One of the biggest remaining mysteries in particle physics is dark matter. It began when scientists observing galaxies found that the observable matter in them (such as stars or clouds of gas) does not produce a large enough gravitational pull to hold itself together; there must be some unseen type of matter present which keeps them together - they called it dark matter.
Physicists have proposed several candidates for dark matter and made many attempts to detect them. No experiment yet has managed to find direct evidence of these candidate particles but as the technology and methods of detection are refined many scientists still have hope that this will happen. A new experiment called MIGDAL at the Rutherford Appleton Laboratory (RAL) will hopefully help in the search for dark matter by revealing a new channel for direct dark matter detection.
Several dark matter (DM) detectors have been built all around the world. These detectors can be described simply as a large tank of a noble gas cooled down into a liquid state. When a DM particle or neutron enters the tank, it can interact with the atoms in the tank and produce a visible signal with a nuclear recoil. The hope is this signal would come from an unrecognised particle that we could call dark matter.
Visualisation of dark matter (Credit: Illustris). For more info on dark matter look here!
It is of course more complicated than this in reality, as we encounter issues due to background signals (i.e. the detector picking up signals from ordinary matter) and the sensitivity of the detector.
To deal with the first issue of background signals these detectors have been constructed deep underground using the Earth itself to provide a thick layer of shielding. This blocks out a significant proportion of ordinary particles whereas the potential DM particles, which are much less likely to interact with matter, can reach the detector.
The second issue of detector sensitivity is one with many innovative solutions. One of these is being explored as part of the MIGDAL experiment at RAL. To understand it, the principle of a DM detector should first be explained. When a DM particle hits an atom in the detector’s tank it causes the atom’s nucleus to recoil which creates a signal, the strength of which depends on the mass of the DM particle.
There is a limit to how sensitive the detector is i.e. there is a threshold for the signal strength below which it won't be detected. If the DM particle is low mass (<10 GeV) it will produce a signal below this experimental detection threshold. In a recent paper physicists realised that an old idea from nuclear physics could create a new channel for direct DM detection which would effectively lower the threshold energy. It’s known as the Migdal effect, and it states that when a nuclear recoil happens, because the atom’s surrounding electron cloud will take some time to catch up with the nucleus there is a small but distinct probability of one of those electrons getting excited and even ionised. This creates the possibility of a DM particle which is below the threshold energy to create a detectable signal but may be able to produce an electron via the Migdal effect. This would widen the range of the energy (and mass) of DM particles that could be detected. The Migdal effect was first proposed in 1939 by a Russian physicist Arkady B. Migdal and has yet to be observed experimentally in nuclear scattering. The newly established Migdal In Galactic Dark mAtter expLoration (MIGDAL) collaboration (which includes members from STFC’s Particle Physics Department) is aiming to observe this effect and explore its use in DM detection. The goal of the MIGDAL experiment is to detect the predicted electron shake-off emission, which is thought to accompany nuclear scattering with low, but calculable probability. The group have devised an experiment to study the Migdal effect by bombarding a low-pressure gas based on CF4 with high energy monoenergetic neutrons.
Detected scintillation light from the gas can be used to capture high resolution track images so that a 3D reconstruction of the electron emission accompanying a nuclear recoil can be produced, an example of what this would look like can be seen below.
Example image of tracks produced due to Migdal effect from neutron bombardment of Fluorine.
The challenge in observing the Migdal effect until now has been that it is a very rare event (occurring approximately once in a million collisions). This probalility increases with a higher energy nuclear recoil, and so to improve the chances of observing it in experiment a high energy neutron source is required. It just so happens that across the road from the Particle Physics Department (PPD) at RAL is the world-class facility, the ISIS Neutron and Muon Source. The upcoming NILE project at ISIS had already procured a DT generator (a desktop fusion device using deuterium and tritium to produce high energy neutrons at 14.1 MeV), planning to use it to test the effect of space weather on electronics. However, when a scientist from that team happened to sit down for a coffee with Pawel Majewski from PPD (and a member of the MIGDAL collaboration) they realised the DT generator would be ideally suited for testing the Migdal effect. Maria Kastriotou, Pawel Majewski, Christ Frost and Carlo Cazzaniga with the NILE instrument - used for the MIGDAL experiment.
The DT generator should provide a clear demonstration of the Migdal effect however, to probe the effect at the low nuclear recoil energies relevant to most dark matter scattering models (up to a few hundred keV) lower energy neutrons are needed from a DD generator (a fusion reactor using only deuterium as a fuel). In 2021 this generator was procured with UKRI’s World Class Laboratories funding.
This project would significantly broaden the range of dark matter particles that could be detected and improve the next generation of dark matter detectors, potentially giving us insight into one of the remaining unsolved problems of physics. Not bad from just a cup of coffee!