Researchers probe deep inside the antimatter of a tiny particle at the center of every atom.

A glittering snowflake. The wings of a butterfly. A beeswax honeycomb. These and many other objects found in nature, art and geometry are examples of symmetry, with each half a mirror image of the other.

Symmetry is all around us. And yet, protons—the tiny, positively charged particles at the center of every atom—are curiously asymmetrical, a fact that has long intrigued scientists.

In a paper recently published in the journal Nature, an international team of scientists—including Ed Kinney, a professor of physics at the University of Colorado Boulder—shared the results of their two-decades-long SeaQuest experiment into the broken symmetry of the proton’s inner antimatter.

The results of this experiment, which took place at the federal Fermi National Accelerator Laboratory west of Chicago in Kane County, Illinois, and was first proposed in 1999, not only deepen our limited knowledge of protons, but may also help inform future accelerator-based particle physics research, which is useful for studying everything from cancer treatment to the formation of the universe. 

“Protons are ubiquitous; they’re everywhere around us, and yet they have an extremely complex internal structure,” said Paul Reimer, an experimental physicist at Argonne National Laboratory and one of the study’s co-authors. “Hadrons, including protons, form most of the visible mass in the universe and yet, we don’t really understand the insides of them.”

Exploring protons

Protons are made up of particles called quarks, as well as their antimatter counterparts called antiquarks. The quarks and antiquarks, which swim around in what researchers have dubbed the “proton sea,” can be categorized even further, with flavors such as up, down, antiup and antidown. 

Scientists understand that protons contain two up quarks and one down quark, a mismatch that explains why protons are positively charged. But, until recently, they didn’t fully understand the ratio of antiquarks, nor whether that ratio changed under varying conditions. Two earlier experiments offered a glimpse into this proton antimatter world, but the results were murky at best.

Quarks and antiquarks, which are held together by a strong nuclear force, are difficult to study because of their fleeting nature. Inside the proton, these pairs sporadically pop in and out of existence before annihilating each other. In high-energy collisions between protons, however, scientists can detect the antiquarks, especially when a quark from one proton annihilates an antiquark from the other. This annihilation process occasionally creates two new oppositely charged particles called muons, which can act as a calling card of the quark-antiquark pair. 

In the SeaQuest experiment, scientists zeroed in on these muons to analyze the ratio of antiup quarks to antidown quarks at various momentums. They discovered that, no matter the antiquarks’ momentum, there are always more antidown quarks than antiup quarks in the proton.

That finding contradicts earlier research conducted at Fermilab in the late 1990s, which suggested that a proton’s antimatter asymmetry reversed at high momentums. With SeaQuest, scientists detected no flip between antidown and antiup quarks. 

These revelations re-open the door for theories that were previously rejected based on earlier results.

“There’s no reason for there to be more of one than the other—nobody knows why,” Kinney said. “There are a number of theoretical models out there—some of them do predict this but have very different mechanisms for explaining it. It’s really not understood right now and, of course, this data was just published a few weeks ago, so it’ll be interesting to see what the theorists now go back and figure out about this.”

Detecting muons

To perform the experiment, researchers shot proton beams through various layers of material, ending with an iron wall that helped them track the muons.

“Muons are really easy to detect—they go through just about anything,” Kinney said. “If you give them something as thick as the earth, they will eventually get stopped. But for regular-sized things, they just can’t be stopped. If you put up a big wall of iron and they get through it, you know it’s a muon.”

Researchers, including several at CU Boulder, created tracking detectors to help analyze the muons as they passed through. 

CU Boulder’s tracking detector was created by graduate student Po-Ju Lin, postdoctoral researcher Joseph Katich, undergraduate student Brian McDonald and physics department technical staffer Eric Erdos, who have all since graduated, retired or moved on to other positions. CU Boulder’s built-from-scratch detector weighed 1,000 pounds and contained roughly 10,000 wires, some that were only 20 microns in diameter (meaning they were barely visible—a single human hair measures 70 to 90 microns in diameter).

Members of Kinney’s lab also regularly traveled to Illinois to help collect data and maintain their detector. They also helped make sense of the experiment’s mountain of data, collected over four years.

Scientists still don’t understand exactly why protons’ antimatter is asymmetrical, but this experiment is a start.

“It’s yet another one of those cases where this very basic particle that makes up all of us has little surprising things to it—it’s not as simple as you would’ve guessed it would be,” Kinney said. “That’s the most exciting part about it. We’ve uncovered a mystery that we now get to explore with further research.”