The Dark Matter Mystery

[For a graphical description of the dark matter problem, check out the comic written and illustrated by Andy Mastbaum, a former undergraduate working on the project: The miniCLEAN Book]

Astronomers have been looking outward, cataloging the contents of the universe, for centuries. At the same time, scientists have also observed (and sometimes created) many different forms of matter in the laboratory. Amazingly, much of what we see in the cosmos appears to be made of particles that can be recreated and studied here on Earth. The result of decades of work, the Standard Model provides the ingredients for everything from stars and planets to cosmic rays.

However, careful measurements of galaxies, clusters, and the cosmic microwave background radiation have revealed a puzzle. The data point toward a universe filled with invisible matter; matter which does not emit, reflect, or absorb light, but nevertheless can be "seen" through its gravitational effects. For lack of a more precise term, this matter has been given the name "dark matter." Study of dark matter in the cosmos has led us to conclude that it cannot be made from any particle in the Standard Model. The WMAP experiment has determined with great precision that the matter/energy content of the universe is composed of 23% dark matter, and only 4% comprises the particles described by the Standard Model.

This great puzzle mirrors the discovery of helium in the Sun by astronomers in 1868. First seen as an unknown emission line in the solar spectrum, it was another 27 years before helium could be isolated in the laboratory and studied. Dark matter has been found at large scales in the universe, and the race is on to detect it in a laboratory environment to learn about its microscopic properties. Does dark matter interact through the weak force, like a neutrino? Is dark matter very light or very heavy? Can new theoretical ideas like supersymmetry explain the new particles that form the dark matter?

Experimental Detection

There are many avenues for expanding our understanding of dark matter:

Until we know more about dark matter, it is important to pursue all of these avenues with as many varied approaches as possible. No single experiment can cover all the possible dark matter options.

One well-motivated theory is that dark matter consists of very heavy particles that interact very weakly with matter. Such particles might each weigh more than an iron atom, but interact more rarely than a neutrino. These hypothetical particles are called Weakly-Interacting Massive Particles, or WIMPs for short.

Many experiments have searched for WIMPs, and even more experiments are planned or under way that will dramatically increase our sensitivity to rare interactions between matter on Earth and WIMPs. The general approach to WIMP detection is to build a very clean detector, low in natural radioactivity, as large as possible, and place it deep underground to shield it from cosmic rays. A WIMP passing through the experiment will sometimes interact with the nucleus of one of the atoms in the detector. The collision will deposit some energy in the detector that can be observed via a variety of technologies. The challenge for any experiment is to distinguish a WIMP collision from events caused by electrons, neutrons, and gamma rays depositing similar amounts of energy in the detector.

The DEAP & CLEAN Family of Detectors

The DEAP & CLEAN collaborations are pursuing a staged approach to WIMP detection, focused on exploiting the unique properties of liquid argon and neon as a scintillator. All noble gases scintillate when charged particles pass through them, and when liquified, have a high enough density to make an effective WIMP target. In addition, the timing of the scintillation light allow nuclear recoils (the signal of WIMPs) to be separated from electron recoils (caused by natural radioactivity) with extremely high efficiency.

Our focus is on purely liquid detectors (also called "single-phase" detectors), rather than liquid+gas detectors, and specifically on neon and argon. The simplicity of a liquid-only detector, and the relatively low cost of argon and neon, make it practical and inexpensive to scale the technology up to 50 tonne targets with minimal R&D.

To demonstrate the technology, the collaboration has a sequence of argon and neon detectors completed, under construction, and planned for the future:

Starting with DEAP-1, the DEAP/CLEAN family of detectors are designed to function as both R&D devices to understand and validate the single-phase technology, and also as dark matter experiments with sensitivities comparable to competing technologies. With this approach, we plan to contribute significantly to the ongoing race to find dark matter, while also preparing ourselves to scale up to the next larger detector.