If or when SLAC’s planned project, the Glowing Obscure Matter Experiment (LDMX), receives funding – a decision from the Department of Energy is expected sometime next year – the project will scan for vivid shadowy matter. The experiment is designed to accelerate electrons toward a tungsten target at End Station A. In the immense majority of collisions between a speeding electron and a tungsten nucleus, nothing compelling will happen. But rarely – once in 10,000 trillion hits, if there is vivid shadowy matter – the electron will instead interact with the nucleus through an unknown shadowy force, creating vivid shadowy matter, greatly depleting the electron’s energy.
This 10,000 trillion is actually the worst-case scenario for delicate shadowy matter. This is the lowest rate at which shadowy matter can be produced consistent with thermal relic measurements. But Schuster says delicate shadowy matter could be created by one in 100 billion impacts. If so, then at the experiment’s planned collision rate, “that’s an excess amount of dark matter you could produce.”
Nelson said LDMX would need to operate for three to five years to definitively detect or rule out the vivid shadowy matter thermal relic.
Ultralight shadowy matter
Other shadowy matter hunters are fine-tuning their experiments for another candidate. Ultralight shadowy matter is similar to axion, but is no longer committed to solving the mighty CP problem. For this reason, it can be much lighter than ordinary axions, as delicate as 10 billionths of a trillionth of the mass of an electron. This diminutive mass corresponds to a wave of enormous wavelength, as long as a diminutive galaxy. In fact, the mass cannot be less, because if it were, even longer wavelengths would mean that shadowy matter could not cluster around galaxies, as astronomers observe.
Ultralight shadowy matter is so incredibly diminutive that the shadowy force particle needed to mediate its interactions is thought to be massive. “There is no name for these mediators,” Schuster said, “because it is beyond any possible experiment. It has to be there [in the theory] for consistency, but we don’t care about them.”
The origin story of ultralight dark matter particles depends on the specific theoretical model, but Toro argues that they formed after the Big Bang, so the thermal relic argument is irrelevant. There is another motivation for thinking about them. Particles arise naturally from string theory, a candidate fundamental theory of physics. These weak particles are created this way six diminutive dimensions According to string theory, they can be curled or “condensed” at any point in our 4D universe. “The existence of delicate axion-like particles is highly motivated by many kinds of string condensation,” said Jessie Shelton, a physicist at the University of Illinois, “and we should take it seriously.”
Instead of trying to create shadowy matter with an accelerator, experiments to search for axions and ultralight shadowy matter listen to the shadowy matter that supposedly surrounds us. Judging by gravitational effects, shadowy 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 shadowy matter to have a density of almost half the mass of a proton per cubic centimeter. Experiments attempt to detect this ubiquitous shadowy matter using powerful magnetic fields. In theory, ethereal shadowy matter sometimes absorbs a photon from a mighty magnetic field and converts it into a microwave photon that an experiment can detect.