The Compact Linear Collider (CLIC) is a TeV-scale high-luminosity linear e + e − collider studied by the international CLIC and CLICdp collaborations hosted by CERN. CLIC uses a two-beam acceleration scheme, in which normal-conducting high-gradient 12 GHz accelerating structures are powered via a high-current drive beam. For an optimal exploitation of its physics potential, CLIC is foreseen to be built and operated in stages. The initial stage will have a centre-of-mass energy of 380 GeV, with a site length of 11 km. The 380 GeV stage optimally combines the exploration of Higgs and top-quark physics, including a top threshold scan near 350 GeV. A higher-energy stage, still using the initial single drive-beam complex, can be optimised for any energy up to 2 TeV. Parameters are presented in detail for a 1.5 TeV stage, with a site length of 29 km. Since the 2018 ESPPU reporting, significant effort was invested in CLIC accelerator optimisation, technology developments and system tests, including collaboration with and gaining experience from new-generation light sources and free-electron lasers. CLIC implementation aspects at CERN have covered detailed studies of civil engineering, electrical networks, cooling and ventilation, scheduling, and costing. The CLIC baseline at 380 GeV is now 100 Hz operation, with a luminosity of 4.5 $\times 10^{34}\text{ cm}^{-2}\text{s}^{-1}$ × 10 34 cm − 2 s − 1 and a power consumption of 166 MW. Compared to the 2018 design, this gives three times higher luminosity-per-power. The new baseline has two beam-delivery systems, allowing for two detectors operating in parallel, sharing the luminosity. The cost estimate of the 380 GeV baseline is approximately 7.2 billion CHF. The construction of the first CLIC energy stage could start as early as ∼2034-2035 and beam commissioning and first beams would follow a decade later, marking the beginning of a physics programme spanning 20-30 years and providing excellent sensitivity to Beyond Standard Model physics, through direct searches and via a broad set of precision measurements of Standard Model processes, particularly in the Higgs and top-quark sectors. This report summarises the CLIC project, its implementation and running scenarios, with emphasis on new developments and recent progress. It concludes with an update on the CLIC detector studies and on the physics potential in light of the improved accelerator performance. The physics potential includes results from the 3 TeV energy stage, which was studied in detail for the CLIC CDR in 2012 and the CLIC Project Implementation Plan of 2018.

Abstract Volume 1 of the FCC Feasibility Report presents an overview of the physics case, experimental programme, and detector concepts for the Future Circular Collider (FCC). This volume outlines how FCC would address some of the most profound open questions in particle physics, from precision studies of the Higgs and EW bosons and of the top quark, to the exploration of physics beyond the Standard Model. The report reviews the experimental opportunities offered by the staged implementation of FCC, beginning with an electron-positron collider (FCC-ee), operating at several centre-of-mass energies, followed by a hadron collider (FCC-hh). Benchmark examples are given of the expected physics performance, in terms of precision and sensitivity to new phenomena, of each collider stage. Detector requirements and conceptual designs for FCC-ee experiments are discussed, as are the specific demands that the physics programme imposes on the accelerator in the domains of the calibration of the collision energy, and the interface region between the accelerator and the detector. The report also highlights advances in detector, software and computing technologies, as well as the theoretical tools/reconstruction techniques that will enable the precision measurements and discovery potential of the FCC experimental programme. The content and structure of this report are guided by the scope and priorities defined in the mandate of the FCC Feasibility Study. It is therefore not intended to serve as an exhaustive review of the full physics potential of FCC. Several topics, already covered in earlier reports such as the FCC CDR, are not reiterated here or are addressed only briefly, in alignment with the study’s focus. This volume reflects the outcome of a global collaborative effort involving hundreds of scientists and institutions, aided by a dedicated community-building coordination, and provides a targeted assessment of the scientific opportunities and experimental foundations of the FCC programme.

