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If or when SLAC’s planned project, the Light Dark Matter Experiment (LDMX), receives funding — a decision from the Department of Energy is expected in the next year or so — it will scan for light dark matter. The experiment is designed to accelerate electrons toward a target made of tungsten in End Station A. In the vast majority of collisions between a speeding electron and a tungsten nucleus, nothing interesting will happen. But rarely — on the order of once every 10,000 trillion hits, if light dark matter exists — the electron will instead interact with the nucleus via the unknown dark force to produce light dark matter, significantly draining the electron’s energy.
That 10,000 trillion is actually the worst-case scenario for light dark matter. It’s the lowest rate at which you can produce dark matter to match thermal-relic measurements. But Schuster says light dark matter might arise in upward of one in every 100 billion impacts. If so, then with the planned collision rate of the experiment, “that’s an inordinate amount of dark matter that you can produce.”
LDMX will need to run for three to five years, Nelson said, to definitively detect or rule out thermal relic light dark matter.
Ultralight Dark Matter
Other dark matter hunters have their experiments tuned for a different candidate. Ultralight dark matter is axionlike but no longer obliged to solve the strong CP problem. Because of this, it can be much more lightweight than ordinary axions, as light as 10 billionths of a trillionth of the electron’s mass. That tiny mass corresponds to a wave with a vast wavelength, as long as a small galaxy. In fact, the mass can’t be any smaller because if it were, the even longer wavelengths would mean that dark matter could not be concentrated around galaxies, as astronomers observe.
Ultralight dark matter is so incredibly minuscule that the dark-force particle needed to mediate its interactions is thought to be massive. “There’s no name given to these mediators,” Schuster said, “because it’s outside of any possible experiment. It has to be there [in the theory] for consistency, but we don’t worry about them.”
The origin story for ultralight dark matter particles depends on the particular theoretical model, but Toro says they would have arisen after the Big Bang, so the thermal-relic argument is irrelevant. There’s a different motivation for thinking about them. The particles naturally follow from string theory, a candidate for the fundamental theory of physics. These feeble particles arise from the ways that six tiny dimensions might be curled up or “compactified” at each point in our 4D universe, according to string theory. “The existence of light axionlike particles is strongly motivated by many kinds of string compactifications,” said Jessie Shelton, a physicist at the University of Illinois, “and it’s something that we should take seriously.”
Rather than trying to create dark matter using an accelerator, experiments looking for axions and ultralight dark matter listen for the dark matter that supposedly surrounds us. Based on its gravitational effects, dark matter seems to be distributed most densely near the Milky Way’s center, but one estimate suggests that even out here on Earth, we can expect dark matter to have a density of almost half a proton’s mass per cubic centimeter. Experiments try to detect this ever-present dark matter using powerful magnetic fields. In theory, the ethereal dark matter will occasionally absorb a photon from the strong magnetic field and convert it into a microwave photon, which an experiment can detect.
Two proposed experiments intend to pursue the dark frequencies: Stanford University’s DM Radio and the ADMX-EFR, or Axion Dark Matter Experiment Extended Frequency Range, at the University of Washington. Both experiments were originally conceived to search for axions, but are now being revised to scan for the lighter variants.
The biggest challenge for both experiments — which also await the Department of Energy’s funding decisions — lies in the faintness of the predicted microwave photons. The tiny signal requires all experimental noise to be removed other than the system’s inherent quantum jitter, a hurdle that researchers think they can overcome.
While these researchers await the faint signals of feeble particles, other dark matter ideas remain in play. Some theoretical physicists are reconsidering a long-sidelined idea that this invisible matter might take the form of primordial black holes created during the Big Bang. One more possibility is that the prevailing theory of gravity isn’t quite right. So far, however, competing gravity theories haven’t garnered much interest.
“At this point,” Schuster said, “let’s be honest, everybody is guessing.”