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Scientists Ponder Universe's Missing Antimatter

John Roach
National Geographic News
July 6, 2005
 
Why is the universe dominated by matter? It is among the most perplexing questions to face particle physicists, scientists who study the tiniest building blocks of the universe.

Theories of physics require that for every particle of matter created at the big bang—the cosmic explosion that marked the beginning of the universe—so too was its antiparticle equivalent, or antimatter, said Persis Drell, a particle physicist at the Stanford Linear Accelerator Center (SLAC) in Menlo Park, California.

"That's fine, but all we see now is matter. Everything on Earth, in the solar system, everything as far out as we can see is made of matter," she said. "What happened to all the antimatter?"

Elvin Harms, head of the antiproton source department at the Fermi National Accelerator Laboratory in Batavia, Illinois, said another way to phrase the question is: Where is the antimatter?

"Is it just this part of the universe that tends to be dominated by matter?" he said, raising the possibility that other parts of the universe are dominated by antimatter.

In recent years researchers have begun to formulate answers to these questions, but their best explanations fall short of accounting for the matter-antimatter imbalance in the universe today.

According to Drell, a new particle collider under construction on the border between France and Switzerland by European Organization for Nuclear Research (CERN) and a second collider proposed by the international science community may provide more answers.

"We are hoping from these particle colliders—the Large Hadron Collider and the [International] Linear Collider—to go another step deeper and make progress," she said.

Particle Annihilation

Matter and antimatter share nearly identical properties except the antiparticle has an opposite electric charge from the particle. For example, an electron has a negative charge, so its antiparticle, the positron, has a positive charge.

Since opposites attract, particles and their antiparticle counterparts are inclined to join together. But when they do, they annihilate each other in a flash of pure energy.

Star Trek fans may recognize the matter-antimatter reaction as the fuel for the spaceship Enterprise. "It's true and cool," Drell said. "If you take an electron and anti-electron they will annihilate, and you'll get pure energy."

The head-scratching problem for particle physicists is, if equal amounts of matter and antimatter were created at the big bang, why is there only matter left around? Why didn't all the matter and antimatter meet up and annihilate, leaving a universe without matter?

According to Drell, shortly after the big bang, most of the matter and antimatter did annihilate, but one in a billion particles survived "and that became the galaxies, the universe, and us as we know it now," she said.

But why? How?

CP Violation

The answer lies at least partially in an effect called charge parity (CP) violation that indicates matter and antimatter decay differently, Drell said.

In experiments conducted at the B factory particle collider at SLAC, Drell and colleagues were able to partially determine the differences in decay between elementary particles and antiparticles.

But the degree of CP violation determined by this experiment—and others—fails to account for the entire matter-antimatter imbalance in the universe.

"We don't have a complete understanding yet. Our current picture and understanding doesn't give enough CP violation," Drell said.

The hope is that ever more sophisticated instruments at the Large Hadron Collider and the International Linear Collider will allow particle physicists to look for other processes and effects that account for the asymmetry between matter and antimatter.

The Large Hadron Collider is scheduled to be turned on in 2007. It will accelerate protons around two 16.7-mile (27-kilometer) rings to near the speed of light and smash them together.

The International Linear Collider is still in the proposal stage. It would create high-energy particle collisions between electrons and positrons. Positrons are the antimatter counterpart to electrons.

The resulting explosions at the colliders will tear the particles apart, allowing scientists to study what makes up the matter and answer questions about the structure of the universe, including what happened to all the antimatter.

Harms of the Fermi National Accelerator Laboratory equated these next-generation particle colliders to "supermicroscopes," allowing scientists to study the collisions with finer resolution.

"They'll allow us to achieve higher energy than what we have now to probe further into the structure of matter," he said.

Currently, the Tevatron collider at Fermilab is the most powerful particle accelerator in the world. It brings protons and antiprotons together in a 4-mile (6.4-kilometer) long underground ring.

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