The CMS and ATLAS experiments and their superconducting magnetic and calorimetric systems, installed in the LHC (Large Hadron Collider) at CERN, have both been equipped with cryogenic installations. The CMS experiment is making use of a large superconducting solenoid magnet, while the ATLAS experiment combines a large superconducting toroid magnetic system, a superconducting solenoid and three liquid argon calorimeters. These cryogenic installations have all been in continuous operation since the LHC physics run started in 2009. Although these systems have been performing with high reliability, specific consolidations have been proposed to further improve their reliability. The consolidations have been mainly implemented during the LHC long shutdown periods of 2013-2014 and 2019-2020. For the CMS experiment, hot-spare helium compressors were installed to allow full redundancy in case of failure. For ATLAS one redundant high-pressure helium compressor and two of the booster helium compressors have been upgraded with new and increased capacity units. A unit of 10’000 litres liquid helium storage was integrated into the cooling process of the ATLAS solenoid allowing significantly improved operation autonomy and recovery in case of a resistive transition of the magnetic toroid system. The CMS oil removal system was upgraded to overcome oil problems that were impairing the performance of the helium refrigerator. Finally, for both ATLAS and CMS infrastructure, an upgraded full flow dryer scheme was added in the global process of helium systems. This paper describes the consolidation program made to these cryogenic systems towards their availability improvement.

In response to the 2020 Update of the European Strategy for Particle Physics , the Future Circular Collider (FCC) Feasibility Study was launched as an international collaboration hosted by CERN. This report describes the FCC integrated programme , which consists of two stages: an electron-positron collider (FCC-ee) in the first phase, serving as a high-luminosity Higgs, top, and electroweak factory; followed by a proton-proton collider (FCC-hh) at the energy frontier in the second phase. The FCC-ee is designed to operate at four key centre-of-mass energies: the Z pole, the WW pair production threshold, the ZH production peak, and the top/anti-top production threshold—each delivering the highest possible luminosities to four experiments. Over 15 years of operation, FCC-ee will produce more than 6 trillion Z bosons, 200 million WW pairs, nearly 3 million Higgs bosons, and 2 million top anti-top pairs. Precise energy calibration at the Z pole and WW threshold will be achieved through frequent resonant depolarisation of pilot bunches. The sequence of operation modes between the Z, WW, and ZH substages remains flexible. The FCC-hh will operate at a centre-of-mass energy of approximately 85 TeV—nearly an order of magnitude higher than the LHC—and is designed to deliver 5 to 10 times the integrated luminosity of the upcoming High-Luminosity LHC. Its mass reach for direct discovery extends to several tens of TeV. In addition to proton-proton collisions, the FCC-hh is capable of supporting ion-ion, ion-proton, and lepton-hadron collision modes. This second volume of the Feasibility Study Report presents the complete design of the FCC-ee collider, its operation and staging strategy, the full-energy booster and injector complex, required accelerator technologies, safety concepts, and technical infrastructure. It also includes the design of the FCC-hh hadron collider, development of high-field magnets, hadron injector options, and key technical systems for FCC-hh.

Volume 3 of the FCC Feasibility Report presents studies related to civil engineering, the development of a project implementation scenario, and environmental and sustainability aspects. The report details the iterative improvements made to the civil engineering concepts since 2018, taking into account subsurface conditions, accelerator and experiment requirements, and territorial considerations. It outlines a technically feasible and economically viable civil engineering configuration that serves as the baseline for detailed subsurface investigations, construction design, cost estimation, and project implementation planning. Additionally, the report highlights ongoing subsurface investigations in key areas to support the development of an improved 3D subsurface model of the region. The report describes development of the project scenario based on the 'avoid-reduce-compensate' iterative optimisation approach. The reference scenario balances optimal physics performance with territorial compatibility, implementation risks, and costs. Environmental field investigations covering almost 600 hectares of terrain - including numerous urban, economic, social, and technical aspects - confirmed the project's technical feasibility and contributed to the preparation of essential input documents for the formal project authorisation phase. The summary also highlights the initiation of public dialogue as part of the authorisation process. The results of a comprehensive socio-economic impact assessment, which included significant environmental effects, are presented. Even under the most conservative and stringent conditions, a positive benefit-cost ratio for the FCC-ee is obtained. Finally, the report provides a concise summary of the studies conducted to document the current state of the environment.

