Physicists announce antimatter discovery

Mother Nature likes matter better than antimatter––a preference physicists technically refer to as charge-parity violation. First observed in 1964 by James Cronin and Val Fitch, the indirect CP violation they studied won them the 1980 Nobel Prize in physics.

Since then, theorists have worked to devise a model of physics that could account for CP violation, but there was no independent evidence to test the models against. Nothing, that is, until Wednesday, Feb. 24, when Chicago graduate student Peter Shawhan announced at a Fermilab seminar the discovery of direct CP violation, an entirely new type of inequality between matter and antimatter.

“It’s an uncharted territory,” said Bruce Winstein, the Samuel Allison Distinguished Service Professor in Physics, who headed up the 21-year effort that led to the discovery. “For 34 years we’ve had one measurement of CP violation, just one manifestation of it. This is the first new one since that time.”

In 1964 at Brookhaven National Laboratory, Cronin and Fitch observed indirect CP violation, the unbalanced mixing of neutral subatomic kaon particles with their charged antiparticles. The Fermilab team has observed direct CP violation.

To study the process, the Fermilab team produces enormous quantities of kaons with the world’s highest-energy proton beam at Fermilab’s TeVatron accelerator. Kaons decay into other types of particles within a tiny fraction of a second after they are produced, so the KTeV detectors must identify and measure their position and energy quickly.

The experiment, called Kaons at the TeVatron at Fermilab, is a collaboration involving 80 physicists from 12 institutions. About 15 of the KTeV physicists are from the University; eight of these Chicago scientists analyzed the data that led to the Feb. 24 announcement.

Winstein began experimenting with CP violation in 1978. Construction on the latest experiment, the third in a successively more accurate series, began in 1992. The experiment began running 24 hours a day in late 1996.

“It’s an extremely high-precision experiment,” said Shawhan, the senior Chicago graduate student on the project. “First we have to design the experiment well. Then we have to be very certain that we understand our detector and our analysis in greater detail than most other high-energy experiments because we’re looking for such a subtle effect. A great deal of work has gone into that effort.”

The experiment attempts to measure a quantity called “epsilon prime divided by epsilon.” If the quantity had turned out to be zero, it would have verified the Superweak Model of CP violation. A nonzero value would favor the Standard Model, to which most physicists subscribe.

The result that Shawhan announced Feb. 24 was 0.00280 with an error of 0.00041. This eliminates the Superweak Model as the sole explanation for CP violation, but a problem remains. “The number that we got was larger than most theorists had predicted,” said Edward Blucher, Assistant Professor in Physics at Chicago and a member of the Fermilab team for five years. Blucher and his students, Jim Graham and Val Prasad, along with graduate student Colin Bown, postdoctoral scientists Rick Kessler and Sasha Glazov, and former member Aaron Roodman, made up the Chicago team for this analysis.

The European laboratory for particle physics, CERN, in Switzerland, found evidence for direct CP violation before the Fermilab team, but the CERN measurements were less precise. “It wasn’t definitive evidence,” Winstein said.

The Chicago researchers initially reacted to the latest result with mixed emotions. “There was a mixture of jubilation, shock and a feeling of, oh my God, did we screw up, all at once,” Winstein said.

But there is no question about the latest Fermilab results, Cronin said: “It’s final.”

The experiment has doubled scientific knowledge about CP violation independent of any theory or speculation, said Cronin, Professor in Physics and Astronomy & Astrophysics at Chicago.

“This wonderful discovery is a beautiful surprise,” Cronin said. “It’s just wonderful because I don’t think anybody expected it. That’s what makes it especially delicious.”