International Collaboration plans towards a multi-TeV muon collider

I.FAST Innovation Fostering in Accelerator Science and Technolog y Horizon 2020 Research Infrastructures GA n° 101004730 DELIVERABLE REPORT International Collaboration plans towards a multi-TeV muon collider DELIVERABLE: D5.1 Document identifier: IFAST-D5.1 Due date of deliverable: End of Month 46 (End February 2025) Report release date: 13/10/2025 Work package: WP5: Strategies and milestones for accelerator research and technology Lead beneficiary: INFN Document status: Final ABSTRACT This report briefly summarizes the achievements performed during the four years-time span by this network set-up during the past European Strategy for Particle Physics Update (2020) and later by the international community within the Muon Collider Collaboration (IMCC) hosted at CERN. The path to promote the innovative accelerator technology of bright muon beams and the submission of the input documents to the on-going update of the European Strategy for Particle Physics is part of the Accelerator R&D Roadmap, approved and implemented by CERN Council and reviewed by the CERN Laboratories Directors Group (LDG) in February 2025. The MUon colliders STrategy network (MUST) supported IMCC to develop a multi-TeV muon collider design and the required technology R&D plans, needed to demonstrate the feasibility of this future project. R&D plans require enabling accelerator technologies (magnets, RF and materials) also studied by other I.FAST WPs, and new dedicated test facilities and a more complex muon cooling demonstrator. INTERNATIONAL COLLABORATION PLANS TOWARDS A MULTI-TEV MUON COLLIDER Deliverable: D5.1 Date: 13/10/2025 Grant Agreement 101004730 PUBLIC 2 / 12 I.FAST Consortium, 2025 For more information on IFAST, its partners and contributors please see https://ifast-project.eu/ This project has received funding from the European Union’s Horizon 2020 Research and Innovation programme under Grant Agreement No 101004730. IFAST began in May 2021 and will run for 4 years. Delivery Slip Name Partner Date Authored by N. Pastrone INFN 12/10/2025 Reviewed by M. Vretenar, L. Celona [on behalf of Steering Committee] CERN 13/10/2025 Approved by Steering Committee 13/10/2025 INTERNATIONAL COLLABORATION PLANS TOWARDS A MULTI-TEV MUON COLLIDER Deliverable: D5.1 Date: 13/10/2025 Grant Agreement 101004730 PUBLIC 3 / 12 TABLE OF CONTENTS 1 INTRODUCTION ................................................................................................................................................... 5 2 STATUS OF THE DESIGN STUDY ..................................................................................................................... 6 3 TECHNOLOGY: R&D PLAN - DEMONSTRATORS AND TEST FACILITIES .......................................... 8 4 FUTURE PLANS AND NEXT STEPS ............................................................................................................... 10 5 REFERENCES ...................................................................................................................................................... 11 INTERNATIONAL COLLABORATION PLANS TOWARDS A MULTI-TEV MUON COLLIDER Deliverable: D5.1 Date: 13/10/2025 Grant Agreement 101004730 PUBLIC 4 / 12 Executive summary The task MUon collider STrategy network (MUST) was conceived to support the effort to design a multi-TeV muon collider and to plan and prioritize the required R&D for this new unique facility. An International Muon Collider Collaboration (IMCC) was promptly envisaged soon after the past European Strategy recommendation (June 2020) to promote the innovative accelerator technology of bright muon beams. Included as an operative panel within the Acceleration R&D Roadmap the international Muon Collider future project devoted all the studies to prove and consolidate the feasibility of the design, evaluating the resources needed to prioritize and complete the required R&D plan, demonstrating the outstanding physics potential of such a challenging facility, at the frontier of fundamental physics exploration. Sustainability and cost were also addressed, and more work is still required to finalize a complete end-to-end design. So far, no showstoppers were identify on the several studies carried on. Both CERN and FNAL were identified as possible sites to host the final facility and/or the ionizing muon cooling demonstrator, a mandatory step to be able to proceed with the overall facility. Actually, a multi-TeV muon collider represents the most compact and efficient collider option to reach the highest energies of interest for future physics discoveries. The challenges due to the muon beams short lifetime affect both the muon production and cooling and the subsequent fast acceleration and collider ring design. The new