Physicists at the University of Sheffield have been working as part of a global team to create the world’s most powerful particle accelerator. The collaboration aimed to demonstrate how to create a muon accelerator, and their results could advance our understanding of the fundamental constituents of matter.

Since the 1930s, particle accelerators have been used to generate high-energy beams which are used in a range of scientific disciplines, from treating cancers to measuring the chemical structure of drugs, and from manufacturing silicon microchips to studying the make-up of matter using colliders such as the Large Hadron Collider at CERN.

So far, protons, electrons, and ions have all been accelerated into beams, but an international team called the Muon Ionization Cooling Experiment (MICE) collaboration, including physicists from the University of Sheffield, has been working towards creating a muon beam.

Muons are subatomic particles that arrive on the Earth’s surface after forming as a by-product of cosmic rays colliding with molecules in Earth’s upper atmosphere — about 10,000 muons reach every square metre of Earth’s surface, every minute. While their electrical charge and spin (an intrinsic property of elementary particles) are the same as those of an electron, muons have a mass 207 times as great. This means they have the potential to create beams with ten times more energy than those made in the Large Hadron Collider.

Muons have several uses, including studying the atomic structure of materials, as catalysts for nuclear fusion reactions, and seeing through materials which are too dense for X-rays to penetrate.

For experiments, muons are produced by smashing a proton beam into a target. The muons are separated at the target and directed through a series of magnetic lenses. The collected muons form a diffuse cloud, so in a collider, the chances of them hitting each other and producing interesting phenomena are low. To combat this, a process known as beam cooling is used. By bringing the muons closer together and making them move in the same direction, the muon clouds become less diffuse. However, the magnetic lenses used could only bring the muons closer together, or get them moving in the same direction — not both at the same time.

To combat this challenge, the MICE collaboration tested a new method. They cooled the muons by putting them through specially designed energy-absorbing materials, while the beam was tightly focused by powerful magnetic lenses. Following this, the denser cloud of muons could be accelerated by a normal particle accelerator in a precise direction. This dramatically increases the likelihood of a muon collision.

Physicists from the University of Sheffield designed and prototyped the target mechanism that produced the muons for the experiment.

Dr Chris Booth, from the University of Sheffield’s Department of Physics and Astronomy, who led the development of the particle-production target, said: “The techniques demonstrated by the MICE experiment will allow new particle accelerators to be constructed which could explore fundamental physics in areas ranging from neutrinos (subatomic particles with no electrical charge and very little mass) to the Higgs boson and the energy frontier that lies beyond the reach of the Large Hadron Collider.”

On Thursday 6 February MICE announced the success of this crucial step in creating a muon beam. They published their results in Nature on 5 February.

The experiment was carried out at the Science and Technology Facilities Council (STFC) ISIS Neutron and Muon Beam facility on the Harwell Science and Innovation Campus in Oxfordshire. The results of the MICE experiment show that this method of beam cooling (ionization cooling) works and can be used to advance development of the world’s most powerful particle accelerator.

Dr Chris Rogers, based at ISIS and the collaboration’s Physics Coordinator, explained: “MICE has demonstrated a completely new way of squeezing a particle beam into a smaller volume. This technique is necessary for making a successful muon collider, which could outperform even the LHC.”

Professor Ken Long, spokesperson for the experiment from Imperial College London, added: “The enthusiasm, dedication, and hard work of the international collaboration and the outstanding support of laboratory personnel at STFC and from institutes across the world have made this game-changing breakthrough possible.”

Featured image: The target mechanism designed by scientists at the University of Sheffield. Photo: STFC.

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