We can finally know the size of the proton

Finally, we’ve determined the size of a proton. More than 15 years after an experiment unexpectedly shook up the world of particle physics, researchers are regaining their grip on one of the most fundamental properties of this fundamental particle.

Look around you, and everything you see is full of protons. The proton is a fundamental building block of our world – and until 2010, we thought we understood it quite well. We knew its structure – it’s made of three quarks – and we knew its size.

Then, measurements based on an exotic hydrogen atom revealed that the proton may actually be about 4 percent smaller than expected. Physicists discovered sources of experimental error as well as theories of new physics phenomena that could solve this “proton radius puzzle.” In 2019, another experiment strengthened the evidence that the proton’s size was larger than had long been estimated.

Now, the issue has finally been settled by a pair of complementary experiments that make the case for the small proton more convincing than ever. He said that the radius of the proton is about 0.84 femtometer or less than one million billionth of a meter.

“When you look at that data, how much money are you willing to bet what the proton radius is? For me personally, right now, with these measurements, the betting odds go up significantly,” says dylan yost At Colorado State University, who worked on an experiment.

To determine this radius, both experiments focused on hydrogen atoms because each of these contains only one proton and one electron. The two particles have opposite electrical charges, so they exert an electromagnetic force on each other, and this interaction affects the energies that can occur within each particle’s atom. But this interaction depends on the size of the proton, meaning that one way to find the proton’s exact dimensions is to measure how an electron in the same atom moves from one energy state to another.

Two research teams did just that by using lasers to control the electrons in hydrogen atoms. Between them, they measured three electron transitions between energies that had never been measured before.

From this, they calculated the radius of the proton, and their numbers not only matched each other, but also matched that important 2010 measurement. “It is (now) very, very unlikely that there is still this proton radius puzzle,” says Lothar Meisenbacher at the University of California, Berkeley, which was part of the second experiment, which was conducted at the Max Planck Institute for Quantum Optics (MPQ) in Germany.

This is no small feat – the kind of experiments Meisenbacher and his colleagues conducted are extremely difficult. Hydrogen atoms must be placed in an ideal vacuum; The lasers required are often expensive and must be calibrated extremely carefully. While three or four weeks may be enough to collect data, it may take years to catalog and understand every possible source of bias and errors that may creep into the final measurements. Additionally, these experiments are highly specific in how they manipulate hydrogen, Masenbacher says, so that the exact reason why their findings may differ is usually very difficult to pinpoint.

But while their findings agree, this diversity of approaches is a strength because effects arising from some specific instrumental perturbation would not show up in experiments, say. juan rojo At the Vrije University Amsterdam in the Netherlands. “The proton radius should be a universal property; no matter how you look at it, it should give the same result. That’s why both of these papers are quite nice, because they provide different approaches to the same number,” he says.

The increased certainty in the proton’s shape is especially important for fine-tuning theories about new particles that might be discovered by studying the behavior of hydrogen’s electron, Yost says. In fact, the MPQ experiment was already precise enough to test the predictions of our best current mathematical model, called quantum electrodynamics, to an accuracy of 0.5 parts per million. The team found no anomalies — nothing that would indicate new forces or particles — but Rojo says the two experiments pave the way for similar studies to become an important part of particle physics research.

While the Giant Particle Collider can discover heavy new particles, these tabletop experiments with hydrogen atoms and lasers can seek out much lighter particles that would otherwise remain hidden, says Yost. “Now that we’re confident that we really understand what’s going on, we can say, OK, what kind of limits can we put on the new physics?” He says.