MuCol Design Study EU project, started in March 2023, focus on the major technological challenges including technologies, Machine Detector Interface and the cooling cell integration. The MUST task served as an initial common ground for growing the international muon-collider collaboration. Several joined annual meetings were held to assess the progress of the study, define priorities and timeline, also aiming at consolidating the collaboration, while the US Snowmass strategy process went to completion by December 2023. Several input documents were submitted to the European Strategy for Particle Physics Update reporting in detail the results achieved by the Design Study supported by this network. The present document summarizes the most relevant studies addressed by the collaboration to assess future plans. INTERNATIONAL COLLABORATION PLANS TOWARDS A MULTI-TEV MUON COLLIDER Deliverable: D5.1 Date: 13/10/2025 Grant Agreement 101004730 PUBLIC 5 / 12 1 Introduction The Muon Collider represents a transformative opportunity in the field of particle physics. By colliding muons - leptons 200 times more massive than electrons - it enables high-energy collisions in a compact footprint while maintaining the clean experimental environment typical of lepton colliders. This dual advantage offers both high precision and high energy reach, uniquely positioning the muon collider to explore physics beyond the Standard Model and to directly probe the dynamics of electroweak symmetry breaking. The concept has gained strong momentum through the International Muon Collider Collaboration (IMCC), with support from institutes and laboratories across Europe, the US, and Asia. A collider of muons could be a compact and efficient way to reach energies of interest for future physics discoveries; but a substantial R&D program is needed to fully prove its feasibility and to assess its cost. This Strategy group as a task of WP5 was conceived as an initial common ground for a growing international muon-collider collaboration, based on the initial work of the CERN Muon Collider Working Group [1]. The International Muon Collider Collaboration, after being launched by the Large Particle Physics Laboratory Directors Group (LDG), soon after the European Strategy of Particle Physics Update in June 2020 (ESPPU2020) [2] was finalized as the IMCC future project hosted at CERN after the review of the Accelerator R&D Roadmap approved by CERN Council at the end of 2021 [3]. Several workshops and meetings on muon colliders were organized in the last four years involving all the community in Europe driven by IMCC and in U.S. with a series of events also promoted by the “Muon Collider Forum” in the context of the Snowmass 2021 U.S. Community Planning Exercise. The final P5 panel report, released on December 2023 [4], confirmed and strengthened the muon collider’s potential for the exploration of the energy frontier, advocating R&D investments with the perspective of hosting a muon collider in the US. The main goals of the iFAST Strategy task 5.1 were to: • Support the effort to design a muon collider and to project and plan the required R&D. • Consolidate the community devoted to developing an international future facility. • Prepare the platform to disseminate the information (website, meetings, simulation tools). All the work achieved so far by the community lead by IMCC (also including the EU project MuCol [5]) and reviewed by the LDG, withing the implementation of the Accelerator R&D Roadmap, was initially conceived and outlined during the past ESPPU2020, and strongly supported by iFAST. This report testifies as the Strategy group MUST in WP5 was a crucial seed for the future highenergy muon collider. The baseline design of the muon production and cooling were developed, and the collaboration is devising the optimum test facilities to prove its feasibility. All major components of the facility have been studied and analyzed, including sitting at CERN and FNAL. At the end of I.FAST the common ground for a growing international muon-collider collaboration has been strengthened and the R&D plan is under review. All the achievements are collected in the input document submitted to the on-going ESPPU2026 [6,7,8]. INTERNATIONAL COLLABORATION PLANS TOWARDS A MULTI-TEV MUON COLLIDER Deliverable: D5.1 Date: 13/10/2025 Grant Agreement 101004730 PUBLIC 6 / 12 2 Status of the design study The present status of the project is based on a schematic layout of the collider facility with a proton driver source, as shown in Figure 1 and contains the following key areas: 1. The proton driver (blue box in the diagram) produces a short, high-intensity proton pulse. 2. This pulse hits the target (indigo) and produces pions. The decay channel guides the pions and forms a beam with the resulting muons via a buncher and phase rotator system. 