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HARVARD GAZETTE ARCHIVES

Gabrielse
Gerald Gabrielse plots ways to slow down antiatoms so physicists can see if they are very different from ordinary atoms. (Staff photo Stephanie Mitchell/Harvard News Office)

Lasers produce slow, cold antiatoms

So where has all the antimatter gone?

By William J. Cromie
Harvard News Office

A new way to make colder atoms of antimatter has been found. It could help bring scientists closer to understanding why we, and everything else, are made out of matter instead of antimatter.

According to the best theories, ordinary matter and its shadowy twin were created in approximately equal amounts when the universe came into existence some 14 billion years ago. If that's true, then where has all the antimatter gone?

"The imbalance is an embarrassing thing in physics, something we don't talk about," admits Gerald Gabrielse, Leverett Professor of Physics at Harvard University. To solve this universal mystery, researchers want to get a good look at antimatter atoms. To get such a view, they need a good way to slow them down.

To start with, this stuff is extremely difficult to make. Antimatter moves much faster than a speeding bullet and it annihilates itself when it hits ordinary matter, like air or the walls of a container. The problems involved in overcoming this were solved in 2002 by using supercold vacuum traps that keep antimatter suspended away from the walls with magnetic fields. Using such "traps," as they are called, antiprotons can be combined with antielectrons to make atoms of antihydrogen.

The process is vastly more difficult than electrons and protons coming together naturally to make plain hydrogen. And added to all the expense and exotic engineering is the fact that such custom-made antiatoms move too fast to get a good look at them.

One way to slow them down is to collide the heavier antiprotons with lighter antielectrons. Gabrielse compares this to "slowing a car by driving it through a garage full of Ping-Pong balls." Two groups have successfully done such experiments, but the antihydrogen atoms they make still move too fast to see if they are fundamentally different from ordinary hydrogen.

In the antimatter business, slow means cold. The colder the atom, the less energy it has and the slower it moves. These experiments are conducted at temperatures hovering around 455 degrees below zero Fahrenheit.

"We need really cold atoms before we can grab onto them tightly enough to keep them from getting away," Gabrielse explains. "So far this is as close as we've come."

Looking for a brake

By "we," Gabrielse means an international group of researchers from the United States, Canada, Germany, and Switzerland that calls itself ATRAP, for Antihydrogen TRAP. They work near Geneva at the Center for European Nuclear Research, the only place where antimatter is made.

The new slowing technique starts with shooting lasers at atoms of cesium, ordinarily a soft silvery-white metal. The extra energy excites electrons orbiting the nucleus or core of the cesium atoms. The excited atoms are in a cold magnetic trap with antielectrons zipping around them. The relatively larger atoms make it highly probable that they will strike an antielectron and form an electron-antielectron pair.

Electrons carry a negative charge, antielectrons a positive one, making it more likely they will get together. But the experimenters have to work very, very fast. Such a pair exists for only a ridiculously small fraction of a second before the two particles annihilate each other.

Those pairs that survive long enough are rapidly shuttled into a second trap full of antiprotons. Collisions in this supercold, high vacuum cause the joining of an antiproton and antielectron to form antihydrogen.

Those who want to know more details about the technique can read the ATRAPpers report in the Dec. 31 issue of Physical Review Letters, or the Jan. 7 issue of Science magazine. It's all very complicated, and this technique actually produces fewer antiatoms than bumping together the antielectrons and antiprotons directly. But the laser method gives more of the control over the product that physicists require. With more refinement, Gabrielse and his colleagues hope that this is the brake they need to answer the biggest question in physics.

What would God do?

When antiatoms are moving slowly enough, they will be probed with another type of laser system, one capable of seeing details of known and unknown subatomic particles in their innards.

Experiments done so far at the largest accelerator (atom smasher) facilities have found some intriguing minor differences between matter and antimatter, but they're not enough to provide a good answer to why the universe is the way it is. Physicists have jiggled their theories of subatomic particles to accommodate these differences. The question persists, however, about whether there may be bigger variations, big enough so that "we would have to revise all our theories about matter at the most fundamental level," in Gabrielse's words.

But what would such differences be like? The short answer is, nobody knows. "We're fishing for a thing whose color, shape, and size we don't know," Gabrielse comments. "To describe it requires some sort of a framework. But there's no framework believed and adopted by all physicists that predicts how things might be different."

If differences are found, "we'll rejoice and we'll cope," Gabrielse continues. Of course, many physicists don't expect to find things that will upset theories they were trained to accept. There might well be another way to explain the imbalance in the universe. Either way, the experiments have to be done.

"It's an irresistible candidate for measurement," Gabrielse declares. "God decides and we measure. That's how it works."







Copyright 2007 by the President and Fellows of Harvard College