Bubble Chambers Aid Search for Dark Matter


The Tevatron collider shut down at the end of September, but Fermilab physicists are still active in the ongoing search to directly detect dark matter.

To aid in the research they’re resurrecting bubble chambers and fixed target experiments dating back to the 1970s.

ANALYSIS: Where is Dark Matter Hiding?

Bubble chambers are basically vessels filled with superheated liquid to detect particles moving through it.  A new experiment underway aims to achieve better calibration for the bubble chambers used in the Chicagoland Observatory for Underground Particle Physics (COUPP) experiment, located 350 feet underground in a Chicago tunnel. It’s called the COUPP Iodine Recoil Threshold Experiment (CIRTE), and it’s designed to improve the sensitivity of the COUPP detector.

The current working model for the actual “stuff” in the universe calls for only about 4 percent regular matter. The rest of the universe is comprised of dark energy (73 percent), which is causing the expansion of the cosmos to accelerate, and dark matter (about 23 percent).

So what is dark matter? One of the leading contenders to that answer is weakly interacting massive particles (WIMPs). WIMPs would be a new type of matter that almost never interacts with regular matter — they only interact through the gravitational and weak nuclear forces (hence the “weakly interacting” moniker).

WIMPs share certain qualities with neutrinos. Like WIMPs, neutrinos interact only rarely with other subatomic particles, although they’re definitely a lot “chattier” than WIMPs. Perhaps that explains why many facilities and detectors currently studying neutrinos can also be used to search for WIMPs.

There are lots of different experiments, using a variety of approaches, designed to search for dark matter particles, with some promising — if hotly debated — preliminary results. But bubble chambers were nearly extinct in the field before COUPP leader Juan Collar hit upon the notion of using them to search for dark matter. They’re great as neutrino detectors, too.

In 2007, COUPP installed a new germanium-based neutrino detector 330 feet below ground in the sewers of Chicago, renting this unusual lab space from the city. The design was modified to detect WIMPs instead of neutrinos.

COUPP’s “detector” is a glass jar filled with a liter or so of a fire-extinguishing liquid (iodotrifluoromethane). When a WIMP hits a nucleus of one of those atoms, it triggers an evaporation of a small amount of that liquid, producing a tiny bubble.

It’s initially too tiny to see, but it grows, and that growth can be recorded with digital cameras.

“The bubbles in the fluid are slow enough that high-speed cameras will capture the changes through continuous still shots. We’re making the world’s most boring movie,” Peter Cooper, the Fermilab physicist heading up CITRE, told Symmetry Breaking.

Once the bubbles reach about one millimeter in size, the COUPP scientists can study the images in earnest, looking for telltale statistical variations between photographs. Ideally, this enables them to distinguish whether a bubble resulted from background radiation, or from a dark matter particle.

COUPP currently operates at SNOLAB in Canada, another underground physics facility. The group has succeeded in placing some fundamental limits on certain properties for WIMPs.

But there’s still quite a bit of uncertainty about how much energy is required to create a bubble in the chamber, which means the experiment isn’t as sensitive as it could be. And that’s where CITRE comes in. Per Symmetry Breaking:

Scientists will fire pions, the lightest meson, in the Fermilab Test Beam Facility at a tiny pen-sized bubble chamber to measure how much energy needs to be deposited in the chamber to form a bubble. The CITRE collaborators will use a fixed-target technique called elastic scattering of pions. The pions interact with iodine, the target nucleus in the COUPP bubble chambers with the most sensitivity to the most popular dark matter candidates. The pions are surrogates for dark matter – the bubble chamber sees them both in the same way by observing the bubble from the recoiling iodine. Unlike dark matter, however, pions can be easily observed with other detectors on both sides of the bubble chamber, allowing COUPP scientists to know how much energy the pion gave to a scattered iodine nucleus.

CITRE scientists are currently running beam tests on various targets before they bring in the bubble chamber, to make sure they fully understand how a bubble, once formed, is likely to behave. The more CITRE can reduce the energy uncertainty, the better the measurements will be. And since the entire chamber has to be recompressed after each single bubble, it’s important to get it right.

With CITRE’s help, dark matter’s days of hiding might just be numbered.

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