Diary of a WIMPy Particle…
Four fundamental forces rule the universe: The “weak” and “strong” nuclear forces, gravity, and electromagnetism.
Everybody knows about gravity. We are all affected by it. In fact, if it has mass, it is affected by gravity. And gravity causes matter to behave in predictable ways. Here on Earth, this means when we throw something into the air, it comes back down. In space, this means less massive objects are drawn towards more massive ones.
Astrophysicists can determine a lot about the universe from gravity. They can estimate how different masses in the universe behave, and they can use those predictions to see how they affect phenomena around them. For the most part, their predictions were accurate, and mass was being affected by gravity just as expected. But, to their shock, they began to encounter problems.
The masses of stars were slightly off. Gravity was behaving as though more mass was present than there supposedly should have been. There were discrepancies in temperatures, velocities, and other physical phenomena. Scientists were baffled—and they found that the universe contains five times more mass than they thought.
But how could we overlook more than eighty percent of the mass in the universe?
And thus was born the theory of dark matter. We can’t see dark matter, because it’s not affected by electromagnetism (EM). This means it doesn’t emit light or any other forms of EM radiation that we can usually measure. There are also theories as to what sort of subatomic particle this dark matter is comprised of. A popular theory suspects they are “weakly interacting massive particles”—or WIMPs. As their name suggests, WIMPs interact only via gravity and the weak force. To draw a comparison, they have similar properties to a neutrino, except with much more mass.
It is this neutrino comparison which motivates physicists to model the behavior of WIMPs after certain interactions that neutrinos undergo. A theoretical phenomenon known as “neutrinoless double-beta (0νββ) decay” assumes that the two neutrinos that enter into the reaction are capable of being their own “anti” particle. In other words, these two neutrinos will eventually end up annihilating each other. (Consider a particle and antiparticle to be the numbers 1 and -1. If you add them together, you get zero.)
Experiments to prove that 0νββ decay truly exists are continuously underway, and also becoming more precise. The ability to successfully conduct and refine this reaction would reveal a great deal of information about the properties and behavior of neutrinos.
In the context of WIMPs—the idea is to take this 0νββ decay model and set “bounds” that tailor it to the slightly different criteria of these dark matter particles—which could, in theory, allow them to detect their existence. In fact, their research claims that this model may be one of the most successful and accurate methods of detecting dark matter.
In this model, the incoming WIMPs (or neutrinos) would start off with different amounts of energy, depending on its speed, and whether it’s in an “excited” state. They also have a “spin” which describes its angular momentum—a bit like pretending the particle is spinning in a certain direction, depending on its spin number. Particles with different spin numbers will interact differently.
These initial characteristics are crucial in predicting how the particles will behave, what the process of the decay will be, and how long it will take. It is important that scientists are absolutely sure of what they are looking for, both in terms of how the particles will behave, as well as their numerical calculations. This ensures the likelihood of not only finding what they are looking for, but actually knowing when they’ve found it. One of the most difficult hurdles in detecting WIMPs (and science in general) is ruling out “background noise” and anomalous occurrences that are inevitably detected, and deciding if what you found is what you’re looking for.
The ability to understand and predict the behavior of this mysterious dark matter will reveal volumes, not only about the universe at large—how it came to exist, how it will continue to behave—but also about the physics of our own daily reality. Dark matter is everywhere—dark matter is right in front of you! There is no doubt it has an impact, and perhaps it is possible to use it to our advantage. But before we can even consider that, we first need to be able to find it.