This March saw the annual workshop of the Compact Linear Collider (CLIC) at CERN, gathering 220 collaborators from over 26 countries to discuss the latest status of the CLIC accelerator and detector studies.

Two collaborations exists to study the feasibility of a future electron-positron linear collider at CERN for the era beyond the High-Luminosity LHC: the CLIC accelerator collaboration, studying a novel two-beam concept to reach high accelerating gradients of 100 MV/m to allow the realisation of a compact and affordable multi-TeV linear collider; and the CLIC detector and physics (CLICdp) collaboration, dedicated towards detailed detector and physics studies with the goal of fully exploiting the physics potential at CLIC.

During the workshop, particular focus was directed towards the recently published updated staging scenario for the CLIC accelerator [1], where construction and operation is pursued in three energy stages with collision energies of 380 GeV, 1.5 TeV and 3 TeV respectively. First beams could be foreseen in 2035; the starting point of a 22-year long physics programme.

“The CLIC accelerator is based on a revolutionary concept for linear acceleration up to multi-TeV energies” says CLICdp spokesperson Lucie Linssen. “It remains the only mature option that could take us to multi-TeV electron-positron collisions”.

Figure 1: A simulated top pair event at 3 TeV showing boosted topology. Dedicated techniques are used to identify features in the jet substructure in order to correctly differentiate top jets from light flavour jets.

Philipp Roloff, one of the physics analysis working group leaders, emphasised in his talk 'Why is it important that CLIC is upgradable beyond the 380 GeV stage?’ the importance of operation at different collision energies. “This staging scenario is crucial to exploit the full physics landscape. At its initial energy CLIC is optimised for Higgs and top measurements and further includes a scan at the top quark pair production threshold. The higher energy stages provide the best sensitivity to new physics through direct and indirect searches. High-energy operation also gives access to rare processes such as double Higgs production which is sensitive to the important Higgs self-coupling."

CLIC Week 2017 hosted a variety of sessions on detector development and optimisation, as well as new results and future prospects of physics studies. CLICdp have recently released a comprehensive paper on Higgs studies [2] and is now moving towards an overview paper on top quark physics covering studies of the top mass at, and beyond, the top pair production threshold, as well as top electroweak and Yukawa couplings, rare decays and more. A summary of the current status and plans for CLIC top quark studies was presented by Filip Zarnecki on Friday March 10; current efforts in the reconstruction of highly boosted top pairs (figure 1) was presented in greater detail during a dedicated parallel session.

During the open plenary session on Wednesday March 8, several talks were given to provide an overview of the CLIC accelerator, detector and physics programme, placed in the context of LHC results. This session also addressed the use of CLIC-related developments in other applications. The spokesperson for the CLIC accelerator collaboration, Philip Burrows, presented the current status of the machine design and the pathway towards the European strategy for 2019 and beyond. Furthermore, a summary talk on the CLIC Test Facility at CERN (CTF3) was given by its former coordinator Roberto Corsini, reporting on the successful demonstration of key technological concepts allowing a CLIC-style accelerator to be built. With the completion of the CTF3 experimental programme the facility has now been approved for conversion into an electron accelerator facility, CLEAR (CERN Linear Electron Accelerator for Research). A status and outlook report of the CLEAR facility was presented on Wednesday afternoon by Erik Adli.

Figure 2: The new single detector concept for CLIC, CLICdet, includes all-silicon vertex and tracker detectors for precision momentum measurements and flavour tagging. The fine-grained calorimeter system is optimised for particle flow reconstruction, with the full detector enclosed by a 4 T superconducting solenoid magnet. Due to the importance of luminosity measurements at lepton colliders, CLICdet features a complex forward region including dedicated radiationhard calorimeters for luminosity monitoring as well as an extended coverage for low polar angles.

With the adoption of a single detector concept based around the interaction point, a new unified detector model has been developed for CLIC (figure 2) [3]. The status of this model was outlined by the vertex and tracker coordinator Dominik Dannheim. The detector features all-silicon vertex and tracking detectors, for which a wide range of detector R&D is currently underway. An overview of the tracking detectors and relevant technologies was presented by Daniel Hynds; this included recent results from a range of emerging silicon detector concepts, such as High Voltage (HV-) and High Resistivity (HR-) CMOS, Silicon-on-Insulator (SOI) and a number of prototype devices fabricated as part of the CLIC research effort.

Many of the technologies under study for the CLIC detector are also of interest to the High Luminosity LHC upgrade, as well as for the HEP community at large. This extends beyond the detector R&D, where software reconstruction techniques developed for particle flow at linear colliders has been applied to current and next-generation neutrino experiments. An overview of such an approach for liquid Argon TPCs was presented by DUNE co-spokesperson Mark Thomson.

CLIC Week 2017 (clicw2017.web.cern.ch) took place March 6-10 at CERN. The Indico pages are open and video recordings exists for the public session on Wednesday March 8.

References:

[1] "Updated baseline for a staged Compact Linear Collider", CERN Yellow Report, CERN-2016-004

[2] “Higgs Physics at the CLIC Electron-Positron Linear Collider”, CLICdp-Pub-2016-001

[3] “CLICdet: The post-CDR CLIC detector model”, CLICdp-Note-2017-001