Sometimes, physicist Yuanning Gao watches TV in a mirror.
“If you’ve never tried it, you should,” he says.
At first, the scenes look pretty normal. But Gao says that if you watch long enough, you’ll start to notice a few oddities. “For example, everyone uses their left hands a lot,” he says. “This is when you realize that it’s not the real world, because most people are right-handed.”
Gao studies the differences between objects and their mirror images—on screen (just for fun) and at a subatomic level (in the lab). As part of his work on the LHCb experiment at CERN, Gao investigates matter and antimatter.
Matter and antimatter are nearly identical, except their charge and magnetic properties are flipped. They also have the curious feature that when they meet, they annihilate into pure energy.
The Big Bang should have created equal amounts of matter and antimatter, which should have annihilated one another completely. But in our universe, matter seems to have won out. Something about the symmetry between matter and antimatter is broken.
On a human scale, broken symmetry can be an inconvenience that makes scissors and power tools awkward for lefties. But at a subatomic level, broken symmetry is essential.
“Our entire existence depends on this broken symmetry,” Gao says.
Scientists are still trying to explain where it comes from.
Recently Gao and his colleagues on the LHCb experiment had a breakthrough: They discovered a heavier cousin of the proton behaving differently than its equal-but-opposite antimatter counterpart. This is the first time scientists have found a matter-antimatter asymmetry in proton-like particles. It took more than a decade of searching, but one exceptionally dedicated graduate student finally caught a lucky break.
Equal but opposite
Xueting Yang was around 8 years old when she encountered the term “antimatter” for the first time.
“I was reading a poster at a science museum about what the universe is made of,” Yang says. “I didn’t know that there’s so much more out there than just the ordinary matter we see around us. It really opened my eyes.”
Yang is now a graduate student working with Gao on the LHCb experiment and trying to understand why there’s more matter than antimatter in the universe.
It’s a question scientists have been puzzling over since shortly after the discovery of antimatter in the 1930s. Their conclusion was that even though matter and antimatter seem like perfect mirror images, there must be a few clandestine differences that allow them to behave differently. In the 1960s, scientists discovered this discrepant behavior, called CP violation, in matter and antimatter particle decays.
When a particle decays, it transforms into lighter byproducts, and there are often hundreds, if not thousands, of possible outcomes (a bit like the spin of a subatomic slot machine). To look for differences between matter and antimatter particles, scientists pick just one possible decay pathway (for instance, a cherry, a lemon and a seven) and then compare how often this combination of decay products shows up for the matter versus the antimatter versions of the same particle.
Over the last 60 years, scientists have made numerous observations of slightly different decay preferences in simple, short-lived particles called mesons, which contain two point-like particles called quarks.
But the universe isn’t made of two-quark mesons; it’s made of three-quark baryons like protons and neutrons.
“This has puzzled us for 20 years,” Gao says. “We really needed to find CP violation in baryons.”
Earlier measurements of CP violation mostly came from colliders called B-factories, which collide electrons and their antimatter counterparts, positrons. But these B-factories couldn’t reach a high enough energy to produce a large number of baryons. “Baryon production is much more difficult than mesons,” Gao says. “For that, we needed a machine like the Large Hadron Collider.”
Even with the help of the LHC, there is another problem: Baryons are more complicated particles than mesons, and it’s much harder for theorists to predict their decay preferences and where CP violation might show up. “We never know which channel will work,” Gao says.
Over the last decade, Gao and his colleagues analyzed six decay channels of the lambda-b baryon, a composite particle similar in structure to a proton, except one of its up quarks is replaced by the heavier b quark.
“We started with the simple channels,” Gao says.
But these simple channels turned up no sign of CP violation. LHCb scientists realized that if they wanted to increase the odds, they needed to be more ambitious. “A theorist recommended a decay channel with four different end products,” Gao says. “But it was a very difficult channel. We needed someone with a lot of patience.”
This was around the time Yang arrived in the group as a PhD student. “She is really very talented and very bright,” Gao says. “I think we found the most hard-working and patient person to do it.”
For three years, Yang and her collaborators chased down and reconstructed the lambda-b baryons that decayed into a proton, a kaon, and two pions (matter decay products for the matter lambda-b baryon; antimatter decay products for the antimatter lambda-b baryon).
"Out of 10,000 lambda-b baryons, only one lambda-b will decay in this way," Yang says. “But that’s actually a rather large fraction."
According to Yang, the concept is straightforward, but the execution is extremely challenging. “It’s like flipping coins and counting how often they land on heads,” she says. “But sometimes, the coins roll under the bed, and other times they’re very dirty and hard to see. You need to do meticulous work to make a good measurement.”
They also needed to consider the impact of our matter universe on the LHC-produced antimatter particles. “Our detector is made of matter, but we needed to detect both matter and antimatter particles,” Yang says. “We know that we reconstruct the matter and antimatter particles with different efficiencies, and we needed to calculate this very precisely to get the final results.”
After two years of preparing the analysis and one year of rigorous review by the LHCb collaboration, in September 2024, Yang and her collaborators were given permission by the experiment to finally run their analysis on the real LHCb data.
When Yang first saw the results, she was gobsmacked. "I was worried about making a mistake, and so I recalculated it a few times before I shared it with my supervisor," she says.
The difference between the matter and antimatter decay rates was rather small—only around 2.5%—but the statistical significance was so high that there is only about a 1 in 3.5 million chance that the results are due to a statistical fluctuation. She and her colleague had found the first CP violation in baryons.
“It was really an exciting moment,” Yang says.
While the results are still consistent with the CP violation allowed in the Standard Model of particle physics, they mark the start of a new epoch of CP violation research in particles that are closely related to the protons and neutrons that make up all the visible matter in the universe.
Yang and Gao both hope that this research will eventually lead us closer towards answering the ultimate question: How are we here?
“This is really just the beginning,” Yang says.
