13 November 2019

Steinar Stapnes considers the past, present and future of CERN’s proposed Compact Linear Collider.

The proposed CLIC collider

There is an increasing consensus that the next large accelerator after the LHC should be an electron–positron collider. Several proposals are on the table, circular and linear. Around 75 collaborating institutes worldwide are involved in the CERN-hosted studies for the Compact Linear Collider (CLIC), which offers a long-term and flexible physics programme that is able to react to discoveries and technological developments.

The 11 km-long initial stage of CLIC is proposed for operation at a centre-of-mass energy of 380 GeV, providing a rich programme of precision Higgs-boson and top-quark measurements that reach well beyond the projections for the high-luminosity LHC. From a technical point of view, operation of the initial stage by around 2035 is possible, with a cost of approximately 5.9 billion Swiss francs. This is similar to the cost of the LHC and of the proposed International Linear Collider in Japan, and considerably  less than that of future circular lepton colliders.

Extensions beyond the initial CLIC energy into the multi-TeV regime allow much improved precision on Standard Model (SM) measurements and greater reach for physics beyond the SM, with upgrade costs of 5 (to 1.5 TeV) and 7 billion Swiss francs (to 3 TeV) for the two further stages. A key part of the CLIC study has been the physics and detector studies showing that beam-induced backgrounds can indeed be mitigated, compatible with the clean experimental conditions expected in electron–positron colliders.

Steinar Stapnes

The question of what follows after the initial stage of a linear electron–positron collider is premature to answer. It is crucial to choose the most flexible approach now, and to develop technically mature and affordable options, encouraging a broad and exciting R&D programme. The option of expanding CLIC from its initial phase is already built into its staging scheme. Novel acceleration technologies can potentially push linear colliders even further in energy, although significantly more work is needed on beam qualities and energy efficiency for such options. High-energy proton and muon colliders are also potential future directions that need to be developed. While for protons the challenges are related to magnet performance, collider size and costs, for muons the technical design concepts need to mature and the radiation and experimental conditions need to be better understood.

CLIC offers a unique combination of precision and energy reach, and has a long history dating back to around 1985. At that time, the LEP tunnel was under construction and the first LHC workshop had just taken place. The motivation then was to move well beyond the W- and Z-boson studies foreseen at LEP to search for and study the top quark, Higgs boson and possible supersymmetric particles in a mass range from hundreds of GeV to several TeV. After the top-quark discovery at Fermilab and Higgs-boson discovery at CERN, we know that CLIC can do exactly that – even though the search arena for new physics is much more open than considered at that time.

Successful formula

High-energy electron–positron collisions, together with proton–proton or proton–antiproton collisions, have been a successful formula for progress in particle physics for half a century. Increasing the energy and luminosity of such machines is challenging. CLIC’s drive-beam concept was instrumental in providing a credible and scalable powering option at multi-TeV energies. A cost optimisation combined with the practical need of radio-frequency (RF) power units for R&D and testing led to the present normal-conducting 12 GHz “X-band” accelerating structures with an accelerating gradient of up to 100 MV/m. In parallel, CLIC’s energy use at 380 GeV has been scrutinised to keep it well below CERN’s annual consumption today, and less than 50% of the estimation for a future circular electron–positron collider.

The question of what follows after the initial stage of a linear electron–positron collider is premature to answer

The next steps needed for CLIC are clear. The project-implementation plan foresees a five-year preparation phase prior to construction, which is envisaged to start by 2026. The preparation phase would focus on further design optimisation and technical and industrial development of critical parts of the accelerator. System verification in free-electron-laser linacs and low-emittance rings will be increasingly important for performance studies, while civil engineering and infrastructure preparation will become progressively more detailed, in parallel with an environmental impact study. Detector preparation will need to be scaled up, too.

The increasing use of X-band technology – either as the main RF acceleration for CLIC or for compact test facilities, light sources, medical accelerators or low-energy particle physics studies – provides new collaborative opportunities towards a technical design report for the CLIC accelerator.

It is for the broader particle-physics community and CERN to decide whether CLIC proceeds. We are therefore eagerly looking forward to the conclusion of the European strategy process next year. For now, it is important to communicate how CLIC-380 can be implemented rapidly involving many collaborative partners, and at the same time provide unique and timely opportunities for R&D to keep future options open.

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