[cdn-nucl-l] Physicists Create Antimatter

September 19, 2002

More Sci- Than Fi, Physicists Create Antimatter

By DENNIS OVERBYE

hysicists working in Europe announced yesterday that they had passed through nature's looking glass and had created atoms made of antimatter, or antiatoms, opening up the possibility of experiments in a realm once reserved for science fiction writers. Such experiments, theorists say, could test some of the basic tenets of modern physics and light the way to a deeper understanding of nature.

Matter and antimatter are like the good and evil twins of nature; they are endowed with equal and opposite characteristics like charge and spin, so if they meet they obliterate each other, releasing a flash of energy upon contact.

Weird as it sounds, antimatter is a normal feature of the real, unfictional universe. Scientists see the creation of antihydrogen atoms as the first step toward testing some physicists' deepest notions about nature, which hold that antimatter should look and behave identically to ordinary matter.

For example, any violation of the expected symmetry between hydrogen and antihydrogen would rock physics to its core.

The new research was conducted by physicists at CERN, the particle physics laboratory outside Geneva.

By corralling clouds of antimatter particles in a cylindrical chamber laced with detectors and electric and magnetic fields, the physicists assembled antihydrogen atoms, the looking glass equivalent of hydrogen, the most simple atom in nature. Whereas hydrogen consists of a positively charged proton circled by a negatively charged electron, in antihydrogen the proton's counterpart, a positively charged antiproton, is circled by an antielectron, otherwise known as a positron.

They then observed the flashes of energy when the new antihydrogen atoms annihilated themselves in collisions with ordinary matter in the walls of the chamber.

At least 50,000 antihydrogen atoms have been created since the experiment began in August, said Dr. Jeffrey S. Hangst, from Aarhus University in Denmark, who coordinated efforts by 39 physicists from 10 institutions in a collaboration named Athena. A paper on their results has been accepted for publication in Nature and was posted on its Web site www.nature.com/nature.

While antihydrogen atoms have been glimpsed in CERN experiments before, this is the first time they are being produced under circumstances that might eventually permit their detailed study, the scientists say.

"It's very exciting to see the production of antihydrogen," said Dr. Rolf Landua, a CERN physicist and Athena member. "It opens up a whole new line of experiments with antimatter."

In publishing its paper, Athena appears to have just beaten a consortium, also based at CERN and known as Atrap, which has also been in the forefront of antimatter research. In an e-mail message, Atrap's leader, Dr. Gerald Gabrielse of Harvard, called the production of antihydrogen an "important and challenging" milestone, adding that his group had seen similar signals.

Dr. Alan Kostelecky, a theoretical physicist at Indiana University, called it "a phenomenal technical achievement," and said that Athena and Atrap were the "the Wright Brothers of antimatter physics." Dr. Kostelecky compared today's announcement to the first powered flight: "Who knows where it will lead. Nobody can foresee it."

In science fiction, antimatter, with its perfect convertibility to energy, is the ultimate rocket fuel, but the CERN scientists see their antihydrogen atoms as a ticket not across the galaxy but in effect to a different mathematical universe, in which positive is negative and left is right.

According to the standard theories of physics, the antimatter universe should look identical to our own. Antihydrogen and hydrogen atoms should have the same properties, emitting the exact same frequencies of light, for example.

But some theorists have speculated that the symmetry between matter and antimatter might be violated in some versions of theories that seek to unite Einstein's theories on gravity with the weird rules of quantum mechanics. The most ambitious such "theory of everything," which portrays particles as strings wriggling in 10-dimensional space-time, seems to preserve the matter-antimatter symmetry, but theorists do not really understand why.

Antimatter has been part of physics since 1927 when its existence was predicted by the British physicist Paul Dirac. The antielectron, or positron, was discovered in 1932. According to the theory, matter can only be created in particle-antiparticle pairs. It is still a mystery, cosmologists say, why the universe seems to be overwhelmingly composed of normal matter.

In modern laboratories like CERN or Fermilab in Illinois, physicists accelerate antiprotons or positrons produced by nuclear reactions to the speed of light and collide them with conventional particles to produce tiny starbursts of primordial energy, recreating forms of matter and energy unseen since the big bang.

The raw material for CERN's antimatter factories is made by shooting protons, or naked hydrogen nuclei, into a iridium target, but "instead of speeding them up," Dr. Landua said, "we slow them down."

To do that, CERN built the Antiproton Decelerator, which uses electric and magnetic fields to slow the particles from near light speed to about one-tenth of that, he said.

From there the antiprotons go into a "catching trap," where most of their remaining energy is absorbed and radiated away by electrons swirling in a magnetic field, lowering them to a temperature of about 15 degrees above absolute zero and speeds of a few hundred feet per second. Meanwhile, positrons from the decay of a form of radioactive sodium are separately slowed and accumulated. The two clouds of oppositely charged particles are then superimposed by adjusting electrical fields in a cylindrical "mixing trap" lined with detectors.

Once an antiproton and positron have joined forces, the resulting antihydrogen atom is electrically neutral and thus no longer caged by the electrical fields in the trap. "The atom drifts to where it wants to go," said Dr. Landua, namely the wall where its components will annihilate with their opposite numbers in a characteristic pattern: the antiproton into a spray of lighter particles called pions, and the positron producing a flash of gamma rays. The Athena team recorded this pattern 131 times and based on simulations, concluded that it had produced at least 50,000 antihydrogen atoms.

The Athena experimenters say they still know very little about their antihydrogen atoms. They say that in the spring, they hope to train a laser on the antihydrogen to make a preliminary measurement of the atom's spectrum so that it can be compared to regular hydrogen.

"Any measurement would be interesting because we know essentially nothing about it," Dr. Hangst said.