November 7, 2013
Axion In A Haystack: The Hunt For Dark Matter Continues
April Flowers for redOrbit.com - Your Universe Online
A research team at the University of Washington, led by Professor Leslie Rosenberg, is hunting for a theorized but never-before-seen elementary particle called an axion. The researchers will use the recently retooled, extremely sensitive detector that is currently in a testing and shakeout phase at UW's Center for Experimental Nuclear Physics Astrophysics.
Physicists in the late 1970s first hypothesized the axion as a solution to a problem in a theory called quantum chromodynamics. Scientists know very little for sure about the axion. It appears to have no other interaction other than reacting gravitationally with matter.
Scientists have believed since the 1930s that there must be some unseen but massive substance - like a gravitational glue - that prevents rotating galaxies from spinning apart. If axions exist, they are candidates for the makeup of cold dark matter that would act as that gravitational glue.
About one-quarter of all the mass of the universe can be accounted for by dark matter, but scientists still aren't sure what exactly dark matter is. Finding axions is tricky because they react so little, and the reactions they are likely to produce are so faint and difficult to detect.
“We have probably the most sensitive axion detector in operation,” Rosenberg said. “It looks for the incredibly feeble interaction between the axion and electromagnetic radiation.”
The Axion Dark Matter Experiment (ADMX) is searching for cold dark matter axions in the halo of the Milky Way by detecting the very weak conversion of axions into microwave photons.
The team is using a detector that employs a powerful magnet surrounding a sensitive microwave receiver that is supercooled to 4.2 kelvins, or about minus-452 F. Thermal noise is reduced by such low temperatures, which also increase the chance that the detector will actually observe axions converting to microwave photons.
The researchers can fine-tune the microwave receiver to the axion mass. This will increase the possibility of detecting an interaction between axions and the detector’s magnetic field. A miniscule amount of electromagnetic power would be deposited into the receiver by a reaction, which could be recorded by computers monitoring the detector.
Previous efforts to locate the axion have been attempted, but the ADMX is garnering greater interest because of recent developments in physics research. The most notable of these developments is that the Large Hadron Collider (LHC), which discovered the elusive Higgs boson in 2012, did not find evidence to support supersymmetry, a proposed resolution for some inconsistencies among theories of particle physics.
The impetus to separate the search for dark matter from work on supersymmetry was provided by the LHC's lack of evidence. The newest version of the ADMX is drawing a great deal of interest from researchers, according to Rosenberg.
“This is a needle-in-a-haystack experiment. Once we find the needle, we can stop immediately,” Rosenberg said. “We could find it in our first week of data-taking, our last week of data-taking, or never.”
The detector assembly was completed in October. The team has since begun weeks to months of commissioning, which involves testing and fine-tuning the equipment before starting the hunt in earnest.