RWTH Aachen Particle Physics Theory

Looking for Dark Matter: Dark(ness) at the end of a tunnel

April 24th, 2019 | by

In the last post, we introduced the “Shake It, Make It, Break It” approach for dark matter detection and talked about shaking dark matter to deduce its properties (what is usually called Direct Detection). Of course, the ideal way to study dark matter would be to create it in our laboratories, which brings us to the second approach: Make It. 

Attempts to make dark matter are carried out in particle colliders. To make dark matter we destroy light (aka visible matter). But before we go down that road, let’s have quick look at the Standard Model of Particle Physics. This model describes the physics of everything visible around us. It tells us the structure of atoms, the workings of three of the four forces governing all interactions in nature, the mechanism by which particles gain mass, the mechanism by which they decay. Essentially, it is a summary of our knowledge of particle physics. And although the standard model has been an incredible success, it appears to be somewhat incomplete. For example, we don’t yet have a particle that mediates gravity, the fourth force. We don’t know conclusively why neutrinos have mass. Or why the Higgs mass appears fine-tuned. The incompleteness of the Standard Model provides a strong motivation for additional particles. If dark matter is composed of particles, it could be one of the missing puzzle pieces in the Standard Model. And much like in an actual puzzle, we can deduce the properties of the missing piece by the space its absence has created. We can figure out which Standard Model particles are likely to talk to dark matter and build robust models around these interactions. With this basis for dark-visible interactions, we can look for them at colliders.

Since dark matter is invisible to our detectors, its presence after a collision can be deduced from the absence of the energy which it carries away.

A particle collider is used to accelerate particles to high energies, smash them together and study the resulting debris to understand the physics of nature at small (length) scales. One can look for dark matter at colliders by figuring out whether this debris matches our expectation from the Standard Model (which doesn’t include dark matter). A simple way to do this (in principle) is by using the law of conservation of energy– in any physical process, the total energy of the system remains conserved. The initial energy of the particle beams is something we know from an experiment’s design. The total energy after a collision can be reconstructed from the energy of the particles we detect. If these two numbers don’t match, we know that some of the energy has been carried away by “invisible” particles which could be potential dark matter candidates. (In practice, this is much harder to do which is one of the reasons we have an entire subgroup of physicists (theorists and experimentalists) devoted to working out the intricacies of collider physics.)

Another way to look for dark matter at colliders is by studying how the Standard Model particles produced in a collision decay. Consider the Z-boson. In the Standard Model, it can decay into quarks, leptons or neutrinos. We know the total decay width of the Z-boson which characterizes the probability that a Z-boson would decay. We can also get measurements on the individual probability of a Z-boson decaying into quarks, leptons, and neutrinos. A mismatch between these probabilities is a hint that a Z-boson decays into something else which is invisible to our detectors. Once again, we can deduce the presence of dark matter by its absence.

We’ve known for quite some time that there is more to the Universe than meets the eye. To understand it, we must exhaust all avenues available to us. Crashing particles being one of them.

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