A new technique for forming high quality particle beams has been achieved in an international collaboration involving the University of Strathclyde.
Physicists have, for the first time, demonstrated an important technique required to harness a type of particle known as a muon in a way which could enable the creation a new class of particle accelerator- required to study fundamental physics.
The new muon accelerator will offer a better understanding of the fundamental constituents of matter. It could potentially replace the Large Hadron Collider (LHC), providing at least a tenfold increase in energy for the creation of new particles.
The MICE (Muon Ionization Cooling Experiment) project has shown that it is possible to channel a sufficient number of muons into a high-energy accelerator to enable research in new areas of particle physics.
Muons have similarities with electrons but are around 200 times their mass. They also have an extremely brief lifespan, of two millionths of a second.
Compared to electrons and protons this short lifetime means all ‘manipulation’ of muons must be done extremely quickly. The MICE project has proven the physical principle of ionisation cooling required to quickly ‘order’ the muon beam. Once ordered the muons can be accelerated which extends their lifetime through relativistic ‘time dilation’.
The experiment was carried out at the Science and Technology Facilities Council (STFC) ISIS Neutron and Muon Beam facility on the Harwell Campus in Oxford. The results have been published in the journal Nature.
Dr Kevin Ronald, a Reader in Strathclyde’s Department of Physics, led the University’s team working on the MICE project. He said: “Muon accelerators open up a range of exciting physics research. To achieve comparable energies with electron beams, other approaches can require much larger diameter circular tunnels. A muon accelerator could address the needs on a much smaller footprint. Muons also offer access to other important physics- including the study of neutrinos.
“The MICE project has shown that the effective ‘temperature’ of a muon beam can be reduced within the lifetime of the muons. The results of this research will inform future accelerator research and the ways in which technology will develop for accelerators.”
Dr Colin Whyte, a Senior Research Fellow in Physics at Strathclyde, played a leading role in the international collaboration as project officer, co-ordinating the multinational team working on the apparatus for the MICE project. He said: “This project provides new options which could be exploited following the LHC. One options would be to build a new accelerator, using this technology, to study the Higgs boson.
“The findings of MICE have taken muon cooling from something that was viewed as theoretically possible to something that has been shown in practice.”
Muons are the product of another type of subatomic particle, pions, which are produced by driving a beam of protons into a target. Pions of the correct momentum can be separated from the debris created at the target by magnets and directed through a further series of magnets where they decay to produce muons. These muons form a diffuse cloud, meaning that they are difficult to accelerate and there is a low chance of them colliding and producing useful interesting physical phenomena.
To make the cloud less diffuse, a process called beam cooling is used. This involves re-ordering the muons, bringing them closer together and moving in the same direction. Magnetic lenses can achieve either of these effects but not both at the same time.
The ultra-short lifespan of muons is a major obstacle to cooling them and previous methods took too long to achieve an effect. In the 1970s, a new method known as ionization cooling was suggested and it was subsequently developed into theoretically operable schemes, but testing this idea in practice remained a significant hurdle.
The MICE Collaboration developed a methodology and equipment to study this new approach to cooling the muons. This involved arranging to pass the muons through specially-designed, energy-absorbing materials such as lithium hydride, a compound of lithium and hydrogen, or liquid hydrogen cooled to around minus 250 degrees Celsius and contained by exceptionally thin aluminium windows.
These energy-absorbers were incorporated into a series of powerful superconducting magnetic lenses used to focus the beam. This combination of energy absorbing elements with strong magnetic focussing allows the required re-ordering of the muon beam. The measurement is so delicate that it required measuring the beam particle-by-particle, using techniques more commonly found in particle physics than the usual accelerator diagnostics.
After cooling the beam, the muons, now filling a much smaller volume and moving in the same direction, can be accelerated much more efficiently by a normal RF particle accelerator with a much greater chance for muon collisions. Alternatively, the accelerated cold muons can be stored so that their decay products can be studied.
Dr Ronald and Dr Whyte work in the Atoms, Beams and Plasmas laboratory in Strathclyde's Department of Physics.
The UK collaboration included Imperial College, the Universities of Glasgow, Liverpool, Oxford, Sheffield and Warwick. Many international partners were also involved.