Roadmaps set a path to post-LHC facilities

In setting out a vision for the post-LHC era, the 2020 update of the European strategy for particle physics (ESPPU) emphasised the need to ramp up detector and accelerator R&D in the near and long term. To this end, the European Committee for Future Accelerators (ECFA) was asked to develop a global detector R&D roadmap, while the CERN Council invited the European Laboratory Directors Group (LDG) to oversee the development of a complementary accelerator R&D roadmap. 

After more than a year of efforts involving hundreds of people, and comprising more than 500 pages between them, both roadmaps were completed in December. In addition to putting flesh on the bones of the ESPPU vision, they provide a rich and detailed snapshot of the global state-of-the-art in detector and accelerator technologies.

Future-proof detectors

Beyond the successful completion of the high-luminosity LHC, the ESPPU identified an e⁺e^– Higgs factory as the highest priority future collider, and tasked CERN to undertake a feasibility study for a hadron collider operating at the highest possible energies with a Higgs factory as a possible first stage. The ESPPU also acknowledged that construction of the next generations of colliders and experiments will be challenging, especially for machines beyond a Higgs factory.

The development of cost-effective detectors that match the precision-physics potential of a Higgs factory is one of four key challenges in implementing the ESPPU vision, states the ECFA roadmap report. The second is to push the limitations in radiation tolerance, rate capabilities and pile-up rejection power to meet the unprecedented requirements of future hadron-collider and fixed-target experiments, while a third is to enhance the sensitivity and affordably expand the scales of both accelerator and non-accelerator experiments searching for rare phenomena. The fourth challenge identified by ECFA is to vigorously expand the technological basis of detectors, maintain a nourishing environment for new ideas and concepts, and attract and train the next generation of instrumentation scientists.

To address these challenges, ECFA set up a roadmap panel, chaired by Phil Allport of the University of Birmingham, and defined six task forces spanning different instrumentation topics (gaseous, liquid, solid state, particle-identification and photon, quantum, calorimetry) and three cross-cutting task forces (electronics, integration, training), with the most crucial R&D themes identified for each. Tasks are mapped to concrete time scales ranging from the present to beyond 2045, driven by the earliest technically achievable experiment or facility start-dates. The resulting picture reveals the potential synergies between concurrent projects pursued by separate communities, as well as between consecutive projects, which  was one of the goals of the exercise, explains ECFA chair Karl Jakobs of the University of Freiburg: “It shows the role of earlier projects as a stepping stone for later ones, opening the possibility to evaluate and to organise R&D efforts in a much broader strategic context and on longer timescales, and allowing us to suggest greater coordination,” he says. 

Attracting R&D experts and recognising and sustaining their careers is one of 10 general strategic recommendations made by the report. Others include support for infrastructure and facilities, industrial partnerships, software, open science, blue-sky research, and recommendations relating to international coordination and strategic funding programmes. Guided by this roadmap, concludes the report, concerted and “resource-loaded” R&D programmes in innovative instrumentation will transform the ability of present and future generations of researchers to explore and observe nature beyond current limits.

“Ensuring the goals of future collider and non-collider experiments are not compromised by detector readiness calls now for an R&D collaboration programme, similar to that initiated in 1990 to better manage the activities then already underway for the LHC,” adds Allport. “These should be focused on addressing their unmet technology requirements through common research projects, exploiting where appropriate developments in industry and synergies with neighbouring disciplines.” 

Accelerating physics 

Although accelerator R&D is necessarily a long-term endeavour, the LDG roadmap focuses on the shorter but crucial timescale of the next five-to-ten years. It concentrates on the five key objectives identified in the ESPPU: further development of high-field superconducting magnets; advanced technologies for superconducting and normal-conducting radio-frequency (RF) structures; development and exploitation of laser/plasma-acceleration techniques; studies and developments towards future bright muon beams and muon colliders; and the advancement and exploitation of energy-recovery linear accelerator technology. Expert panels were convened to examine each area, which are at different stages of maturity, and to identify the key R&D objectives.

The high-field-magnets panel supports continued and accelerated progress on both niobium-tin and high-temperature superconductor technology, placing strong emphasis on its inclusion into practical accelerator magnets and warning that final designs may have to reflect a compromise between performance and practicality. The panel for high-gradient RF structures and systems also identified work needed on basic materials and construction techniques, noting significant challenges to improve efficiency. Longer term, it flags a need for automated test, tuning and diagnostic techniques, particularly where large-scale series-production is needed. 

Energy consumption and sustainability are key considerations in defining R&D priorities and in the design of new machines

In the area of advanced plasma and laser acceleration, the panel focused on rapidly evolving plasma-wakefield and dielectric acceleration technologies. Further developments require reduced emittance and improved efficiency, the ability to accelerate positrons and the combination of accelerating stages in a realistic future collider, the panel concludes, with the goal to produce a statement about the basic feasibility of such a machine by 2026. The panel exploring muon beams and colliders also sets a date of 2026 to demonstrate that further investment is justified, focusing on a 10 TeV collider with a 3 TeV intermediate-scale facility targeted for the 2040s. Finally, having considered several medium-scale projects under way worldwide, the energy-recovery linacs panel identifies reaching the 10 MW power level as the next practical step, and states that future sustainability rests on developing 4.4 K superconducting RF technology for a next-generation e⁺e^– collider. 

In addition to the technical challenges, states the report, new investment will be needed to support R&D and test facilities. Energy consumption and sustainability are explicitly identified as key considerations in defining R&D priorities and in the design of new machines. Having identified objectives, each panel set out a detailed work plan covering the period to the next ESPPU, with options for a number of different levels of investment. The aim is to allow the R&D to be pushed as rapidly as needed, but in balance with other priorities for the field.

Like its detector R&D counterpart, the report concludes with 10 concrete recommendations. These include the attraction, training and career management of researchers, observations on the implementation and governance of the programme, environmental sustainability, cooperation between European and international laboratories, and continuity of funding. 

“The accelerator R&D roadmap represents the collective view of the accelerator and particle-physics communities on the route to machines beyond the Higgs factories,” says Dave Newbold, LDG chair and director of particle physics for STFC in the UK. “We now need to move swiftly forwards with an ambitious, cooperative and international R&D programme – the potential for future scientific discoveries depends on it.”