Muon colliders have a great potential for high-energy physics. They can offer collisions of point-like particles at very high energies, since muons can be accelerated in a ring without limitation from synchrotron radiation. However, the need for high luminosity faces technical challenges which arise from the short muon lifetime at rest and the difficulty of producing large numbers of muons in bunches with small emittance. Addressing these challenges requires the development of innovative concepts and demanding technologies.
The Update of the European Strategy for Particle Physics recommended to integrate an international design study for a muon collider in the European Roadmap for accelerator R&D.
In response to this, the Laboratory Directors Group, which represents the large European Particle Physics Laboratories has initiated an International Muon Collider Collaboration to study the concept.
Develop a muon collider concept based on the proton driver and considering the existing infrastructure. This includes the definition of the required R&D program, based on previously achieved results, and covering the major issues such as cooling, acceleration, fast ramping magnets, detectors, . . . .
Schematic layouts of Muon Collider complexes based on the proton driver scheme and on the low emittance positron driver scheme emphasizing synergies is sketched below.
The functional elements of the muon beam generation and acceleration systems are:
– a proton driver producing a high-power multi-GeV, multi-MW bunched Hbeam,
– a buncher made of an accumulator and a compressor that forms intense and short proton bunches,
– a pion production target in a heavily shielded enclosure able to withstand the high proton beam
power, which is inserted in a high field solenoid to capture the pions and guide them into a decay
– a front-end made of a solenoid decay channel equipped with RF cavities that captures the muons
longitudinally into a bunch train, and then applies a time-dependent acceleration that increases the energy of the slower (low-energy) bunches and decreases the energy of the faster (high-energy) bunches,
– an “initial” cooling channel that uses a moderate amount of ionization cooling to reduce the 6D phase space occupied by the beam by a factor of 50 (5 in each transverse plane and 2 in the longitudinal plane), so that it fits within the acceptance of the first acceleration stage. For high luminosity collider applications, further ionization cooling stages are necessary to reduce the 6D phase space occupied by the beam by up to five orders of magnitude,
– the beam is then accelerated by a series of fast acceleration stages such as Recirculating Linacs Accelerators (RLA) or Fixed Field Alternating Gradient (FFAG) and Rapid Cycling Synchrotron (RCS) to take the muon beams to the relevant energy before injection in the muon collider Ring.