The hunt for antihelium: finding a single heavy antimatter nucleus could revolutionize cosmology.

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.

The asymmetry, however, might have been much larger at the high energies of the early moments of the Big Bang. But theories of high-energy particle physics don't make strong predictions about the size of the asymmetry, and no experimental data pins it down. So, physicists don't know whether such an asymmetry could create the universe as a matter-only place.

AN ANTIGALAXY FAR, FAR AWAY Another way to solve the problem of the predominance of matter in the nearby universe is to suppose that the antimatter might still be around but sequestered in distant antimatter-dominated regions.

In 1979, Floyd Stecker of the Goddard Space Flight Center suggested that the matter-antimatter asymmetry could have arisen spontaneously in the first moments after the Big Bang, swinging one way in some regions of the universe and going the opposite way in other regions. That would have produced an excess of matter in some places and an excess of antimatter in others. As the universe rapidly expanded, each of those regions would have ballooned.

"All of [these scenarios] are viable options right now, and I don't think there's anything to choose between them," comments physicist Mark Trodden of Syracuse University in New York. "I think it is entirely reasonable that there were mechanisms [in the first moments after the Big Bang] that would have led to very large regions of matter and antimatter."

Because light interacts with matter and antimatter in the same way, planets, stars, and galaxies made of antimatter would look exactly the same as those made of normal matter. "You can't tell whether you're looking at a galaxy or an antimatter galaxy," Stecker says.

However, detecting even a single nucleus of antihelium near Earth would be strong evidence that antimatter stars and galaxies are out there somewhere. Energetic collisions among the cosmic rays--each a high-speed proton or other nucleus that flies about the universe at nearly the speed of light--can produce occasional antiprotons. "There's even a small chance of making antideuterium," Stecker says, "but it's almost impossible to make an antihelium. It would be a very small probability over the whole lifetime of the universe."

To make an antihelium near Earth, cosmic-ray collisions would have to create four particles--two antiprotons and two antineutrons--at nearly the same time. Those particles would have to converge on a single point, traveling slowly enough to join into a nucleus rather than bounce apart.

In 1997, Pascal Chardonnet of the Theoretical Physics Laboratory in Annecy-le-Vieux, France, calculated the odds of this happening. One antihelium nucleus would be created for every million trillion or so cosmic-ray protons, which themselves are rare. At that rate, one of the current experiments would take 15 billion years on average to encounter a single antihelium made that way. Since the universe is only about 13.7 billion years old, any antihelium that experiments might detect is far more likely to come from a region of primordial antimatter than from a cosmic ray collision.

"If they did see an antihelium, it would be very dramatic evidence" that distant antimatter galaxies do exist, Stecker says.

NEEDLE IN A HAYSTACK The search for cosmic antihelium began in 1979. The late Robert L. Golden, founder of the Particle Astrophysics Lab at New Mexico State University in Las Cruces, launched a high-altitude balloon carrying a detector designed to look for antiparticles, especially antiprotons. The flight was the first to find antiprotons among the cosmic rays, but it turned up no trace of antihelium.

Scientists knew that stumbling upon an antihelium was a long shot. Fortunately, such missions detect antiparticles of all kinds, so the flights gather useful data about antiprotons and other particles.

The antiparticle detectors work by recording the paths of cosmic rays passing through the instrument. The particles either leave a trace on layers of silicon, or the ionized gas that they produce affects the voltages within grids of wires. A strong magnetic field bends the paths of charged particles, with positive charges going one way, negative the other. By measuring the direction and degree of curvature and other characteristics, the scientists distinguish between oppositely charged particles with the same mass and thus distinguish matter from antimatter.

Searches for antiparticles continued with a series of balloon flights beginning in the mid-1980s as well as a 1998 space shuttle mission. With stronger magnets, larger collecting areas, longer flights, and improved electronics, these experiments had steadily increasing sensitivity and detected more and more antiprotons, but none detected antihelium.

Therefore, no more than one antihelium per 1.5 million cosmic rays of normal helium could be reaching Earth, concluded a 2002 analysis by Makoto Sasaki of the BESS team.

Last June, a team of Italian researchers led by Piergiorgio Picozza of the University of Rome launched an antimatter-observing satellite into Earth orbit. The researchers designed the satellite, called the Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics (PAMELA), to steadily gather antiparticles for 3 years. They calculated its sensitivity at one antihelium among roughly 100 million nuclei of normal helium.

