Dark matter makes up 24% of the universe and the gravitational effect of its presence is the glue that holds galaxies together, but what is it?
This is one of the questions occupying Professor Davide Costanzo, experimental particle physicist and team leader of the Sheffield ATLAS group. On the 10th of February 2021 Professor Costanzo gave an illuminating seminar on the topic of dark matter investigations at the Large Hadron Collider (LHC) hosted by the Nuclear and Particle Physics Society (NPPS). He gave an overview of the theories, which mostly centre around dark matter being a new kind of particle that hasn’t been detected before, and highlighted the neutralino as a possible candidate. The neutralino is a hypothetical particle which arises from supersymmetry. Though this is a popular theory it does not yet have any experimental evidence to back it up.
There is evidence that dark matter exists on an astronomical scale, but no experiment here on Earth has detected anything that might be a dark matter particle. Many have tried to detect it directly using, for example, underground tanks of xenon and watching for dark matter particles colliding with the quarks in the xenon atoms. So far nothing has been found. Enter the LHC and a different approach to the dark matter search. Instead of trying to observe dark matter particles in the wild, physicists at the ATLAS experiment, including Professor Costanzo, are attempting to produce and detect evidence of the particles as a product of proton-proton collisions.
ATLAS is the largest particle detector apparatus at the LHC in Geneva, with around 2000 physicists working on it. It allows them to study the momentum of the particles produced in high energy collisions, and therefore identify them and establish their properties. If a pair of dark matter particles are produced in a collision between quarks they will not be indicated in any way by the ATLAS detector, since dark matter is by nature weakly interacting. However since momentum is conserved, if any events are recorded where a jet of particles is ejected in one direction with nothing to balance this in the other direction – a monojet event – then the missing momentum may indicate a new particle which could be dark matter.
Many of these monojet events have already been analysed and the missing momentum has been compared to outcomes consistent with the Standard Model of particle physics. One alternative outcome involving particles we already know about is the production of a particle called a Z-boson which then decays into two neutrinos, which are also undetectable by ATLAS. To date all of the analysed monojet events have agreed with processes already described by the Standard Model. Whilst this endeavour has not provided any evidence of a dark matter particle, it has allowed physicists to set a lower limit for the expected mass of this theoretical particle.
This does not mean the LHC will not find dark matter particles in future. Its third run is scheduled to begin in March 2022 and from 2025 to 2027 it will be shut down and upgraded to the High Luminosity LHC (HL-LHC). The aim is to enable it to collect ten times more data than it was originally designed to. Even if the new upgraded HL-LHC doesn’t find evidence of the nature of the dark matter particle, assuming it is indeed a particle, there is always a bigger collider! The Future Circular Collider (FCC) is a theoretical collider that could run at energies up to almost 8 times greater than is currently achievable by the LHC. Since dark matter particles are predicted to be heavy then higher energies may be necessary to produce them, and in this case we may have to wait until the 2040s for the next collider to be built to unravel this and other particle mysteries.
Keep an eye on the NPPS Facebook page for future seminars.