The MIGDAL Collaboration aims to unambiguously detect the Migdal effect under the most favourable conditions. The prediction of this effect dates back to the work in 1939 by Russian physicist Arkady Migdal. He noticed that quantum mechanics predicts that if a particle gives a sudden jolt to the nucleus of an atom, there is a small probability that the atom will emit a high-energy electron.

This effect was largely ignored by the particle physics community but in the last decade or so, various theoretical physicists realised that it could have significance in the search for dark matter. For over 30 years, physicists have been running laboratory experiments to search for dark matter particles, which current theories say are constantly streaming through all of the Earth. Usually, these experiments search for dark matter particles that scatter elastically with an atomic nucleus. This scattering collision gives the nucleus some energy, which can be detected with these exquisitely sensitive detectors operating deep underground. The theoretical physicists realised that the sudden collision of the dark matter particle with the nucleus can also be followed by the emission of an electron, exactly through the mechanism predicted by Arkady Migdal. Although this happens only rarely, many of the most sensitive dark matter detectors find it easier to detect electrons rather than a recoiling atomic nucleus, so the Migdal effect helps in the search for dark matter particles with a mass between the electron and proton masses, which are predicted by many particle-physics theories of dark matter.

We know the Migdal effect is promising for dark matter searches but it has never before been observed in the context of nuclear scattering. The MIGDAL experiment aims to detect the tell-tale emission of the low-energy electrons ejected from atoms and molecules upon the scattering of fast neutrons providing a unique and unambiguous signal of the Migdal effect. A beam of fast neutrons will be used as a proxy for the dark matter particles as both dark matter and neutrons are electrically neutral and the way they interact with the atomic nucleus shares many similarities. It is also relatively straightforward to buy equipment that produces a neutron beam while no-one has yet been able to produce a dark matter beam in the laboratory!

The MIGDAL experiment will employ an Optical Time Projection Chamber to detect neutron interactions in a gaseous medium at pressure far below the normal atmospheric pressure. Our detector allows three-dimensional tracks to be reconstructed through the following detector sub-systems: i) track ionisation is drifted to a double Gas Electron Multiplier (GEM) and converted to an optical signal which is imaged by a fast CMOS camera; ii) the amplifier charge is collected at an anode plane segmented into readout strips to obtain the perpendicular coordinate; iii) a photomultiplier tube detects both the primary and secondary scintillation light to provide the absolute depth coordinate. All of this to say that we have designed a system to capture and measure the tracks from the recoiling nucleus and the emitted electron that originate at a common vertex, as they must if the electron is emitted from the atomic nucleus that has been hit by a neutron. Operating the experiment at a small fraction of atmospheric pressure will allow the electron tracks to go further, giving us a better chance of ‘photographing’ them in the detector.

The experiment will be installed at the Neutron Irradiation Lab for Electronics (NILE) facility at ISIS facility at the Rutherford Appleton Laboratory in the UK, and be exposed to intense D-D and D-T neutron generators in a series of runs at varying detector conditions and utilising different gas mixtures based on carbon tetrafluoride (CF4), a molecular gas involving carbon and fluorine. Carbon tetrafluoride serves as the base gas because of its high scintillation yield and spectrum compatibility with our CMOS camera system. The experimental work will be complemented by a much-needed theoretical framework extending existing models.

The MIGDAL Collaboration benefits from the expertise of scientists working in six countries. The project started in October 2019 with funding by the UK’s Science & Technology Facilities Council. Additional funding and contributions have been provided by the Particle Physics Department at the Rutherford Appleton Laboratory.