Volume 1 of the FCC Feasibility Report presents an overview of the physics case, experimental programme, and detector concepts for the Future Circular Collider (FCC). This volume outlines how FCC would address some of the most profound open questions in particle physics, from precision studies of the Higgs and EW bosons and of the top quark, to the exploration of physics beyond the Standard Model. The report reviews the experimental opportunities offered by the staged implementation of FCC, beginning with an electron-positron collider (FCC-ee), operating at several centre-of-mass energies, followed by a hadron collider (FCC-hh). Benchmark examples are given of the expected physics performance, in terms of precision and sensitivity to new phenomena, of each collider stage. Detector requirements and conceptual designs for FCC-ee experiments are discussed, as are the specific demands that the physics programme imposes on the accelerator in the domains of the calibration of the collision energy, and the interface region between the accelerator and the detector. The report also highlights advances in detector, software and computing technologies, as well as the theoretical tools /reconstruction techniques that will enable the precision measurements and discovery potential of the FCC experimental programme. This volume reflects the outcome of a global collaborative effort involving hundreds of scientists and institutions, aided by a dedicated community-building coordination, and provides a targeted assessment of the scientific opportunities and experimental foundations of the FCC programme.

In response to the 2020 Update of the European Strategy for Particle Physics, the Future Circular Collider (FCC) Feasibility Study was launched as an international collaboration hosted by CERN. This report describes the FCC integrated programme, which consists of two stages: an electron-positron collider (FCC-ee) in the first phase, serving as a high-luminosity Higgs, top, and electroweak factory; followed by a proton-proton collider (FCC-hh) at the energy frontier in the second phase. FCC-ee is designed to operate at four key centre-of-mass energies: the Z pole, the WW production threshold, the ZH production peak, and the top/anti-top production threshold - delivering the highest possible luminosities to four experiments. Over 15 years of operation, FCC-ee will produce more than 6 trillion Z bosons, 200 million WW pairs, nearly 3 million Higgs bosons, and 2 million top anti-top pairs. Precise energy calibration at the Z pole and WW threshold will be achieved through frequent resonant depolarisation of pilot bunches. The sequence of operation modes remains flexible. FCC-hh will operate at a centre-of-mass energy of approximately 85 TeV - nearly an order of magnitude higher than the LHC - and is designed to deliver 5 to 10 times the integrated luminosity of the HL-LHC. Its mass reach for direct discovery extends to several tens of TeV. In addition to proton-proton collisions, FCC-hh is capable of supporting ion-ion, ion-proton, and lepton-hadron collision modes. This second volume of the Feasibility Study Report presents the complete design of the FCC-ee collider, its operation and staging strategy, the full-energy booster and injector complex, required accelerator technologies, safety concepts, and technical infrastructure. It also includes the design of the FCC-hh hadron collider, development of high-field magnets, hadron injector options, and key technical systems for FCC-hh.

Particle physics experiments make use of magnetic fields up to $4 \,\mathrm{T}$ to bend electrically charged particles such that their charge and momentum can be determined. The particle energy measurement requires a low amount of material, or material that is highly transparent to particles inside the calorimeter volume. The conflict between the small volume of space reserved for a magnet and having a field of several teslas inside the detector is often resolved by using superconducting magnets. Up to now, large particle physics detector magnets have been constructed with low temperature superconductors, but there are clear benefits from using high temperature superconductors in future particle physics detector designs, such as allowing for an elevated operating temperature and the reduced amount of superconductor needed. In addition to the HTS material itself, additional material is needed to support the Lorentz forces, and to temporarily carry the current in case of a quench since these magnets are always one-of-a-kind and they need to operate reliably and without damage in case of a failure scenario. The stabilizer has to be a low-density material for high particle transparency, such as aluminium. Since the density of the superconductor is a factor of 4 higher than the density of aluminium, a reduction of superconducting material also means an improvement of the particle transparency: the density of a material is directly related to its particle transparency. This paper presents a conceptual design for high temperature superconducting detector magnets and a study of the type of aluminium stabilizer used.

