The Compact Linear Collider (CLIC) workshop is the main annual gathering of the CLIC accelerator and detector communities. This year it attracted more than 220 participants to CERN, 22-26 January. CLIC is a proposed electron-positron linear collider, envisaged for the era beyond the High-Luminosity LHC (HL-LHC), that would operate a staged programme over about 25 years with collision energies at 0.38, 1.5, and 3 TeV. This year, the meeting focused on preparations for the update of the European Strategy for Particle Physics.

The footprint of CLIC for the three collision energies considered: 380 GeV, 1.5 TeV, 3 TeV (credit: CLIC)

The initial CLIC energy stage is optimised to provide high-precision Higgs boson and top-quark measurements, with the higher-energy stages enhancing sensitivity to effects from beyond-Standard Model (BSM) physics. Following a 2017 publication on Higgs physics, the workshop heard reports on recent developments in top-quark physics and the BSM potential at CLIC, both of which are attracting significant interest from the theory community.

A comprehensive report on top quark physics studies which can be carried out at the three energy stages is nearing completion. It includes studies of top-quark production at threshold and beyond, searches for Flavour Changing Neutral Current (FCNC) top-quark decays, and studies of top-quark pair production in Vector Boson Fusion (VBF) as well as the ttH process. The latter allows the direct extraction of the top Yukawa coupling. Centre-of-mass energies above 1 TeV provide a significant increase in the sensitivity reach for BSM physics. The report includes the phenomenological interpretation of the results from the top-quark production studies for top-philic operators in an Effective Field Theory (EFT), and prospects are given for specific models such as top-quark compositeness, where CLIC has discovery potential to scales far beyond its centre-of-mass energy.

A simulated top-quark pair event at 3 TeV (tt→qqbμνb) – the upper part of the detector features an isolated muon (magenta) along with a b-jet containing an additional muon – the lower part of the detector features a boosted top-quark jet (credit: CLIC)

Speakers reported on the progress in the validation and performance of the new CLIC detector model, using a new software suite for event simulation and reconstruction. The software suite is developed in common with ILC and is also used for ongoing FCC-hh tracker optimisation studies and for the design and optimisation of a CLIC-like detector for FCC-ee. To ensure that the detector performance meets the challenging requirements on position resolution, timing capabilities and low-mass features, a new approach to tracking has been commissioned, and the particle flow analysis and flavour-tagging capabilities have been consolidated.

The new CLIC detector model “CLICdet” (credit: CLIC)

Updates were presented on the broad and active R&D programme on the vertex and tracking detectors, which aims to find technologies that simultaneously fulfil all the CLIC requirements. Several technology options are being pursued, including thin planar sensors, integrated HV-CMOS and HR-CMOS. Detector modules were constructed and tested at the SPS using a dedicated high-precision beam telescope. The silicon pixel R&D is pursued in synergy with the ATLAS and ALICE detector upgrades and the Medipix/Timepix developments. Reports were given on test beam campaigns and on ideas for future developments. Many of the tracking and calorimeter technologies under study for the CLIC detector are also of interest to the HL-LHC, where the high granularity and time-resolution needed for CLIC are equally crucial.

A silicon pixel detector prototype for the CLIC vertex detector (active HV-CMOS + CLICpix2 ASIC) (credit: CLIC)

For the accelerator, studies with the aim of reducing the cost and power have particular priority, in order to present the initial CLIC stage as a project requiring resources comparable to what was needed for LHC. Key activities in this context are high efficiency RF systems, permanent magnet studies, optimised accelerator structures and overall implementation studies related to civil engineering, infrastructure, schedules and tunnel layout.

A key aspect of the ongoing accelerator development is moving towards industrialisation of the component manufacture, by fostering wider applications of the CLIC 12 GHz X-band technology with external partners. As a new initiative, the CLIC Workshop saw the kick-off meeting for the CompactLight project recently funded by the European Commission’s Horizon 2020 programme. This three-year project brings together leading European institutions and companies to design an optimised X-ray free-electron laser based on X-band technology, to pave the way for significantly more compact and power-efficient accelerator facilities. 2017 also saw the realisation of the CERN Linear Electron Accelerator for Research (CLEAR), a new user facility for accelerator R&D. Its programme includes CLIC high-gradient and instrumentation studies. Presentations at the Workshop addressed the programmes for instrumentation and radiation studies, plasma-lensing, wake-field monitors and high-energy electrons for cancer therapy.

As input for the update of the European Strategy for Particle Physics the CLIC accelerator and detector and physics collaborations will prepare summary reports, which will focus on the 380 GeV initial CLIC project implementation, and include plans for the project preparation phase in 2020-2025.