Bright light/dark matter: Free-electron lasers enter the realm of particle physics
Nuclear and particle physics are concerned with the smallest bits of our universe: from the particles that zip along and wink out of existence in less than seconds to those that gather by the untold billions to form matter that lasts eons as brightly shining stars.
Scientists have studied these particles for the last hundred years or so by producing them in ever-more-powerful machines and by building ever-larger arrays of sensitive devices to detect them. But in all of those studies, they have yet to spot at least one form of matter – dark matter.
Dark matter is thought to provide the extra "oomf" of gravity that glues stars together into galaxies, among other niceties. Although we have yet to see this matter, astronomers have inferred its presence by noting its effects on the matter in our universe that we can see.
"So there is more and more evidence for dark matter from space telescopes. The only question is to figure out what it is," says Andrei Afanasev, a senior staff scientist at Jefferson Lab and an assistant professor at Hampton University. He says these views of the effects of dark matter and our knowledge of how the universe works have even led to predictions of how bits of dark matter may be produced and studied by humans for the first time.
One of these predictions has spurred Jefferson Lab scientists to join the hunt for glimpses of dark matter and, by doing so, these physicists are breaking new ground in high-energy particle physics.
In spring 2007, the JLab's Free-Electron Laser was used to hunt for a predicted dark matter particle. This was the first demonstration that high-intensity light sources can be used to perform experiments complementary to high-energy particle physics machines, like the newly completed Large Hadron Collider, or LHC.
The goal of the experiment was to test whether one predicted dark matter particle, the axion, could be produced and detected using the "light shining through a wall" technique. In the experiment, photons that comprise the laser light from the FEL were passed through a strong magnetic field, where some photons were predicted to convert into axions. The photons and axions then encountered a wall (actually, a thick block) of aluminum.
"If the axions are created, they should pass through several inches of aluminum with no problem," Afanasev explains. Photons, on the other hand, would be blocked.
Beyond the wall was another strong magnetic field. If axions were created in the first magnet and traveled through the wall, this second magnet would reconvert the axions back into photons; in a sense, shining light through a wall. But Afanasev says that no evidence for regenerated photons was seen.
"We saw backgrounds, which we carefully analyzed and verified. And at the end of the measurement, we realized that we could not claim any axion signal in this range," he says.
The experiment was carried out by the multi-institutional LIPSS (Light Pseudoscalar and Scalar Search) collaboration at the Jefferson Lab FEL, the world's most powerful tunable laser. Jim Boyce, a Jefferson Lab FEL staff scientist and technology transfer manager, says the FEL has provided a clear result that may not have been possible elsewhere in such a short period of time.
"The FEL is so powerful, it produces so many photons, you stand a chance of seeing something were it there. And you can see it in a reasonable amount of time - a predicted one per hour," Boyce explains.
This result is in agreement with published results from other collaborations of particle physicists searching for axion-like particles, including the PVLAS, BMV, GammeV and BFRT collaborations.
Nevertheless, Boyce says this result is exciting "because it establishes new boundaries, new regions where people speculate these particles could exist."
Indeed, when combined with results from other research efforts in this arena, the LIPSS collaboration's results place new limits of fundamental importance on theories in particle physics, cosmology and astrophysics.
"Our work touches on some of the same physics that the LHC touches on," says O. Keith Baker, a professor at Yale University and an LHC experimenter, as well as the lead spokesperson on this research effort. "What we've demonstrated is the versatility of the FEL: that we can do these kinds of experiments and the advantages of doing these experiments with a free-electron laser."
As for the next step, Baker says most of the experimental collaborations conducting this type of research are planning second-generation experiments.
"What began as a search for the axion has now expanded to tests of dark energy, string theory and new ideas of how to increase sensitivity to the axion," he explains.
The collaboration's research effort was kicked off by a grant from the Office of Naval Research, which provided most of the research funding. Additional funding was provided by Yale University, Hampton University and Jefferson Lab.
"We went from an idea to a running experiment in less than a year, and I think a lot of the credit should go to the FEL staff, who pulled together to make this happen," Baker says.