WIMPs – the search for dark matter is expanding. And going small.

Components of a low-mass dark matter detector now in development. Credit: Marilyn Chung/Berkeley Lab

Dark matter is abundant in the universe, and even though we can’t detect it is by far the most common form of matter, making up about 85 percent of the universe’s total. It hides in plain sight so we don’t know what it’s made of but we can infer it by its gravitational pull on known matter.

Theorized weakly interacting massive particles, or WIMPs, have been among the ideas for what comprises dark matter, but they haven’t shown up where scientists had expected them. It may be time to look beyond popular notions with a lot of funding, such as big projects like Europe’s CERN, and use lots of smaller nets.

The reason to avoid a heavily funded popularity contest is that dark matter could be much “lighter,” or lower in mass and slighter in energy, than previously thought. It could be composed of hypothetical wavelike ultralight particles known as axions. It could be populated by a wild kingdom filled with many species of as-yet-undiscovered particles. And it may not be composed of particles at all.

Some have gotten behind building low-mass dark matter experiments, which could expand our current understanding of the makeup of matter as embodied in the Standard Model of particle physics. That was proposed in a United States Department of Energy report, “Basic Research Needs for Dark Matter Small Projects New Initiatives”, signaling that America may be ready to get back into the particle physics game.

The report proposes a focus on small-scale experiments – with project costs ranging from $2 million to $15 million – to search for dark matter particles that have a mass smaller than a proton. Protons are subatomic particles within every atomic nucleus that each weigh about 1,850 times more than an electron. This new, lower-mass search effort will have “the overarching goal of finally understanding the nature of the dark matter of the universe,” the report states.

The report highlights three major priority research directions in searching for low-mass dark matter that “are needed to achieve broad sensitivity and … to reach different key milestones”:

  1. Create and detect dark matter particles below the proton mass and associated forces, leveraging DOE accelerators that produce beams of energetic particles. Such experiments could potentially help us understand the origins of dark matter and explore its interactions with ordinary matter, the report states.
  2. Detect individual galactic dark matter particles – down to a mass measuring about 1 trillion times smaller than that of a proton – through interactions with advanced, ultrasensitive detectors. The report notes that there are already underground experimental areas and equipment that could be used in support of these new experiments.
  3. Detect galactic dark matter waves using advanced, ultrasensitive detectors with emphasis on the so-called QCD (quantum chromodynamics) axion. Advances in theory and technology now allow scientists to probe for the existence of this type of axion-based dark matter across the entire spectrum of its expected ultralight mass range, providing “a glimpse into the earliest moments in the origin of the universe and the laws of nature at ultrahigh energies and temperatures,” the report states.

This axion, if it exists, could also help to explain properties associated with the universe’s strong force, which is responsible for holding most matter together – it binds particles together in an atom’s nucleus, for example.

Searches for the traditional WIMP form of dark matter have increased in sensitivity about 1,000-fold in the past decade.

In a related effort, the U.S. Department of Energy this year solicited proposals for new dark matter experiments, with a May 30th deadline. Perhaps the program we will be talking about in the future is among those that got the nod.