A joint Fermilab/SLAC publication

GammeV: dark energy and chameleons on a shoestring


Call it the Energizer Bunny of particle physics: The GammeV collaboration reins in costs, works fast and just keeps going.

The 10-person Fermilab collaboration formed in April 2007 to look for candidates for dark matter and dark energy. Although several members work on other experiments as well, they were drawn to GammeV’s unique attributes. Its small size allows for large individual roles in building and analysis, plus a chance to search for exotic particles while exploring areas of physics often overlooked by larger collaborations.

By scrounging parts, including an Accelerator Division laser, a Tevatron dipole magnet, and QuarkNet circuit boards, from old experiments, the collaboration kept to a budget of $30,000. The collaboration took less than a year to propose, build and publish results from its first experiment setting new exclusion limits on axion-like particles in the milli-eV mass range.

The collaboration tweaked its equipment and produced in less than a year another set of results, this time on the chameleon particle.

What's a chameleon particle?  A special Results of the Week article in today's issue of Fermilab Today explains:

In the chameleon dark-energy model, the observed acceleration of the universe is caused by a light, spin-zero particle that evades other experiments. This evasion is due to the fact that the properties of the hypothetical chameleon particle---namely its mass---depend upon the environment, hence the chameleon moniker. GammeV collaborators exploit this effect to trap these chameleon particles in a jar-like vacuum chamber.

Collaborators generate chameleon particles by interacting polarized laser light with a magnetic field. Some of the photons oscillate into chameleon particles, which pass through the jar but bounce off of the walls of the vacuum chamber--including the optical windows at each end. When the laser is turned off, the jar empties as the chameleons reconvert to detectable photons. This afterglow is a telltale signature of the chameleon particle.

While they didn't find a signal in this round of work, the results did put constraints on some of the properties of the evasive particle, including its mass and its coupling to photons.

Next on the agenda is a possible proposal to upgrade the chameleon experiment during the next year. Once again the experiment would carry a discounted price tag by taking advantage of the old equipment with a few internal modifications, mainly to the vacuum pump system, and another recycled magnet.

The first chameleon search looked at exclusion plots on “fringe or extreme” models of chameleon particles as dark-energy candidates. The second experiment is expected to probe exclusion plots for a much wider range of models.

How will the collaboration follow that up? Members hope to eventually upgrade their original axion study, but that would require a much larger budget and three to five years of work.

This story first appeared in Fermilab Today on February 6, 2009.

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