3. Several cooling stages (purple) reduce the longitudinal and transverse emittance of the beam using a sequence of absorbers and RF cavities in a high magnetic field. 4. A system of a linac and two recirculating linacs accelerate (light red) the beams up to 63 GeV followed by a sequence of high-energy accelerator rings which reach 1.5 TeV or TeV. 5. Finally, the beams are injected at full energy into the collider ring (red). Here, they will circulate and collide within the detectors until they decay. Fig. 1 Conceptual layout of the muon collider. The studies addressed the most critical items of each key area of the facility to identify any possible issue and to prepare a detailed R&D plan to define priorities and required resources. All details of the project were collected and presented as input to the on-going ESPPU2026 on different documents [6,7,8]. The potential of a muon collider to reach around 10 TeV parton-parton collisions with high luminosity makes it an exciting opportunity for the near and more distant future. Machine parameters evaluating available technologies were studied to achieve feasible luminosity goals, both for a site independent option, or re-using existing tunnels at CERN. Two energy steps were always taken as reference in the design study. Main parameters are collected in the following table. Table 1 Main paramenters. INTERNATIONAL COLLABORATION PLANS TOWARDS A MULTI-TEV MUON COLLIDER Deliverable: D5.1 Date: 13/10/2025 Grant Agreement 101004730 PUBLIC 7 / 12 IMCC also developed a timeline that allows to operate a muon collider by 2050. This demands that all needed technologies be matured in approximately 15 years. This timeline is essentially driven by technical considerations and assumes that sufficient funding is available and that the R&D is successful. This technically-limited, success-oriented timeline can serve the community and the decision makers as a basis to define the strategy and the budget for the future of the study. It is shown in Figure 2. Fig. 2 Conceptual Technically limited timeline for the initial muon collider stage, assuming a firm commitment to implement the project as soon as possible after the High-Luminosity LHC. The timeline of the project phases beyond the planned 10 years of R&D are affected by the siting choice and detailed planning by the host laboratory. The proposed R&D plan is marked in dark blue. Civil engineering studies at CERN indicate that the surface installations of the accelerator facility could be constructed fully on CERN land – as in Figure 3 - and that the SPS and LHC tunnels could be reused to host the accelerator rings, thus minimizing the overall civil engineering. The proton complex would be located on the Meyrin site. The beam would be transported through the SPS tunnel to the Prevessin site where the cooling and initial linacs would be located in cut-and-cover tunnels. The beam is injected into the SPS then the LHC and finally into a new 10 km long collider ring. A similar siting study is underway for Fermilab. Fig. 3 Muon Collider CERN site layout reusing existing SPS and LHC tunnels. INTERNATIONAL COLLABORATION PLANS TOWARDS A MULTI-TEV MUON COLLIDER Deliverable: D5.1 Date: 13/10/2025 Grant Agreement 101004730 PUBLIC 8 / 12 The power consumption for CERN implementations at 3.2 and 7.6 TeV and the 10 TeV siteindependent design are about 117 MW, 182 MW, and 201 MW, respectively. 3 Technology: R&D plan - Demonstrators and Test Facilities During the iFAST project the IMCC collaboration proposed the R&D plan, discussed and reviewed in dedicated meetings also by LDG and the International Advisory Group within the MuCol project. The following main areas were studied and require further development. • The detector design should optimize the performance and minimize the impact of beam induced background. • The muon cooling technology demonstration program should prove the feasibility of design: it requires to develop the components such as the HTS solenoids, the cavities and the absorbers. Test of a full cooling cell with RF power will allow verification of the integrated performance. The demonstrator facility will test several of the cooling cells with beam to demonstrate the technology. • An intense program will establish superconducting magnet performance through the construction and test of models and prototypes. It will focus on HTS solenoids for the muon production target and the muon cooling. These have strong synergy with applications in society such as fusion reactors. A model of the collider ring dipoles will also be constructed and establish the field at large aperture. • A start-to-end model of the collider. The completion of the lattice design along the whole complex and its optimization will guide the component development. The