Researchers at the University of Oxford, together with colleagues at the UK Science and Technology Facilities Council (STFC) and at several other laboratories, have announced results from a new search at the European X-ray Free Electron Laser (European XFEL) Facility at Hamburg for a hypothetical particle that may make up the dark matter of the Universe. The experiment is described in a study published in Physical Review Letters .
This experiment looks for axions, a particle which was proposed to solve a major problem in particle physics: why neutrons, although composed of smaller charged particles called quarks, do not possess an electric dipole moment. To explain this, it was suggested that axions, tiny and incredibly light particles, can "cancel out" this imbalance. If observed, this process would provide direct evidence for new physics beyond the Standard Model.
Additionally, axions turn out to be a natural candidate for dark matter, the mysterious substance that constitutes most of the structure of the Universe.
The researchers used the world's largest and most powerful X-ray laser: the European X-Ray Free-Electron Laser (European XFEL) located in Schenefeld near Hamburg, Germany. This facility features a 3.4-kilometer-long tunnel housing a superconducting linear accelerator and photon beamlines, enabling the generation of ultrashort X-ray flashes at a rate of 27,000 per second.
These are directed through thin slabs of precisely oriented germanium crystals, which have an intense internal electric field. To moving particles, the electric field appears as a strong magnetic field (~10^3 Tesla), enabling photons to convert into axions, and back again.
An opaque titanium sheet inserted between the crystals acts as a barrier to photons, allowing only the axions being searched for to pass through. These are then detected when they convert back into photons in the crystal on the other side – known as the 'light-shining-through-walls' technique.
In this proof-of-principle study, the researchers demonstrated that their setup has sensitivity to axions that is already competitive with other experiments using particle accelerators. It paves the way for future experiments in which researchers will focus on axions in the milli- to kilo-electron volt mass range. They aim to improve the sensitivity by a factor of several hundred so as to be able to detect axions with properties predicted by the theory of Quantum Chromodynamics.
Lead author Dr Jack Halliday, an experimental plasma physicist at STFC, says, "This experiment underscores the versatility of XFEL technology in addressing some of the most challenging questions in fundamental physics and pushing the boundaries of our understanding of the universe."
Principal Investigator Professor Gianluca Gregori says, "This study is the culmination of a long-standing collaboration in the Department of Physics at Oxford between myself (Atomic and Laser Physics), Professor Subir Sarkar (Theoretical Physics), and the late Professor Ian Shipsey (Particle Physics). This experiment required a difficult interpretation of a non-standard measurement, and it was thanks to the wide expertise brought together by such a team that we were able to address it successfully".
