In search of the Axion

[C1ay]

And then there's the "dark matter" question: Just what is that mysterious dark matter that makes up 23 percent of the universe?

 

Lawrence Livermore physicists are working on the answer, and they think it comes down to a strangely ubiquitous particle...

 

lefthttp://hypography.com/gallery/files/9/9/8/Axion_assembly_thumb.jpg[/img]...A particle that hasn't truly been found yet in experiments, but when detected, will help define how our galaxy came together, the nature of quantum physics and how much of the dark matter is made up of these elusive particles.

 

Scientists believe the axion is a very light particle with no electric charge and only the feeblest interactions with anything else. To find the axion, Lab researchers have built the Axion Dark Matter Experiment (ADMX) in the shell of a 1953 Lab building that once housed a fusion research program called Project Sherwood.

 

The axion experiment is quite a bit different. It involves creating intense magnetic fields to detect the elusive particle. The experiment is designed to detect an axion by its decay into a single, real microwave photon in the presence of a magnetic field. It consists of three basic components: a powerful 8-tesla superconducting magnet, a high-Q and tunable cavity and ultrasensitive microwave amplifiers as the front end of a radio receiver.

 

"So if the axions are out there, they can convert to microwave photons in the presence of the magnetic field," said Gianpaolo Carosi, a postdoctoral researcher working on the Livermore experiment. "We can check if the signal is just radio interference by turning off the magnet. If the signal disappears with the magnet off, it is a strong indication that it must be coming from axions."

 

Though 23 percent of the universe consists of dark matter, no one really knows its make-up. Most theories contend the dark matter is a remnant from the Big Bang.

 

"We believe (axions) were created abundantly in the early universe," said Karl van Bibber of the Laboratory Science and Technology Office, who co-leads ADMX with Leslie Rosenberg of the Physics and Advanced Technologies Directorate.

 

At the Earth's distance from the center of our galaxy, there are 100 trillion axions in each volume of space the size of a sugar cube.

 

"You, me, cars, planets, stars, etc., make up only a tiny fraction of the mass in the universe. The vast majority is some new, unseen stuff," Rosenberg said. "The discovery of axions would therefore be a scientific advance on par with the leap from earth-air-fire-and-water to atoms. We would suddenly understand what nature has hidden from us for so long."

 

The axion was first proposed as a way to explain a difficult problem in particle physics: the absence of charge-parity (CP) symmetry violation in strong, or nuclear, interactions. Charge-parity symmetry refers to the notion that the world around us would look the same in a "mirror world" where all electric charges are reversed © and left and right are exchanged (P). Such violations are seen in weak interactions, and violations in strong interactions are expected to be considerably larger. Sensitive experiments, however, have failed to find any evidence of strong-interaction CP violation. To explain this observation, in 1977 Stanford University physicists Roberto Peccei and Helen Quinn proposed a new symmetry of nature that resulted in a particle dubbed the axion.

 

The Livermore axion experiment began in 1995 with funding from the Department of Energy's Office of Science. The Laboratory's Directed Research and Development (LDRD) Program supported the work that laid the groundwork for the experiment. The goal for the experiment was to extend LDRD efforts on one major front: increased power sensitivity. Livermore's plan called for increasing the sensitivity of the amplifiers and the size and magnetic-field strength of the cavity volume.

 

Increasing the sensitivity of the amplifiers comes in the form of SQUIDs - not the eight-armed marine creature, but a new amplifier based on a microstrip-coupled superconducting quantum interference device fabricated by LLNL scientist Darin Kinion. DOE's Office of High Energy Physics funded the experiment upgrade with SQUID amplifiers. The cavity also has grown from the size of a coffee can, in the original experiments at the University of Florida and Brookhaven National Laboratory, to an oil drum-sized cylinder in the Livermore version.

 

The Livermore group - which also is made up of researchers from UC Berkeley, the University of Florida and the National Radio Astronomy Observatory - expects to begin the new experiments with SQUIDs later this summer.

 

There are two hypothetical elementary particles that are the main candidates for the make-up of dark matter: a stable weakly interacting massive particle (WIMP), and the axion. Several groups have taken up looking for WIMPs, but Livermore is sticking to axions for now.

 

"They are ubiquitous and weakly interacting, but their credibility is very high at this time," van Bibber said. "If we detect them, they would represent a unique window back to the time of the Big Bang. We only expect to see 100 axion conversions per second, which is a tiny portion of them."

 

Other Livermore participants include project manager Steve Asztalos and two undergraduate summer students, Karl Twelker and Ben Westbrook.

 

Westbrook got involved when he heard van Bibber give a talk about the experiment at his school, the University of San Francisco.

 

"This is exciting," Westbrook said. "We're probing some unknown waters because we don't even know if they exist."

 

But not knowing whether axions exist doesn't slow down the team for a minute.

 

"I believe axions are there to be found," Rosenberg said. "The case for axions has only grown stronger in time. I also can't ignore my sense of aesthetics: axions feel right and I would be greatly surprised if they didn't exist."

 

Source: Lawrence Livermore National Laboratory

[Pyrotex]

The search for axions in multi-Tesla magnetic fields is interesting. but this leads to the question: do axions also interact with mega-Tesla magnetic fields found near some neutron stars? If they do, should we not see microwave spikes in the spectrum from such neutron stars? Given the number of high field neutron stars in the universe, how long would it take for a significant amount of axions to decay? How much energy (in the form of microwaves) would be added to our universe over, say, 13.5 Billion years?

 

Using permanent magnets, would it be possible to build a microwave oven that runs on axions? :D :D :)