Physicists at Colorado State University have solved the “proton radius puzzle” by measuring its size with unprecedented precision: about 0.84 femtometers.
A team of researchers fromColorado State University (CSU), led by associate professor Dylan Yost, has just published in the journal Physical Review Letters a measure which closes a ten-year-long chapter.
They succeeded in determining with formidable precision the size of the protonthe particle at the heart of each hydrogen atom. In doing so, they did not just correct a value: they above all ruled out the specter of a “new physics” that some hoped to see emerge from this anomaly.
This new value, smaller than the old reference (0.876 fm), confirms the predictions of the Standard Model and rules out the hypothesis of a new physical force.
So, what was this famous “proton radius puzzle”?
The heart of the problem was a stubborn discrepancy in measurements. For years, when scientists measured the proton radius using a standard hydrogen atom (one proton and one electron), they got a certain value.
But in 2010, another experiment, using “muonic” hydrogen (where the electron is replaced by a muon, its cousin much heavier), gave a different result: the proton seemed slightly smaller.
This difference, although tiny, of the order of a quadrillionth of a meter, was a real grain of sand in the cosmic gears. As both measurement methods were valid, this inconsistency left the door open for bold theories.
Perhaps an unknown force, not described by current theory, interacted differently with electrons and muons, causing this distortion, suggesting the need to redefine our physical knowledge and predictions.
How did researchers manage to settle the debate?
The CSU team opted for an extremely fine approach. Rather than resorting to exotic particles, they perfected the measurement on the classic hydrogen atom.
Into a vacuum chamber, they projected a beam of hydrogen atoms and bombarded it with ultraviolet lasers. The idea is that the size of the proton subtly affects energy levels of the electron that orbits around.
By measuring with extreme precision the energy required to make the electron “jump” from one level to another, we can deduce the size of the nucleus.
Ultraviolet laser beams to determine the dimensions of the hydrogen atom
The major challenge, as Ryan Bullis, doctoral student and lead author of the study, explains, was the speed of atoms. « They move so fast that the interaction with the laser is too brief, which can drown out the signals we are looking for ».
His solution was to develop a spectroscopy technique unprecedented using two laser beams simultaneously to refine the reading. This method not only made it possible to resolve the problem of the size of the proton but it also constituted a rigorous test of quantum electrodynamics (the theory describing the interaction between light and matter).
Does this discovery sound the death knell for a “new physics”?
The new measure of 0.84 femtometer aligns perfectly with the value obtained via muonic hydrogen and, above all, with the predictions of Standard Modelthe great rule book of particle physics.
The conclusion is that the initial discrepancy did not come from a mysterious force, but more likely from slight inaccuracies in past experiments.
The result is, in a way, almost disappointing. No new exotic force, no gaping rift in our vision of the cosmos. Science has thus closed the enigma that it itself had opened.
Why use a tabletop laser when you have giant accelerators?
This experience demonstrates the power of complementary approaches in fundamental research. While behemoths like Large Hadron Collider (LHC) are designed to find heavy particles and strong interactions, small-scale precision experiments, like that of the CSU, are expert at finding weak and light interactions.
These experiments can indicate where to look for an anomaly, but it takes both approaches to fully probe the limits of the Standard Model. The team will now apply their method to more complex atoms, such as deuteriumto ensure that they too behave as predicted by the theory.
