Scientists for the first time have seen a specific particle of matter spontaneously turn into its antimatter twin--a discovery that might require some rewriting of the fundamental theory that governs nature at the subatomic level.
Particle physics is like studying fine wristwatches by slamming them together to see which parts fall out. Except that the particles set loose by giant accelerators tend to exist for the briefest wisps of time--only billionths or even trillionths of a second. As a result, scientists can only observe the results of the decay of these particles, collecting data on their mass and electrical charge. Most of the time, these properties fit the Standard Model, the grand theory that has defined the nature of matter for nearly 4 decades. But sometimes, researchers see a particle behave in a new way that could crack open the door to an entirely new category of forces governing particle interactions.
That's what happened when an international group of physicists called the BaBar Experiment used California's Stanford Linear Accelerator Center (SLAC) to glimpse a heavy particle called a neutral D-meson turning into antimatter before it decayed. A D-meson consists of two smaller elements called quarks, one with a property called charm and the other with a property called anti-up. In an article to be published in Physical Review Letters, the BaBar team trained two high-energy particle beams directly at each other and then examined billions of collisions, which produced about 1 million D-mesons. About 500 showed the telltale signs that D-mesons had converted to antimatter--the same particle but containing the opposite constituents: one anti-charm quark and one up quark.
Scientists have observed other kinds of mesons turning into antimatter counterparts since the 1950s, but this is the first time anyone has detected a D-meson making the switch, a process called mixing. Researchers had thought the event had never been witnessed before because the D-meson's two constituent quarks were linked in a way that prevents the jump to antimatter.
The discovery by the BaBar group has challenged this assumption, says physicist and SLAC team leader Hassan Jawahery of the University of Maryland, College Park. The D-meson's jump to antimatter could be due to the particle's interactions with forces that are currently unknown, he says: "[it] could be the result of the Standard Model, or it could be new physics."
Physicist Jon Rosner of the University of Chicago agrees that it's too soon to know the significance of the find. Although the detection of D-meson mixing is "a long-sought goal of particle physics," he says, "the theoretical predictions for this effect--even those based on the Standard Model--have been all over the map," so it's "good to see the mixing actually observed."