How do we make dark matter in the laboratory?
Everything we know about dark matter — so far — comes from its gravitational influence on other things: stars, gas, photons in the early universe, and so on. We’d like to detect dark matter on Earth, but to do so, we’re stuck with the properties of dark matter in our galaxy that Nature has given us. Specifically, dark matter is slow (with typical speeds about a thousandth of the speed of light), and its mass density is fixed (about one proton’s worth of mass per 3 cubic centimeters). Already we can see that there are a couple failure modes for dark matter detection: if its mass is too light, its slow speed might not give it enough kinetic energy to make anything exciting happen in a detector, and if there isn’t enough of it around, the event rate might be too slow.
The solution to both these problems is for us to make our own dark matter! This strategy has been around for a while, and is usually known as “collider detection,” where the signature at a high-energy collider like the LHC is a large amount of missing energy which would otherwise seem to violate conservation of momentum. But strictly speaking, this is detecting the absence of something, rather than the something (dark matter) itself. Recently, there has been a revival of interest in using a type of collider experiment more common in the 1960’s to detect lighter-mass dark matter. Here, a beam of electrons or protons collides with a fixed target (of graphite, beryllium, lead, or something else), rather than having two beams of electrons or protons collide with each other. The price we pay is that the total center-of-mass energy of the collision is much lower than the beam energy, but the reward is that the luminosity of the beam can be much higher, leading to much larger event rates. Most of the beam particles just stop inside the target without doing anything interesting, so these experiments are also known as beam dumps. But even if only a tiny fraction of these collisions produce dark matter, we can dial both the speed (by changing the beam energy) and the density (by changing the flux of beam particles) of the dark matter, and set up a detector downstream which is tailored to the properties we now have experimental control over. Instead of just seeing missing energy as at the LHC, we would see the telltale signatures of dark matter scattering at the detector.
There is a nice synergy with neutrino physics, since neutrino “beams” are made by using precisely this setup, and one can piggyback a dark matter experiment on top of a neutrino experiment which is already running. I’m interested in ways we can leverage the amazing infrastructure which has developed around neutrino physics to look for dark matter, and also in designing new experiments using beam dumps which are specifically tailored to look for models of dark matter which escape other experimental probes.
DAEδALUS and dark matter (video). Perimeter Institute, December 3, 2014.