Particle physicists have long coveted the advantages of a muon collider, which could offer the precision of a LEP-style electron–positron collider without the energy limitations imposed by synchrotron-radiation losses. The clean neutrino beams that could be produced by bright and well-controlled muon beams could also drive a neutrino factory. In a step towards demonstrating the technical feasibility of such machines, the Muon Ionisation Cooling Experiment (MICE) collaboration has published results showing that muon beams can be “cooled” in phase space.
“Muon colliders can in principle reach very high centre-of-mass energies and luminosities, allowing unprecedented direct searches of new heavy particles and high-precision tests of standard phenomena,” says accelerator physicist Lenny Rivkin of the Paul Scherrer Institute in Switzerland, who was not involved in the work. “Production of bright beams of muons is crucial for the feasibility of these colliders and MICE has delivered a detailed characterisation of the ionisation-cooling process – one of the proposed methods to achieve such muon beams. Additional R&D is required to demonstrate the feasibility of such colliders.”
MICE has delivered a detailed characterisation of the ionisation-cooling processLenny Rivkin
The potential benefits of a muon collider come at a price, as muons are unstable and much harder to produce than electrons. This imposes major technical challenges and, not least, a 2.2 µs stopwatch on accelerator physicists seeking to accelerate muons to longer lifetimes in the relativistic regime. MICE has demonstrated the essence of a technique called ionisation cooling, which squeezes the watermelon-sized muon bunches created by smashing protons into targets into a form that can be fed into the accelerating structures of a neutrino factory or the more advanced subsequent cooling stage required for a muon collider – all on a time frame short compared to the muon lifetime.
An alternative path to a muon collider or neutrino factory is the recently proposed Low Emittance Muon Accelerator (LEMMA) scheme, whereby a naturally cool muon beam would be obtained by capturing muon–antimuon pairs created in electron–positron annihilations.
Playing it cool
Based at Rutherford Appleton Laboratory (RAL) in the UK, and two decades in the making, MICE set out to reduce the spatial extent, or more precisely the otherwise approximately conserved phase-space volume, of a muon beam by passing it through a low-Z material while tightly focused, and then restoring the lost longitudinal momentum in such a way that the beam remains bunched and matched. This is only possible in low-Z materials where multiple scattering is small compared to energy loss via ionisation. The few-metre long MICE facility, which precisely measured the phase-space coordinates of individual muons upstream and downstream of the absorber (see figure), received muons generated by intercepting the proton beam from the ISIS facility with a cylindrical titanium target. The absorber was either liquid hydrogen in a tank with thin windows or solid lithium hydride, in both cases surrounded by coils to achieve the necessary tight focus, and maximise transverse cooling.
A full muon-ionisation cooling channel would work by progressively damping the transverse momentum of muons over multiple cooling cells while restoring lost longitudinal momentum in radio-frequency cavities. However, due to issues with the spectrometer solenoids and the challenges of integrating the four-cavity linac module with the coupling coil, explains spokesperson Ken Long of Imperial College London, MICE adopted a simplified design without cavities. “MICE has demonstrated ionisation cooling,” says Long. The next issues to be addressed, he says, are to demonstrate the engineering integration of a demonstrator in a ring, cooling down to the lower emittances needed at a muon collider, and investigations into the effect of bulk ionisation on absorber materials. “The execution of a 6D cooling experiment is feasible – and is being discussed in the context of the Muon Collider Working Group.”
Twists and turns
The MICE experiment took data during 2017 and the collaboration confirmed muon cooling by observing an increased number of “low-amplitude” muons after the passage of the muon beam through an absorber. In this context, the amplitude is an additive contribution to the overall emittance of the beam, with a lower emittance corresponding to a higher density of muons in transverse phase space. The feat presented some extraordinary challenges, says MICE physics coordinator Chris Rogers of RAL. “We constructed a densely packed 12-coil and three-cryostat magnet assembly, with up to 5 MJ of stored energy, which was capable of withstanding 2 MN inter-coil forces,” he says. “The muons were cooled in a removable 22-litre vessel of potentially explosive liquid hydrogen contained by extremely thin aluminium windows.” The instrumentation developed to measure the correlations between the phase-space coordinates introduced by the solenoidal field is another successful outcome of the MICE programme, says Rogers, making a single-particle analysis possible for the first time in an accelerator-physics experiment.
“We started MICE in 2000 with great enthusiasm and a strong team from all continents,” says MICE founding spokesperson Alain Blondel of the University of Geneva. “It has been a long and difficult road, with many practical novelties to solve, however the collaboration has held together with exceptional resilience and the host institution never failed us. It is a great pride to see the demonstration achieved, just at a time when it becomes evident to many new people that we must include muon machines in the future of particle physics.”