Magazine article Science News

The Hunt for Antihelium: Finding a Single Heavy Antimatter Nucleus Could Revolutionize Cosmology

Magazine article Science News

The Hunt for Antihelium: Finding a Single Heavy Antimatter Nucleus Could Revolutionize Cosmology

Article excerpt

When summer comes to Antarctica this December, a group of physicists there will launch an enormous balloon carrying a scientific instrument through Earth's atmosphere to the edge of space. If all goes well, the detector will count cosmic rays for 20 days. The researchers hope to find among those rays, evidence of galaxies that are millions of light-years away and made entirely of antimatter.

The idea may sound far-fetched, but antimatter regions of the universe wouldn't contradict any laws of physics. In fact, such antigalaxies would address one of the great unanswered questions of cosmology--why the Big Bang seems to have produced more matter than antimatter.

Several teams of scientists around the world are engaged in the search for far-off antimatter galaxies, which has been going on for almost 30 years. The clue that they seek is tiny: a single helium nucleus made of antimatter that has drifted intact across the gulf of inter-galactic space. Physicists agree that finding even one stray antihelium would be compelling evidence of antimatter galaxies.

While the odds of success are slim, "the scientific payoff ... would be very large," says John Mitchell, lead scientist for the upcoming Antarctic mission, called the Balloon-Borne Experiment with a Superconducting Spectrometer (BESS).

And there's urgency in the search for antihelium. Given the expense and sensitivity of the three current research programs, failure to find an antihelium would make it hard to justify another generation of bigger, better, and costlier instruments, says Robert Streitmatter, a colleague of Mitchell at NASA's Goddard Space Flight Center in Greenbelt, Md.

So, the current efforts could be the best hope for discovering antimatter galaxies--if they exist.

IN THE BEGINNING The possibility of stars and planets made of antimatter arose after 1930, when the British physicist Paul Dirac theorized that electrons should have twins with the same mass but an opposite spin and electric charge. Two years later, physicists confirmed the existence of these particles, called positrons. According to current theory, all fundamental particles have twins, known as antimatter, with some of their properties reversed.

Further experiments supported the idea that particles and their antimatter twins are always created in pairs, which in turn suggested that the universe must contain equal amounts of matter and antimatter. When Dirac received the Nobel prize in 1933, he said, "We must regard it rather as an accident that the Earth ... contains a preponderance of negative electrons and positive protons. It is quite possible that for some of the stars it is the other way about."

Through the 1950s, "it was absolutely the belief that there were equal amounts of matter and antimatter in the universe, possibly separated," Streitmatter says. By the mid-1960s, however, work on the Big Bang theory had begun to cast doubt on that idea. When a particle and its antiparticle meet, they annihilate each other in a flash of energy. So if, in the hot, dense, early moments of the Big Bang, particles and antiparticles had existed in equal numbers, they would have subsequently destroyed each other, leaving a universe filled with only radiation.

Today, most physicists argue that production of matter in the newborn universe must have slightly outpaced the production of antimatter. With even a small excess of matter, some would have been left over after all the antimatter had been annihilated.

Scientists propose that the asymmetry responsible for the initial imbalance also shows up when exotic particles called kaons and B mesons decay into other particles in an accelerator (SN: 8/5/00, p. 86). But the degree of asymmetry detected in accelerator experiments is too small to explain the cosmic preponderance of matter. The measured asymmetry would produce enough surplus matter for only about one galaxy in our entire visible universe. …

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