If SLAC’s planned project, the Light Dark Matter Experiment (LDMX), receives funding (a Department of Energy decision is expected within the next year), it will search for light dark matter. The experiment is designed to accelerate electrons toward a target made of tungsten at 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 billion impacts, if light dark matter exists) will the electron interact with the nucleus through the unknown dark force to produce light dark matter, significantly draining the electron’s energy.
Those 10,000 trillion are actually the worst case scenario for light dark matter. It is the lowest rate at which dark matter can be produced to match thermal relic measurements. But Schuster says light dark matter could arise in more than one in every 100 billion impacts. If so, then with the experiment’s planned collision rate, “that’s an excessive amount of dark matter that can be produced.”
Nelson said LDMX will need to run for three to five years to definitively detect or rule out thermal relic luminous dark matter.
Ultralight Dark Matter
Other dark matter hunters have their experiments tuned for a different candidate. Ultralight dark matter is similar to an axion, but is no longer required 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 the electron. That tiny 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, even longer wavelengths would mean that dark matter could not concentrate around galaxies, as astronomers observe.
Ultralight dark matter is so incredibly tiny that the dark force particle needed to mediate its interactions is thought to be massive. “No name has been given to these mediators,” Schuster said, “because it is outside of any possible experiment. It has to be there (in theory) to maintain consistency, but we don’t worry about that.”
The origin story of ultralight dark matter particles depends on the particular theoretical model, but Toro says they would have emerged after the Big Bang, so the thermal relic argument is irrelevant. There is a different motivation to think about them. Particles are naturally derived from string theory, a candidate for the fundamental theory of physics. These weak particles arise from the way six tiny dimensions it could be coiled or “compacted” at every point in our 4D universe, according to string theory. “The existence of light axion-like particles is strongly motivated by many types of string compactions,” said Jessie Shelton, a physicist at the University of Illinois, “and it’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 more 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 centimeter. Experiments attempt 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.
