Overview: Discovering the Quantum Universe

Light on Dark Matter

There are many possible candidates for the particles that make up dark matter. When particle physicists suspect that the underlying theory for something is a complex, they call it a "moose".

Illustration:
Michael S. Turner
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Related Questions:

• What is dark matter?
• How can we make it in the laboratory?

Dark, adj. 1a. Lacking or having very little light. b. Lacking brightness.8. Difficult to understand; obscure. 9. Concealed or secret; mysterious.

The past decade has witnessed the startling discovery that over 95 percent of the universe is not made of ordinary matter, but instead consists of unknown dark matter and dark energy. Astrophysical observations have demonstrated that only four percent of the universe is made of matter like that here on Earth. Seventy-three percent is dark energy, and 23 percent is dark matter.

Dark energy is a mysterious force that fills the vacuum of empty space, accelerating the expansion of the universe. Physicists don’t know what dark energy is, how it works, or why it exists. They do know that it must ultimately have an explanation in terms of particle physics. Are dark energy and the Higgs field related? The discovery of supersymmetry would provide a possible connection. Supersymmetry provides a natural context for both the Higgs field and dark energy.

Definitive evidence for the dark universe has come from many sources, including astrophysical observations of clusters of galaxies that would have flown apart if visible matter were the only thing holding them together. As close to home as the Milky Way, visible matter alone would not hold the stars in their orbits. Dark matter holds the universe together.

What is this dark matter that binds the galaxies and keeps the universe from flying apart? Although dark matter is not made of the same stuff as the rest of the world, physicists have clues to its identity. Cosmological measurements favor "cold" dark matter – heavy particles moving at low speeds – as a major component. For now, though, the dark side of the universe remains a mystery.

Moreover, there is no reason to think that dark matter should be any simpler than visible matter, with its multiple quarks and leptons. New particles do not normally appear in isolation. The 1932 discovery of the positron, for example, signaled a new world of antimatter particles. Today, the challenge is to explore the world of dark matter by creating dark matter particles in the laboratory.

If dark matter is made up of weakly interacting massive particles (something like heavy versions of the neutrinos), cosmological calculations suggest that they would have Terascale masses, in the energy region of the LHC and the ILC. Is this Terascale conjunction a coincidence? Most theories of Terascale physics, although developed with different motivations, posit particles that may contribute to dark matter. For example, an oft-invoked dark-matter candidate is the lowest-mass supersymmetric particle, the neutralino, theorized to reside at the Terascale. The LHC and the ILC have the potential to produce dark matter particles identical to the dark matter present in the universe.

Besides accelerator experiments, other experiments are watching for individual dark matter particles in highly sensitive detectors deep underground. Astrophysics experiments, in turn, are seeking the cosmic remnants of dark matter annihilation in space. However, none of these experiments can positively identify dark matter without help from accelerator experiments.

Accelerator experiments will be able to place dark matter particles into context. For example, the LHC may identify a dark matter candidate in particle collisions. A linear collider could then zero in to determine its mass and interaction strength – to take its fingerprints and make a positive identification. By a fine-tuned scan of the energy scale, a linear collider could also flush out any potential dark matter candidates that might be hiding in the multitude of LHC collisions.

A linear collider’s measurements would allow calculation of a dark matter candidate’s density in the universe. In parallel, increasingly sophisticated cosmological observations will measure dark matter’s density to a corresponding accuracy. A match between the collider and cosmological measurements would provide overwhelming evidence that the candidate particle really is dark matter.