dylan Yost explains that this result eliminates the possibility that a new force or particle was responsible for the previous discrepancy. According to him, the coincidence with theory strengthens confidence in the Standard Model, even if surprises could still come from elsewhere.

To achieve this precision, the team developed an innovative method using two laser beams simultaneously. Student Ryan Bullis, lead author of the study, explains that hydrogen atoms move very quickly and interact little with the laser, making signals difficult to capture. The double laser pulse allowed refining the measurement.


This work illustrates how laboratory experiments can complement large particle accelerators. Dylan Yost compares their approach to a check engine light: it indicates where to look for possible anomalies. Both types of experiments are necessary to probe the Standard Model and discover any new physics.

Building on these results, the team plans to apply the same technique to other atoms, such as deuterium. Dylan Yost says he is satisfied that hydrogen behaves as expected, but reminds that future experiments will certainly allow even greater precision. The quest for fundamental physics continues, between theory and experiment.

Laser spectroscopy of atoms

Laser spectroscopy is a technique that uses laser light to probe the structure of atoms. By varying the laser frequency, electrons can be made to jump from one energy level to another. These transitions are very sensitive to the properties of the atomic nucleus, such as its size.

In the Colorado team's experiment, researchers created a beam of hydrogen atoms in a vacuum chamber. By bombarding these atoms with ultraviolet lasers at precise frequencies, they were able to measure with great accuracy the energy required to excite the electrons.

To deduce the proton radius, the researchers then compared these measurements to theoretical calculations. The difficulty lay in the fact that the atoms move rapidly, which reduces the interaction time with the laser. The solution was to use two laser beams simultaneously to increase precision.