Arc suppression Snubbers offer safety in high energy superconducting magnet systems. Large superconducting magnets may have arcing across their breakers at the moment of slow dump initiation resulting from a sudden current redistribution in the powering circuit. The ATLAS Experiment at CERN experiences such arcing with its 7.3 H Toroidal magnet powering circuit. To discharge the magnets, the current is redistributed to a parallel branch called the Run Down Unit (RDU) by means of a breaker. Due to the physical layout and other metallic components in the vicinity, the power supply and RDU branches have undesirable and unavoidable parasitic inductance, causing a voltage spike over the main breakers at the moment of opening. The resulting arcing deteriorates the breaker contacts, resulting in increased operating temperatures and a more frequent need for maintenance. The arc suppression system developed takes the form of an RC Snubber (Resistor-Capacitor). The Snubber offers the current a low-impedance path, thus smoothing out the current redistribution and suppressing the voltage spike. This initial charging period of the capacitors gives the current enough time to overcome the parasitic inductance, therefore allowing the RDU to smoothly ramp up without a sudden voltage spike. To test the concept, a 1/50th scale demonstrator was developed and showed successful results. After the demonstrator, endurance testing of the electrolytic capacitors was performed to ensure no degradation of the charging characteristics for an equivalent of 20 years of operation. The Snubber was manufactured and implemented with successful results on the ATLAS Toroidal powering circuit.

Various superconducting detector solenoids for particle physics have been developed in the world. The key technology is the aluminum-stabilized superconducting conductor for almost all the detector magnets in particle physics experiments. With the progress of the conductor, the coil fabrication technology has progressed as well, such as the inner coil winding technique, indirect cooling, transparent vacuum vessel, quench protection scheme using pure aluminum strips and so on. The detector solenoids design study is in progress for future big projects in Japan and Europe, that is, ILC, FCC and CLIC, based on the technologies established over many years. The combination of good mechanical properties and keeping a high RRR is a key point for the development of Al-stabilized conductor. The present concern for the detector solenoid development is to have been gradually losing the key technologies and experiences, because large-scale detector magnets with Al-stabilized conductor has not been fabricated after the success of CMS and ATLAS-CS in LHC. Complementary efforts are needed to resume an equivalent level of expertise, to extend the effort on research and to develop these technologies and apply them to future detector magnet projects. Especially, further effort is necessary for the industrial technology of Al-stabilized superconductor production. The worldwide collaboration with relevant institutes and industries will be critically important to re-realize and validate the required performances. Some detector solenoids for mid-scale experiment wound with conventional copper-stabilized Nb-Ti conductor require precise control of magnetic field distribution. The development efforts are on-going in terms of the magnetic field design technology with high precision simulation, coil fabrication technology and control method of magnetic field distribution.

A procedure to measure the magnetic field inside the CMS yoke elements is considered. Fast discharge of the CMS coil can be used to induce voltages in flux loops installed around selected elements of the CMS yoke. By sampling the voltage induced in any one loop, and integrating the voltage waveform over the time of the discharge, the total initial flux in the loop can be measured. The average value of the magnetic field in the yoke element normal to the plane of the flux loop is obtained by dividing the measured value of the flux by the known area enclosed by the loop and the number of turns in the loop.

In the context of EP R&D, CERN is working on conceptual designs for a possible multi-purpose superconducting magnet test beam facility to be used for future detector and electronic device testing. The facility may serve as a replacement of the existing M1 or Morpurgo H8 magnets. The new facility is envisioned to serve all the testing requirements for the next generation of high energy physics particle detectors. Two separate concepts are proposed to satisfy the replacement needs. The magnet will take either the form of a split solenoid, allowing dual use as a dipole or solenoid, or a skateboard tilted racetrack design, allowing dipole function. Both designs allow for at least $1 \,{\mathrm{m}^{\mathrm{3}}}$ testing volume and a magnetic field up to 4-T. It is envisioned to use Niobium Titanium Rutherford cables with a Nickel-Aluminium stabiliser. The operation temperature will be 4.5 K with two-phase helium cooling. The stray fields are minimised to stay below 12 mT at a distance of 5 m from the central point.

