Consider the humble hydrogen atom. Atomic No. 1 – the first element listed on the periodic table. The simplest and most abundant element in the known universe.
Each of these tiny atoms consists of a single positively charged proton in its nucleus with a single negatively charged electron circling it. Together, they form both a simple and well-understood system. That makes it a great platform for fundamental research into the complex particle interactions that make up the universe as we know it.
But one key detail about hydrogen atoms has been up for debate over the last 10 years: the exact radius of the proton found in the core. That gap is known as the proton radius puzzle.
Physics researchers at Colorado State University have settled that question with a new and incredibly precise measurement of the particle's radius. The finding was recently highlighted in Physical Review Letters. The work further confirms a foundational physics theory known as the Standard Model, which explains how the universe behaves, while also opening a door for further study.
When scientists previously measured a hydrogen proton's radius using electrons, they got one answer. But when they used different, heavier particles to double check their findings, the radius seemed slightly smaller. For the researchers, this was like discovering that a house appears to have two different sizes depending on whether it is measured with an old-fashioned tape measure or a modern laser. The answer should be the same, and both approaches are equally valid.
The discrepancy seemed to suggest that previous experiments to measure a proton's size may have some sort of unresolved or systematic issue. Either their equipment was not precise enough or the fundamental rules physicists use to describe the universe needed updating.
The new, incredibly precise measurement by physicists at CSU establishes a proton's radius as about 0.84 femtometers or less than one quadrillionth of a meter. That differs from the previously accepted size of 0.876 femtometers. The slight discrepancy is akin to measuring the entire length of the U.S. and learning you were off by the size of a virus in the total. However, this small clarification – independently and almost simultaneously confirmed by a team at the Max Planck Institute using a different measurement method – seems to finally resolve the puzzle discrepancy.
The amendment to the proton's accepted measurement might be minuscule, but the confirmation has large implications for our understanding of the universe and its functions.
Dylan Yost led the project and is an associate professor in the Department of Physics at CSU. He said findings from their rigorous testing match expectations from the Standard Model, which precisely predicts how particles including electrons, muons and protons interact. The findings also indicate that the ongoing mismatch likely came from subtle issues in earlier measurements or in dealing with certain constants that were developed from experimental data.
"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. That would have significantly changed the Standard Model and is something researchers have been looking for," he said. "That doesn't seem to be the case in this instance though."
For years, Yost's team has been developing table-top spectroscopy experiments with lasers to make these types of measurements. For this project, the team produced a beam of atomic hydrogen in a vacuum chamber and then used lasers to stimulate their electrons between different levels of energy. Because the proton's size subtly affects how electrons behave in their orbit, the researchers could infer the radius by precisely measuring how the electrons responded to the laser during these energy transitions. The experiment also served as an important test of quantum electrodynamics, the theory that describes how light and matter interact at the atomic scale.
Ph.D. student Ryan Bullis is the primary author on the paper. He said a key challenge in the project was developing a technique to study these key energy transitions in detail.
"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," he said. "We developed a new technique that uses two laser fields at the same time to increase the precision of our measurements."
He added that it was incredibly rewarding to pursue and then implement the technique – a first of its kind for this purpose – as part of his thesis.
Yost said the team's small-scale approach to the project offered plenty of flexibility. They could change their equipment rapidly or prioritize different measurements depending on findings and success with new techniques. He also stressed that their approach was particularly useful in search of light and weakly interacting particles compared to instillations like the Large Hadron Collider that are better at finding heavier particles and stronger interactions with permanent large-scale infrastructure.
However, he said both types of experiments are needed to continue pushing the limits of the Standard Model.
"The two approaches fill different needs. With our experiments, we can find and study fundamental physics without large particle accelerators. Our work is like a check-engine light coming on, telling the driver they need to investigate a potential problem," he said. "Our work can tell you where to look or what is working, but you need both teams to continue to fully examine and probe the Standard Model in search of new physics."
Yost said his team will now use the methods from this project to study and measure more complex versions of hydrogen, such as deuterium.
"We can set hydrogen aside for now because we can be satisfied that it behaves as it should. That allows us to look at other elements and interactions to be sure they are doing what we think they should be doing," he said. "There is always a chance that future capabilities will allow us to be even more precise. But we are ready to dig back in and continue to bridge the gap between theory and experiment in the field of atomic, molecular and optical physics."
