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LHCb discovers matter-antimatter asymmetry in charm quarks

A new observation by the LHCb experiment finds that charm quarks behave differently than their antiparticle counterparts.

Illustration representing matter and antimatter particles
Artwork by Sandbox Studio, Chicago with Ana Kova

Scientists on the LHCb experiment at the Large Hadron Collider at CERN have discovered a new way in which matter and antimatter behave differently.

With 99.9999 percent statistical certainty, LHCb scientists have observed a difference between the decays of matter and antimatter particles containing charm quarks. This discovery opens up a new realm to study the differences between matter and antimatter and could help explain why we live in a matter-dominated universe.

“This is a major breakthrough in experimental physics,” says Sheldon Stone, a professor at Syracuse University and collaborator on the LHCb experiment. “There’s been many attempts to make this measurement, but until now, no one had ever seen it. It’s a huge milestone in antimatter research.”

Every structure in the universe—from the tiniest speck of dust to the mightiest star—is built from matter. But there is an equally qualified material for the job: antimatter. Antimatter is nearly identical to matter, except that its charge and magnetic properties are reversed. Precision studies of antihydrogen atoms, for example, have shown that their characteristics are identical to hydrogen atoms to beyond the billionth decimal place.

Matter and antimatter cannot coexist in the same physical space because if they come into contact, they annihilate each other. This equal-but-opposite nature of matter and antimatter poses a conundrum for cosmologists, who theorize that the same amount of matter and antimatter should have exploded into existence during the birth of our universe. But if that’s true, all of that matter and antimatter should have annihilated one another, leaving nothing but energy behind.

Particle physicists are looking for any tiny differences between matter and antimatter which could help explain why matter won out over antimatter in the early universe.

Lucky for them, antimatter is not a totally extinct species. “We don’t usually see antimatter in our world,” says Ivan Polyakov, a postdoc at Syracuse University and internal LHCb reviewer for this new analysis. “But it can be produced when ordinary matter particles are smashed together at high energies, such as they do inside the Large Hadron Collider.”

The main way scientists study the tiny and rare particles produced during the LHC’s collisions is by mapping how they decay and transform into more-stable byproducts.

“This gives us a sort of family lineage for our ­particle of interest,” says Cesar da Silva, a scientist  from Los Alamos National Lab and also a LHCb collaborator. “Once stable particles are measured by the detector, we can trace their ancestors to find the primordial generation of particles in the collision.

“Because of quantum mechanics, we cannot predict what each single unstable particle will decay into, but we can figure out the probabilities for each possible outcome.”

The new LHCb study looked at the decays of particles consisting of two bound quarks—the internal structural components of particles like protons and neutrons. One version of this particle (called D0 by scientists) contained a charm quark and the antimatter version of the up quark, called an anti-up quark. The other version contained the reverse, an up quark and an anti-charm quark.

Scientists on the LHCb experiment identified tens of millions of both D0 and anti-D0 particles and counted how many times each transformed into one set of byproducts (a pair of particles called pions) versus another possible set (a pair of particles called kaons).

With everything else being equal, the ratio of these two possible outcomes should have been identical for both D0 and anti-D0 particles. But scientists found that the two ratios differed by about a tenth of a percent—evidence that these charmed matter and antimatter particles are not totally interchangeable.

“They might look nearly identical from the outside, but they behave differently,” Polyakov says. “This is the puzzle of antimatter.”

The idea that matter and antimatter particles behave slightly differently is not new and has been observed previously in studies of particles containing strange quarks and bottom quarks. What makes this study unique is that it is the first time this asymmetry has been observed in particles containing charm quarks.

Previous experiments—including BaBar, Belle and CDF—endeavored to make this same measurement but fell short of collecting enough data to to tease out such a subtle effect. The huge amount of data generated since the start of LHC Run 2 combined with the introduction of more advanced methods to tag the particles of interest enabled scientists to collect enough matter and antimatter D0 particles to finally see these decay differences beyond a shadow of a doubt.

The next step is to see how this measurement fits with the theoretical models, which are still a little fuzzy on this prediction.

“Theorists will need to figure out if the Standard Model can explain this,” Stone says. “We’re pushing our field and this result will certainly be in the history books.”