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"annihilationoccurs.
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 antimatterspecifically,
antihydrogento 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
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
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
antimatterand not just antiparticlescould 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-livedantihydrogen 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 Foundations
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
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 SDSCs Blue Horizon and the Pittsburgh Supercomputing
Centers 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!"