If or when SLAC’s planned project, the Light Dark Matter Experiment (LDMX), receives funding — a decision from the Department of Energy is expected within about a year — it will scan for light, dark matter. The experiment is designed to accelerate electrons toward a target made of tungsten in terminal A. In the vast majority of collisions between a fast-moving 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, increasing the electron’s energy is significantly depleted.
That 10,000 trillion is actually the worst-case scenario for light, dark matter. It is the slowest rate at which you can produce dark matter that matches measurements of thermal remnants. But Schuster says light dark matter could be created in more than one in 100 billion impacts. If so, then, given the experiment’s planned collision frequency, that’s “an excessive amount of dark matter to produce.”
LDMX will take 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 tailored their experiments to a different candidate. Ultralight dark matter is axion-like but no longer required to solve the strong CP problem. This allows it to be much lighter than regular axions, as light as 10 billionths of a trillionth of the electron’s mass. That small mass corresponds to a wave with an enormous wavelength, 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 observe.
Ultralight dark matter is so incredibly minuscule that the dark force particle required to mediate its interactions is thought to be enormous. “No name has been given to these mediators,” Schuster said, “because this is beyond any possible experiment. It must be there [in the theory] for consistency, but we’re not worried about that.”
The origin story of ultra-light dark matter particles depends on the specific theoretical model, but Toro says they would have formed after the Big Bang, so the thermal relic argument is irrelevant. There is another motivation for thinking about it. The particles follow naturally from string theory, a candidate for the fundamental theory of physics. These weak particles arise from the way six small dimensions can be coiled or ‘condensed’ at any point in our 4D universe, according to string theory. “The existence of light axion-like particles is strongly motivated by many types of string compactifications,” says Jessie Shelton, a physicist at the University of Illinois, “and it is 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 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 a proton 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 can be detected by experiment.