A step closer to building the first muon collider

6 February 2020

For the first time scientists have observed muon ionization cooling – a major step in being able to build the world’s most powerful particle accelerator. This new muon accelerator will give us a better understanding of the fundamental constituents of matter.

Since the 1930s, accelerators have been used to make ever more energetic proton, electron, and ion beams. Such beams have been used in practically every scientific field, from colliding particles in the Large Hadron Collider to measuring the chemical structure of drugs, treating cancers and the manufacture of the ubiquitous silicon microchip.

Now, the Muon Ionization Cooling Experiment (MICE), an international collaboration which includes scientists from Oxford’s Department of Physics, has made a major step forward in the quest to create an accelerator for an entirely different sort of particle – the muon. A muon accelerator could replace the Large Hadron Collider (LHC) and provide at least a ten-fold increase in energy for the creation of new particles. Beams of muons accelerated to high energies and stored in rings would provide intense sources of neutrinos – the most enigmatic known particles in the Universe – for precision studies of their properties.

Accelerating muons

Until now, the question has been whether you can channel enough muons into a small enough volume to be able to accelerate them and then use them to study physics in new, unexplored systems. This new research, published in Nature on 5 February 2020, shows that it is possible. The results of the experiment, carried out using the MICE muon beam-line at the Science and Technology Facilities Council (STFC) ISIS Neutron and Muon Beam facility on the Harwell Campus in the UK, clearly show that ionization cooling works and can be used to squeeze muons into a tiny volume.

Muons have many uses – they can be used to study the atomic structure of materials, they can be used as a catalyst for nuclear fusion and they can be used to see through really dense materials which X-rays can't get through. The research team hopes that this technique can help produce good quality muon beams for these applications as well as for fundamental physics.

Putting the theory to the test

The Oxford MICE group was instrumental in many aspects of the experiment from the bottoms-up calculations of the underlying physical processes which determine the efficiency of ionization cooling to the top-down design of the superconducting magnet channel; some components of the apparatus were constructed in the Oxford’s Department of Physics workshops.

Professor Emeritus John Cobb of Oxford University Department of Physics and the John Adams Institute for Accelerator Science, who jointly led the design and commissioning of one of the superconducting magnets, said: ‘Even though the physics underlying ionization cooling has been understood for many years, its practical implementation is extremely difficult. MICE successfully confronted the formidable technical challenges of designing and operating a system of coupled, high-field superconducting magnets, subject to potentially enormous forces, which tightly enveloped a delicate vessel containing many litres of – potentially explosive! – liquid hydrogen. The practical experience gained will be invaluable to the further development of muon accelerators.'

Demonstration of cooling by the Muon Ionization Cooling Experiment, Mice collaboration, Nature, 5 February 2020.