Question: Is It Possible To Make Dark Matter?

Susan Cartwright answered on 14 Jun 2015:

My friend Dan certainly hopes so – his work on the ATLAS experiment at CERN involves searching for dark matter particles created in the proton-proton collisions produced by the LHC. I, personally, am not holding my breath – on the other hand, I wouldn’t be astonished if it happens.

The key point here is that nobody knows what the dark matter actually is – all we know is that none of the particles in our Standard Model of particle physics has the right properties, so it’s something new. Theorists are unhappy with the Standard Model anyway – too many important numbers, such as most of the particle masses, have to be put in “by hand” and are not predicted – and many different extensions of the Standard Model naturally produce particles with the right properties to be dark matter. Some of these can be made, usually in the LHC, while others can’t.

Probably the most popular candidate for dark matter arises from a theory called supersymmetry. In this theory, every particle in the Standard Model has a more massive, and hence as yet undiscovered, “partner” particle that differs from it by half a unit of “spin” (spin is a quantum property related to, but not identical to, the ordinary spin of a spinning coin or planet; like many quantum properties, it can’t take any value it wants, but only multiples of a certain unit – in this case, Planck’s constant divided by 2 pi). Being massive, these partner particles decay into the lighter particles of the Standard Model that we know, with the exception of the very lightest supersymmetric particle, which cannot decay because there is a conserved quantum number that forbids it (just as the electron cannot decay because it is the lightest particle with non-zero electric charge, and charge is a conserved quantity – you’re not allowed to destroy it). This lightest supersymmetric particle, or LSP, is a good candidate for dark matter: supersymmetric particles would be produced along with ordinary particles in the ultra-high energies of the very early universe (by E = mc^2), and although the heavier supersymmetric particles would soon decay the LSP couldn’t, and would still be around.

If the LSP is indeed the dark matter particle, we expect that it and its fellow supersymmetric particles might be produced at the LHC, provided that their masses are low enough that the LHC energy is enough to produce them (again, E = mc^2). The LSP itself would not be detected in experiments – dark matter, like neutrinos, interacts very feebly with ordinary matter, and the LSP would pass through the ATLAS detector as if it were empty space – but because energy and momentum are conserved there would be “missing energy” in the event – a “gap” in the distribution of the observed particles corresponding to the presence of particles that aren’t observed. This “missing energy” signature is searched for by ATLAS and CMS as a sign of supersymmetry, and if they see it they will have created dark matter particles. No such signature was observed in LHC run I, but they have just started collisions at higher energy, so they may see something over the next year or so that would not have been possible at lower energies.

The LSP is not the only form of dark matter than could be created in the LHC: other, less popular, candidates like “Kaluza-Klein particles” (predicted by some theories that invoke extra space dimensions) could also be produced, and would be seen in the same way. However, other potential dark matter particles could not be made like this. One such particle is the “axion”, which is postulated by some theorists as an explanation for why particles and antiparticles are not more different than they are (there is a very slight difference, but the structure of the Standard Model suggests that there should be a much bigger difference, which is not seen). The axion is made in the early universe, but by a process which is not the same as the ordinary(!) conversion of energy into matter by E = mc^2. It is possible that the axion may be discovered “in the wild” – most probably by an experiment called ADMX, which another of my friends down the corridor works on – but I don’t know of any way of creating it.