The next flight of the BESS instrument, scheduled for December, will have similar sensitivity. The detector will circle above Antarctica for only 20 days, but its collection area is 300 square centimeters, compared with PAMELA's 21 [cm.sup.2].

The largest and most sensitive of the current detectors, the Alpha Magnetic Spectrometer 02 (AMS-02) with its 500-[cm.sup.2] collection area, may never get a chance to fly. An international team of researchers, led by physicist Samuel Ting of the Massachusetts Institute of Technology, designed the $1.5 billion instrument to ride to orbit aboard the shuttle and attach to the International Space Station.

AMS-02 is nearing completion at the European particle physics laboratory near Geneva, Switzerland. But NASA administrator Michael Griffin announced in February that the agency needs all remaining shuttle flights through the scheduled end of the shuttle program in 2010 to complete construction of the space station.

The managers of AMS-02 are searching for another launch vehicle, and the retrofitting and launch would add another $250 million to $1 billion to the cost of the mission. Or the test may be delayed indefinitely. If it ever flies, AMS-02 will improve the sensitivity of the search to about one antihelium in a billion helium nuclei.

A PERILOUS JOURNEY Extreme sensitivity is necessary because antimatter stars and planets, if they exist, are certainly far away. An antihelium nucleus found in Earth's vicinity would have had a long and arduous trip.

At the boundary between a matter-dominated region of the universe and an antimatter region, some of the particles and antiparticles would meet and annihilate each other. That process would create gamma rays, which would emanate from the boundary.

Yet "we see no big ridges of gamma rays from any of the gamma-ray telescopes," says Steve Stochaj of New Mexico State University in Las Cruces.

Given the sensitivity of these telescopes, an antimatter-dominated region of the universe couldn't be closer than about 65 million light-years. That places the antimatter not only well outside the Milky Way but also far beyond the Local Group, which contains the Milky Way and about 50 other galaxies.

"It wouldn't be easy for antimatter nuclei to get here from a very distant galaxy" Stecker says. After escaping its galaxy, the particle would follow the magnetic field lines emanating from the galaxy. Scientists know little about the magnetic fields between galaxies--the field lines might be twisted into knotty loops, so that antimatter particles could never get far from their galaxies of origin.

If it did traverse intergalactic space, the antihelium nucleus would eventually cross a threshold between the antimatter region and the neighboring matter region. Then, the antihelium would be in constant danger of annihilation. However, "it would have to hit another nucleus to annihilate, and that's a rare event," Stoehaj says.

Finally, if an antihelium nucleus is to be recorded, it must approach the neighborhood of the Milky Way. There it would have to contend with the stream of particles flowing out of our galaxy. And once inside the Milky Way, after traveling for millions of years, the nucleus would still have to encounter a detector that's less than a meter across--quite a small target to hit from so far away.

Even with the enormous number of particles that would be flowing out of an antimatter galaxy, the odds of an antihelium nucleus reaching a detector near Earth are small. But because so much is uncertain about the conditions in deep space between galaxies, "there simply is no good calculation" of those odds, Streitmatter says.

This uncertainty makes it difficult for the researchers to know when to stop looking. There's always the possibility that an instrument just slightly more sensitive would make the discovery.

END GAME? Some of the researchers say that the current round of experiments is probably the last. "[We've reached] the practical limit to the effort that anyone's ever going to put into this problem," Streitmatter says. "Unless [an antihelium] is found" he adds, "and then the race is on."

Some physicists doubt whether anything will ever be found. "I don't see any reason why, in our conventional understanding of cosmology, we would see any antihelium in the universe," says Sean Carroll of the California Institute of Technology in Pasadena.

Carroll agrees that the known laws of physics don't rule out the possibility of distant antimatter galaxies, but he says that available observations of the cosmos don't compel him to wonder whether they do. Some scientists have suggested that any region of antimatter might be so large that antihelium particles would never make it to Earth.

However, Carroll notes, "we're often surprised in physics," and physicists shouldn't avoid long-shot experiments. "I think [searching for antihelium is] an interesting observation to do," he says. "You never know. And if they do find it, it would be tremendously exciting."

Most important, evidence of distant antimatter galaxies would shape scientists' understanding of the first few moments after the Big Bang, when small and poorly understood asymmetries assured that matter would survive long enough to form stars, planets, and ultimately life.

"A solid detection of an antihelium would send the theorists scrambling" Stochaj says.