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 at end station A. In most collisions between an accelerating electron and a tungsten nucleus, nothing interesting will happen. But rarely—on the order of once in every 10,000 trillion impacts, if bright dark matter exists—the electron will instead interact with the nucleus via the unknown dark force to produce bright dark matter, greatly depleting the electron’s energy.
That 10,000 trillion is actually the worst-case scenario for light dark matter. This is the lowest rate at which you can produce dark matter to match the thermal relic measurements. But Schuster says that bright dark matter can arise in one out of every 100 billion impacts. If so, then at the experiment’s intended collision rate, “that’s an excessive amount of dark matter that you can produce.”
LDMX will need to operate 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 tuned their experiments to a different candidate. Ultralight dark matter is similar to axion, but is no longer bound to solve the strong CP problem. Because of this, it can be much lighter than ordinary axions, as light as 10 billionths of a trillionth the mass of an electron. This small mass corresponds to a huge wavelength wave, as long as a small galaxy. In fact, the mass cannot be smaller, because if it were, the even longer wavelengths would mean that dark matter could not be concentrated around galaxies, as astronomers note.
Ultralight dark matter is so incredibly tiny that the dark force particle required to mediate its interactions is believed to be massive. “There is no name given to these mediators,” Schuster said, “because it’s beyond any possible experiment. It should be there [in the theory] for consistency, but we don’t worry about them.’
The origin story of ultralight dark matter particles depends on the particular theoretical model, but Thoreau says they would have appeared after the Big Bang, so the thermal-relic argument is irrelevant. There is a different motivation for thinking about them. Particles naturally derive from string theory, a candidate for the fundamental theory of physics. These weak particles arise from the ways in which six small dimensions can be collapsed or “compacted” at any point in our 4D universe, according to string theory. “The existence of light axion-like particles is strongly motivated by many kinds of string compactifications,” said Jesse Shelton, a physicist at the University of Illinois, “and that’s something we should take seriously.”
Instead of trying to create dark matter using an accelerator, experiments looking for axions and ultralight dark matter listen to the dark matter that supposedly surrounds us. Based on its gravitational effects, dark matter appears to be most densely distributed near the center of the Milky Way, but one estimate suggests that even here on Earth, we can expect dark matter to have a density of almost half the mass of a proton per cubic a centimeter. Experiments attempt to detect this ever-present dark matter using powerful magnetic fields. In theory, ethereal dark matter will occasionally absorb a photon from the strong magnetic field and convert it into a microwave photon that can be detected by experiment.
