Dark Matter Mapped Out

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.