[Editors] MIT detector to aid dark-matter search

[Elizabeth Thomson]

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New MIT detector will aid dark matter search
--Calibration tool will reveal when hypothetical particles are detected
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For Immediate Release
WEDNESDAY, DEC. 10, 2008

Contact: Elizabeth A. Thomson, MIT News Office
E: thomson at mit.edu, T: 617-258-5402

Photo Available

CAMBRIDGE, Mass.--Several research projects are underway to try to  
detect particles that may make up the mysterious “dark matter”  
believed to dominate the universe’s mass. But the existing detectors  
have a problem: They also pick up particles of ordinary matter —  
hurtling neutrons that masquerade as the elusive dark-matter particles  
the instruments are designed to find.

MIT physicist Jocelyn Monroe has a solution. A new detector she and  
her students have built just finished its initial testing last week at  
Los Alamos National Laboratory. When deployed in the next few months  
alongside one of the existing dark-matter detectors, the new device  
should identify all of the ordinary neutrons that come along, leaving  
anything else that the other detector picks up as a strong candidate  
for the elusive dark matter.

“Dark matter experiments are very hard,” explains Monroe, who worked  
on the project with undergraduates Dianna Cowern and Rick Eyers and  
with graduate students Shawn Henderson and Asher Kaboth. “They are  
looking for a tiny signal, from a phenomenon that happens very  
rarely,” namely the collision of a dark-matter particle with one of  
ordinary matter, producing a tiny, brief flash of light.

Such flashes can be detected by putting a tank of liquid deep  
underground to shield it from most stray particles, then lining the  
tank with photomultiplier tubes that can pick up even the faintest  
bursts of light.

The problem is, even buried a mile underground, calculations show such  
detectors will pick up far more collisions from particles of ordinary  
matter than from those made of the still-unknown particles of dark  
matter. To be precise, the ordinary collisions should happen about 10  
billion billion times (19 orders of magnitude) more often than the  
dark-matter collisions. So learning how to rule out those ordinary  
collisions is the key to finding the unknown matter.

“We’re really trying to characterize the background,” Monroe explains.  
“We’re making a precise measurement of the energy spectrum of the  
neutron background.” By understanding the nature and intensity of this  
background, it will be possible to design more effective shielding  
material to keep them away from the detectors.

And by running the two detectors at the same time, anytime a signal is  
seen in the neutron detector, any signal seen simultaneously in the  
dark-matter detector can be safely ignored. Only when the dark matter  
detector sees something and the neutron detector doesn’t will there be  
a chance that one of the elusive dark-matter particles has been found.

Nobody knows what the dark matter is made of, but astronomers are sure  
it’s there because of the way its gravitational attraction pulls on  
other, visible matter in space. That allows them to determine just how  
much of the mystery matter is out there — more than five times as much  
as the amount of ordinary matter — but not what it’s made of.

Theorists have come up with a variety of candidates, but the leading  
contenders are a class of subatomic particles known as WIMPS — weakly- 
interacting massive particles. These are the types of particles,  
including one called the neutralino, which should be detectable by the  
deep underground experiments.

“I think probably in the next five years, someone will see a  
candidate” for a dark-matter particle, Monroe says. Although some  
experiments have already claimed to see possible evidence of dark  
matter, so far those claimed results “are surprising and unconfirmed,”  
Monroe says, and have not been accepted by most scientists.

To test the new detector, Monroe and her students took it to Los  
Alamos National Laboratory, where it was exposed to a neutron source  
so that its sensitivity could be precisely calibrated. Once the  
analysis of that test is completed, the device will be sent out to an  
underground laboratory, most likely at the planned Deep Underground  
Science and Engineering Laboratory. This facility, though not funded  
yet, would be set up in the Homestake Mine, a very deep old gold  
mining complex in South Dakota, and one of its multidisciplinary goals  
is provide the world’s deepest location for the detection of cosmic  
dark matter.

The research is partly funded by the National Science Foundation.

--END--

Written by David Chandler, MIT News Office
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