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Lighting up the dark universe

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The CHASE detector. The end of the magnet (orange) can be seen on the right.

Exploring our dark universe is often the domain of extreme physics. Traces of dark matter particles are searched for by huge neutrino telescopes located underwater or under Antarctic ice, by scientists at powerful particle colliders, and deep underground.  Clues to mysterious dark energy will be investigated using big telescopes on Earth and experiments that will be launched into space.

But an experiment doesn't have to be exotic to explore the unexplained. At the International Conference on High Energy Physics, which ended today in Paris, scientists unveiled the first results from the GammeV-CHASE experiment, which used 30 hours' worth of data from a 10-meter-long experiment to place the world's best limits on the existence of dark energy particles.

CHASE, which stands for Chameleon Afterglow Search, was constructed at Fermilab to search for hypothetical particles called chameleons. Physicists theorize that these particles may be responsible for the dark energy that is causing the accelerating expansion of our universe.

"One of the reasons I felt strongly about doing this experiment is that it was a good example of a laboratory experiment to test dark energy models," says CHASE scientist Jason Steffen, who presented the results at ICHEP. "Astronomical surveys are important as well, but they’re not going to tell us everything." CHASE was a successor to Fermilab's GammeV experiment, which searched for chameleon particles and another hypothetical particle called the axion.

results that show...

Preliminary results from the GammeV-CHASE experiment, which rules out the existence of chameleon dark energy particles with a wide range of masses. The blue area on the graph, and the area between the two red lines, are areas of exclusion from GammeV-CHASE. Slide presented at ICHEP on July 23, 2010.

To create chameleon particles, the experiment shone a laser beam into a magnetic field. If chameleons exist, they would be created when photons from the laser beam scatter off photons from the magnetic field. The laser was left on for a certain amount of time to build up enough chameleons, then turned off to allow the chameleon particles to convert back into photons. The "afterglow" photons would then be recorded by the experiment's photo-detectors.

It took about six weeks for CHASE's 10 scientists to collect the 30 hours of data they needed to search for a dark energy discovery. According to the preliminary results presented at ICHEP, the subsequent data analysis didn't reveal any chameleons, which allowed the experiment to rule out a wide range of dark energy models that predict such particles.

"This was the first experiment employing this particular technology that was sensitive to all chameleon dark energy models," notes Steffen.

The experiment, which took less than 18 months from design to the end of data taking, is already being dismantled. Although the CHASE team doesn't plan a successor right now, the search continues at the ADMX experiment in Washington, which is analyzing chameleon search data gathered using a different method.

"It would have been great to find something, but I can’t say that I was really sitting on the edge of my chair expecting to see chameleons," adds Steffen. "It would take new developments in how we approach the problem for us to make serious plans to continue to search for them."

CHASE's final results will be presented August 13 in a seminar at Fermilab.