New laser-assisted measurements find that the fundamental building block of matter, the proton, is about 4 percent smaller than previously thought. The new size could poke holes in one of the pillars of the standard model of particle physics.
“It’s a big deal,” commented physicist Jeff Flowers of the National Physical Laboratory in the U.K., who was not involved in the new work. “It’s given us a glimpse of a chance that there’s a real theoretical leap forward to be made.”
The potentially threatened theory, called quantum electrodynamics or QED, describes how charged particles interact with light. Since the late 1940s, the theory has been wildly successful at predicting where electrons in atoms will spend most of their time. The calculations are especially accurate for the simplest atom, hydrogen, which consists of just one proton and one electron.
But the distance between the electron and the proton depends slightly on the proton’s size, similar to how a planet’s distance from its star depends on the star’s mass. In the last decade, the accuracy of hydrogen studies and the precision of theoretical predictions have gotten so good that physicists can no longer ignore the proton’s girth.
“If you want to compare theory and experiments, you need to know the charge radius of the proton,” said physicist Randolf Pohl of the Max-Planck Institute for Quantum Optics in Germany, a coauthor of the new study. The results appear in the July 8 issue of Nature.
To get the most accurate measurement yet, Pohl and a huge international group of collaborators built an exotic form of hydrogen and blasted it with intense laser light to see how the electrons reacted.
Before Pohl’s study, the most accurate value for the proton’s radius — about 0.8768 femtometers, or less than a quadrillionth of a meter — came from studies of ordinary hydrogen.
According to quantum mechanics, an electron can orbit only at certain specific distances, called energy levels, from its proton. The electron can jump up to a higher energy level if a particle of light hits it, or drop down to a lower one if it lets some light go. Physicists measure the energy of the absorbed or released light to determine how far one energy level is from another, and use calculations based on quantum electrodynamics to transform that energy difference into a number for the size of the proton.
Instead of electrons, Pohl’s group used muons, negatively charged particles about 200 times heavier than electrons. Because of their extra bulk, muons orbit closer to the proton, and their energy levels are more sensitive to the proton’s size.
The team created hundreds of muons per second and rammed them into a diffuse hydrogen gas using the world’s strongest muon source, a powerful particle accelerator at the Paul Scherrer Institute in Switzerland. The muons smacked electrons out of the hydrogen, and got caught in orbit around the leftover proton.
Only 1 percent of the “muonic hydrogen” created this way was useful, Pohl said. These atoms live for just two microseconds. Because there are so few and their lives are so short, the team had to use a “horrendously intense laser” to probe their energy levels, Flowers said. As soon as the atoms formed, the laser zapped them with a precise amount of energy that the physicists could change over the course of the experiment. If the muons took in the right energy, they jumped up to a higher energy level, and almost immediately emitted an X-ray as they decayed back down.
The physicists looked for an excess of X-rays after the laser flashed to figure out which energy made the muons change levels. Then they used equations similar to those used in earlier hydrogen experiments to calculate the proton radius. The measurement was 10 times more accurate than had ever been achieved before.
“With muonic hydrogen, the size of the uncertainty is drastically smaller,” said Flowers. “This new method is a much better method. The trouble is, they don’t give you the same answer.”
The new value for the proton’s radius is 0.84184 femtometers, way too far from the previous value to be a fluke.
There are three possible explanations for the difference. First, one of the experiments could have goofed. Pohl is confident that his group’s experiment is sound.
“Our experiment is elegant and simple,” he said. “Accuracy is easy to achieve. That’s why we firmly believe that our measurement is not wrong.”
Alternatively, the theoretical equation used to derive the radius from the data may have had an error. This is what Pohl suspects.
“As experimentalists, we think something is wrong with theory. But the theorists claim firmly that it’s not their fault,” he said laughingly. “Time will tell us what is the real reason.”
The most exciting possibility is that the experiment picked up on some previously unknown physical effects or undiscovered particles, like what high-energy physics experiments like the Large Hadron Collider are searching for.
“If this holds up, in the sense that further experiments find the same thing, then it’s a hint that there’s some extra terms in the interaction of the atom and its environment,” Flowers said. “They may be new particles,” he added, though he cautioned that it’s too early to do more than speculate. “At the moment, it’s anybody’s guess.”
Image: CREMA collaboration/PSI
- Next-Gen Atom Smashers: Smaller, Cheaper and Super Powerful …
- Quantum Computer Simulates Hydrogen Molecule Just Right
- World’s Most Intense X-Ray Laser Takes First Shots
- World’s Largest Laser Ready to Fire Up
- Texans Build World’s Most Powerful Laser