When I mention the words ‘dark side’, one may directly think to the Force, to Star Wars and the great series of movies from the 70ies / 80ies. Pale copies of such movies appeared later, but I am belonging to the 30+ category so that I grant me the right to intentionally ignore them.
Let us stop here the Star Wars parenthesis. I will not talk about movies, but about particle physics. And in particular about dark matter.
[image credits: the DarkSide collaboration]
DARK MATTER AND THE WAY TO DETECT IT DIRECTLY
I will start this post by recapping some basics about dark matter. This consists of a form of matter that does not interact via electromagnetic interactions.
- It is invisible (electromagnetic interactions are connected to light). That is why it is dark.
- It is sensitive to gravity in a similar way than matter is. That is why it is called matter.
There are several way to search for it, and for more extensive information, I refer to the hitchhiker guide to the quest for dark matter that I wrote two months ago.
Among the various ways to look for dark matter, there is one that is relevant for this post: dark matter direct detection. Dark matter travels through Earth and the idea is to build a giant detector and observe the collision of a dark matter particle with the detector material. This is summarized in the figure below:
[image credits: mine]
In this picture, one dark matter particle (noted DM) interact with one particle of the Standard Model (noted SM) that is part of the detector. Something is happening in the blue blob (let us skip any detail) and the two colliding particles are then recoiling against each other. The trajectory of the dark matter particle has been modified and there is a nuclear recoil in the detector that can be measured. These nuclear recoils are our dark matter signals.
Such events are rare, but having a giant detector and observing during a long time allow us to have a non-negligible probability for having some them occurring. So far, nothing has been observed and the properties of the dark matter particles turn out to be more and more constrained.
WHAT ABOUT THE DARK SIDE AND DARKSIDE
There are many detectors that have been built to directly detect dark matter, and one of the technologies that has been considered relies on liquid argon found underground.
A collision of an argon nucleus with a dark matter particle generates a recoil that gives rise, secondarily, to a sequence of reactions whose measurements allow to characterize the initial event. In particular, the tridimensional picture of the event can be reconstructed.
Such a detector has actually been recently built, as shown in the picture below.
[image credits: Inspire ]
This detector is called the DarkSide-50 detector, and the first results came out last year!