Matter's Victory Over Antimatter Leaves Puzzling Aftermath
The Bullet Cluster, located about 3.8 billion light years from Earth, formed after a violent collision of two giant clusters of galaxies. This image combines an X-ray image from Chandra with optical data from the Hubble Space Telescope and the Magellan telescope in Chile.
Credit: X-ray: NASA/CXC/CfA/M.Markevitch et al.; Optical: NASA/STScI; Magellan/U.Arizona/D.Clowe et al.

This story was updated at 11:55 p.m. ET.

The recent announcement that just a smidge more matter than antimatter was created during eight years of atom-smashing by a particle accelerator in Illinois is an encouraging step for scientists trying to figure out the universe around us. But researchers still have a tough job ahead to determine if what they saw is enough to explain the cold, hard fact that today's universe actually exists.

The balance between certain matter and antimatter particles tipped toward normal matter by just 1 percent during the particle-smashing run at the 4-mile Tevatron collider at the Department of Energy's Fermi National Accelerator Laboratory (Fermilab) in Batavia, Ill. It's highly unlikely that the 1 percent came about due to chance, according to statistical analysis, researchers said.

"We know that what we measured is more than what was previously known," said Stefan Soldner-Rembold, a physicist at the University of Manchester in the United Kingdom. "Whether this is now sufficient to explain cosmological models, well, that's something theorists have to calculate."

Physicists have long suspected that something tilted the balance in favor of matter over antimatter, despite the original universe starting with equal amounts of both in theory. Yet the Standard Model of particle physics barely allows for any matter-antimatter asymmetry or imbalance — certainly not enough to give rise to today's universe.

"We know there's this asymmetry in the Standard Model, but for the quantity measured it's really negligibly small," Stefan Soldner-Rembold explained.

By the same token, any bigger imbalance discovered between matter and antimatter "would be due to some new effect," Soldner-Rembold told

The research team working on Fermilab's DZero experiment hopes that their new finding might be enough to explain how matter won out.

A collection of clues

The idea that antimatter acts as the mirror image of matter is known as charge-parity (CP) symmetry. But scientists had already found a few instances in the past where slight behavioral differences lead to changes in the overall balance of matter and antimatter particles.

A first clue about the possibility of breaking the rules of CP symmetry came during an experiment in 1964, when physicists found differences in the decay of particles called kaons and their anti-kaon counterparts. Past experiments have also measured B mesons, but not in the same way as the latest study at the Tevatron collider.

For now, scientists can draw encouragement from having taken a step closer to finding definitive proof of how matter won the battle against antimatter.

"It gives more credence [to a theory] if you have more than one hint pointing in the same direction," Soldner-Rembold noted.

Watching the smash-up

Finding even the 1 percent difference represented a tricky task.

First, it required the Tevatron collider, which can create antiparticles that might otherwise only arise from rare events such as nuclear reactions or cosmic rays from dying stars. That allows physicists to study high-energy collisions between matter and antimatter particles, such as protons and antiprotons.

A collision between protons and antiprotons creates the B meson particle and its antimatter twin. Those heavy, short-lived particles then decay almost immediately into pairs of particles known as muons, as well as their antimatter counterparts known as antimuons.

The scientists gauged the numbers of positively-charged muons and negatively-charged antimuons by watching which way the particle paths curved as they zoomed between a pair of magnetic fields. The research team also reversed the magnetic fields every two weeks, to make sure that no slight differences among the detector parts skewed their calculations.

"When you perform an experiment, you have to be really sure these background differences are small," Soldner-Rembold said.

By ruling out the background differences, physicists showed that the chance of their finding being consistent with a known effect is below 0.1 percent. That represents perhaps the strongest evidence yet of how differences in the behavior of matter versus antimatter can tip the scales.

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