By Merry Maisel

Where have you been if you can read the word "antimatter" without thinking of Star Trek? The fuel for the starship Enterprise is antimatter. It has been the stuff of science fiction for decades, because when antimatter meets matter, what physicists would call "a very energetic process"–annihilation–occurs. The matter and the antimatter are destroyed, transformed into a burst of energy. It was such bursts, seen in particle experiments in the early 1930s, that proved the existence of antiparticles, which had been a curious and contested consequence of the quantum theory elucidated by Nobel laureate Paul A.M. Dirac. Ever since, theorists and experimentalists have been looking for ways to produce, slow, and contain antimatter–specifically, antihydrogen–to study it. Such studies will answer many fundamental questions of physics. One of the theorists is Chi-Yu Hu, a professor of physics at California State University, Long Beach. She has used several massively parallel computers, including the IBM Blue Horizon supercomputer at the San Diego Supercomputer Center (SDSC) to make one of the most accurate calculations to date of the ideal conditions for creating "cold" antihydrogen.

In setting down the theory that combined quantum mechanics with relativity, Dirac predicted in 1930 that negatively charged electrons had counterpart, positively charged anti-electrons. The tracks of these positrons were first detected in cloud-chamber experiments performed by Carl Anderson at Caltech in 1931.

Knowing that electrons had antiparticles, Dirac then predicted that each of the fundamental particles of matter, whether charged or neutral, would have an antiparticle and that, just as ordinary particles combine to form atoms, antiparticles might combine to make atoms of antimatter.

This prediction was confirmed when the first large particle accelerator produced antiprotons at the Berkeley Radiation Laboratory in 1955. Today the known antiparticles are as numerous as the known particles, and positron-emitting radionuclides are used daily in the medical scanning technique known as PET (positron emission tomography). Still, antiparticles, and especially the heavier ones, like antiprotons, are not easy to produce and control, and what can be produced at very high energies in particle accelerators and colliders is not easy to slow down or "cool" to the low temperatures at which it may be trapped and studied.

To make the first atoms of antihydrogen, for example, physicists collided antiproton beams with xenon atoms to make short-lived electron-positron pairs and found that some of the positrons orbit the antiprotons, forming antihydrogen. The physicists at CERN, the European Laboratory for Nuclear Physics in Geneva, found nine antihydrogen atoms in their data in 1995. This was a great advance because it answered the question of whether antimatter–and not just antiparticles–could be produced.

Now there were more questions: is antihydrogen an exact mirror image of hydrogen? Is the acceleration due to gravity the same for antihydrogen as it is for hydrogen? And does the prediction hold that particles and their antiparticles have equal masses and lifetimes, and equal and opposite charges and magnetic moments?

To answer these questions, the CERN scientists (and others around the world) have worked to find a way of producing "colder"–slower moving, longer-lived–antihydrogen atoms that could be trapped and isolated for study. The low-energy behavior of the atoms and their constituents, within the apparatus for accelerating, creating, decelerating, and trapping them, is a probabilistic combination of individual and group behavior that is in fact a rich chemistry of action and reaction.

In September and October 2002, two international teams at CERN called ATHENA and ATRAP, using the same antiproton source (the new CERN Antiproton Decelerator) and different detection techniques, were able to produce low-energy antihydrogen for the first time. Not just one or two atoms, but tens of thousands. The energy of the antiatoms is still higher than is convenient for study, and the experimentalists are working on ways to slow them down further.

While the experimentalists work on the apparatus, theorists like Chi-Yu Hu are trying to pinpoint the best conditions for cold antihydrogen production.

"What we wanted to do was find the largest cross-section for the formation of low-energy antihydrogen," Hu said. A "cross-section" in particle physics is the probability that a collision will occur between or among different particles traveling at different speeds (having different energies or wavenumbers). A barn door, for example, presents a very large "cross-section" to an incoming mosquito. The probability that the mosquito will hit the barn door is high. And just that image governed the name given to the unit of atomic cross-section: it is the barn, equal to the cross-section of a uranium atom, 10-24 square centimeter.

Hu and her group were looking for the conditions under which a slow-moving antiproton could present a very large barn door to a plodding positron, whereupon low-energy (cold) antihydrogen would form. The process she chose to study is the collision of an antiproton with a "positronium" atom (a positron bound to an electron). In this process, the antiproton can bind to the positron, producing antihydrogen and an excess electron that flies off, leaving the cold antiatom.

Hu has been using the National Science Foundation’s Partnership for Advanced Computational Infrastructure resources for many years in pursuit of "exact solutions, with no intermediate approximations, of quantum three-body scattering with an excited target." The three-body scattering is the chemistry of action and reaction referred to above, modeled by quantum-mechanical calculations of target-projectile interactions and dynamics.

The method she uses takes advantage of the "modified Faddeev equation," a form of the basic Schrödinger wave equation that is particularly suited for computation on large-scale, distributed memory, parallel processing machines like SDSC’s Blue Horizon and the Pittsburgh Supercomputing Center’s Lemieux. The scheme allows Hu and her colleagues to evaluate all of the available "channels" or pathways for antihydrogen formation.

"Our calculations have employed all of the processors on Blue Horizon," Hu said, "The immense power of the machine allows us to calculate the energy at which the three-body collision produces the largest antihydrogen formation cross-sections," and with this information the experimentalists may be able to optimize their equipment to foster the greatest number of interactions at that energy. "We made nearly 200 runs on Blue Horizon at SDSC and Lemieux at PSC," Hu said, "and we anticipate much larger runs as we push toward more highly excited targets.

"What we found was good news," she said. "Both the cross-sections and the differential cross-sections for the formation process we examined were broader than those that had previously been predicted, which means that it may become much easier to obtain antihydrogen in amounts sufficient for study." The results have been published in Physical Review Letters 88: 063401 (2002), and the Journal of Physics B: Atomic, Molecular, and Optical Physics 34: 331-338 (2001) and 35: 3879-3886 (2002).

"Hu has identified an interesting resonant process that may be worth pursuing," said Clifford Surko, an antimatter pioneer and professor of physics at the University of California, San Diego. "The route to antihydrogen that she calls out requires colder positronium atoms than the teams are able to produce at present, but that may well be in the offing." The apparatus at CERN, which has been shut down for several months, will begin a new series of experiments shortly.

Not yet, then, the antimatter engines of Star Trek. "The experimentalists and the theorists still have a lot of work to do," Hu said, "but we will be testing both the fundamental laws of physics and our ability to simulate physical processes with greater and greater accuracy."

In the meantime, the scope and importance of the questions antimatter can answer have both increased. With the development of cosmology and deep-space observation over the last few decades, a paradox has arisen: if matter and antimatter are exactly symmetric, as physicists suppose, then equal amounts of both should have been created in the Big Bang. So why, then, does there appear to be only matter (and no antimatter) in the universe as far as astronomers can detect? If antihydrogen is shown to violate the postulated initial symmetry, that will challenge the "standard model" of particle physics and the fundamental theories of space and time that go with it. The only way to find out will be to produce slow-moving antihydrogen, cold enough for scientists to examine its optical spectrum. "Both theorists and experimentalists should be closing in on these questions in the next few years, and then maybe," she said with a wink, "we can consider fuel for spaceships!"