Laser-powered hydrogen experiment resolves long-running proton size mystery

Physicists at Colorado State University have measured the radius of a hydrogen proton with unprecedented precision, helping resolve a decade-long discrepancy that had raised questions about one of the most fundamental particles in nature.
The team determined the proton’s radius to be about 0.84 femtometers, or less than one quadrillionth of a meter. The result differs from the previously accepted value of 0.876 femtometers and aligns with more recent measurements that suggested the proton is slightly smaller than scientists once thought.
The finding helps close the so-called “proton radius puzzle,” a long-running debate that emerged when different experimental methods produced conflicting measurements of the proton’s size.
For years, physicists obtained one value when measuring hydrogen atoms using electrons. But experiments using muons, heavier cousins of electrons, consistently pointed to a smaller proton radius. The mismatch prompted speculation that unknown physics could be influencing the results.
Precision ends debate
The new measurement suggests otherwise.
According to the researchers, the result agrees with predictions from the Standard Model, the framework that describes how fundamental particles interact. The study also reduces the likelihood that a previously unknown force or particle was responsible for the discrepancy.
“Our test shows precise agreement with theory on the size of a proton to parts-per-trillion levels of accuracy, eliminating the possibility of a new force or particle being responsible for the discrepancy in this case,” said Dylan Yost, associate professor in Colorado State University’s Department of Physics.
“That would have significantly changed the Standard Model and is something researchers have been looking for,” he added.
To make the measurement, the researchers generated a beam of atomic hydrogen inside a vacuum chamber and used ultraviolet lasers to excite electrons between different energy levels. Because the proton’s size subtly influences how electrons behave around the nucleus, the team could infer the proton’s radius by precisely measuring those energy transitions.
The experiment also served as a test of quantum electrodynamics, the theory describing interactions between light and matter.
New laser method
One of the biggest challenges was obtaining clean measurements from fast-moving hydrogen atoms, which interact with laser light for only a short period.
To overcome this limitation, the team developed a new technique that uses two laser fields simultaneously.
“These atoms move very fast and do not interact with the laser for long, which can wash out the signals that we are looking for,” said Ryan Bullis, a Ph.D. student and lead author of the study.
“We developed a new technique that uses two laser fields at the same time to increase the precision of our measurements.”
The result was independently confirmed by a team at the Max Planck Institute using a different measurement approach, further strengthening confidence in the revised proton size.
Researchers say the laser techniques developed during the project will now be applied to more complex forms of hydrogen, including deuterium, to probe other aspects of atomic physics.
Yost said the work demonstrates how precision tabletop experiments can complement large facilities such as particle accelerators in the search for new physics and deeper tests of existing theories.
The study was published in the journal Physical Review Letters.