development of simulation tools that include the relevant beam physics, such as collective effects and imperfections as well as the relevant mitigation techniques, will enable robust luminosity predictions. A study of the machine availability will establish the integrated luminosity performance and guide component and accelerator design. • Experiments in combination with further design work will verify the target robustness. Conceptual designs of the superconducting and normal-conducting cavities along the whole complex. Experimental verification of the performance limits will allow us to optimize the complex design. In particular, the construction of an infrastructure to test RF in a high magnet field will enable experimental optimization and verification of the normalconducting muon cooling RF. • The development of high-power, high-efficiency klystrons will be instrumental for the muon cooling cell test and enable cost effective design. • The performance of the fast-ramping magnet systems and power converter for the RapidCycling Synchrotrons (RCS) should be demonstrated. • Site and environmental impact studies, including civil engineering, allow optimization for power consumption and material usage as well as minimizing the impact of the machine for the local environment. • An overall optimization of the complex for cost, power consumption and risk will be performed and is particularly essential since we cannot base ourselves on experience with INTERNATIONAL COLLABORATION PLANS TOWARDS A MULTI-TEV MUON COLLIDER Deliverable: D5.1 Date: 13/10/2025 Grant Agreement 101004730 PUBLIC 9 / 12 previous similar projects. This optimisation will also cross the boundaries between the different systems. To optimise the use of the limited resources, the simulation studies and code development focused on the most critical areas of the collider. The beam parameters were defined at the interfaces between areas to allow their individual design and optimisation in collaboration with the required technology development. Key examples of the studies are the design and simulation of the proton accumulation and combination complex, the target, the 6D and final cooling, the RCS chain, the collider ring and the machine-detector interface. The CERN-based RFTrack code has been modified to include the muon-matter interaction to simulate the muon cooling and the beambeam simulation code GUINEA-PIG has been modified to simulate muon beam collisions and background generation. The completion of a fully integrated study of the collider is an important focus of the proposed R&D but requires sufficient resources. Important studies were performed to identify realistic performance specifications for novel components and optimise the lattice designs and the beam performance accordingly. Some key examples are: • The magnet team developed a model that allows to predict the achievable field as a function of technology, aperture, operating temperature and cost. This model informed the lattice designs. • The radial build of the collider ring magnets is the result of the work of a team of experts for lattice design, collective effects, beam-matter interactions, cryogenics, vacuum and magnets. • Similarly, the target design includes beam-matter interactions, cryogenics and magnet design. • Beam dynamics and vacuum experts studied the heat deposition in the final cooling absorbers and determined the hydrogen vapor density acceptable for the windows. • In the RCSs, the fast-ramping magnet-power converter systems and the RF systems need to be synchronized to meet the beam dynamics requirements. Models of the power consumption and cost of the systems were derived and implemented into a code that was used to optimise the system. A bottom-up cost model of the collider exists, which covers the key cost drivers. Also, the power consumption has been estimated bottom-up considering the key drivers. Following completion of the start-to-end simulation, an integrated optimisation will be performed that should yield improvement in cost, power and performance of the system and reduce the risk. One of the major achievements of the task was to coordinate the institutes to work on the design and implementation of a cooling cell essential to reduce the transverse and longitudinal emittance of the muon beam before acceleration. A cooling cell is composed of a low-Z material absorber, which will reduce both the longitudinal and the transverse momentum with minimal particle loss, a Radio-Frequency (RF) cavity will re-accelerate the particles to restore their longitudinal momentum, and solenoids, that help focusing the beam and maintain the right (small) value of the beta function at the absorber position. The team is preparing to integrate the components of the first prototype of the cooling cell as in Figure 4.