The conceptual design study of a hadron Future Circular hadron-hadron Collider (FCC-hh) with a center-of-mass energy of the order of 100 TeV assumes using in the experimental detector the superconducting magnetic system with a central magnetic flux density of an order of 4 T. A superconducting magnet with a minimal steel yoke was proposed as an alternative to the baseline iron-free solenoids. In a present study, both designs are modeled with Cobham's program TOSCA and compared. All the main parameters are discussed.

As part of the European Strategy for Particle Physics there is an ongoing development towards a Future Circular Collider (FCC-ee) where electron-positron collisions could be used to study the entire electro-weak sector in a low background environment. Particle detectors are used to study these collisions and a strong magnetic field is required to measure the particles’ momenta. Currently, two detector concepts are being studied: the Innovative Detector for Electron-positron Accelerators (IDEA) and the CLIC-Like Detector (CLD). Both these detectors include a superconducting solenoid magnet with a central field of $2 \,\mathrm{T}$ of which the designs are presented here. The IDEA magnet has an stored magnetic energy density of 14 kJ/kg and in CLD this is 12 kJ/kg. Taking into account their respective free-bore diameters of $4.2 \,\mathrm{m}$ for IDEA and $7.2 \,\mathrm{m}$ for CLD this results in very challenging designs for which the mechanical and quench studies are presented. Their results are promising, but extensive R&D on these magnets would be needed in the future to reach the goals set out in the Conceptual Design Report (CDR).

A new model for the CLIC detector has been defined based on lessons learnt while working with the CDR detector models and after a series of simulation studies. The new model, dubbed "CLICdet", also incorporates the experience from various R&D activities linked to a future experiment at CLIC. This note describes the studies and thoughts leading to the new detector model, and gives details on all of its sub-detector systems. 2 Working Hypotheses

In response to the 2013 Update of the European Strategy for Particle Physics (EPPSU), the Future Circular Collider (FCC) study was launched as a world-wide international collaboration hosted by CERN. The FCC study covered an energy-frontier hadron collider (FCC-hh), a highest-luminosity high-energy lepton collider (FCC-ee), the corresponding 100km tunnel infrastructure, as well as the physics opportunities of these two colliders, and a high-energy LHC, based on FCC-hh technology. This document constitutes the third volume of the FCC Conceptual Design Report, devoted to the hadron collider FCC-hh. It summarizes the FCC-hh physics discovery opportunities, presents the FCC-hh accelerator design, performance reach, and staged operation plan, discusses the underlying technologies, the civil engineering and technical infrastructure, and also sketches a possible implementation. Combining ingredients from the Large Hadron Collider (LHC), the high-luminosity LHC upgrade and adding novel technologies and approaches, the FCC-hh design aims at significantly extending the energy frontier to 100TeV. Its unprecedented centre of-mass collision energy will make the FCC-hh a unique instrument to explore physics beyond the Standard Model, offering great direct sensitivity to new physics and discoveries.

In response to the 2013 Update of the European Strategy for Particle Physics, the Future Circular Collider (FCC) study was launched, as an international collaboration hosted by CERN. This study covers a highest-luminosity high-energy lepton collider (FCC-ee) and an energy-frontier hadron collider (FCC-hh), which could, successively, be installed in the same 100 km tunnel. The scientific capabilities of the integrated FCC programme would serve the worldwide community throughout the 21st century. The FCC study also investigates an LHC energy upgrade, using FCC-hh technology. This document constitutes the second volume of the FCC Conceptual Design Report, devoted to the electron-positron collider FCC-ee. After summarizing the physics discovery opportunities, it presents the accelerator design, performance reach, a staged operation scenario, the underlying technologies, civil engineering, technical infrastructure, and an implementation plan. FCC-ee can be built with today’s technology. Most of the FCC-ee infrastructure could be reused for FCC-hh. Combining concepts from past and present lepton colliders and adding a few novel elements, the FCC-ee design promises outstandingly high luminosity. This will make the FCC-ee a unique precision instrument to study the heaviest known particles (Z, W and H bosons and the top quark), offering great direct and indirect sensitivity to new physics.

In response to the 2013 Update of the European Strategy for Particle Physics (EPPSU), the Future Circular Collider (FCC) study was launched as a world-wide international collaboration hosted by CERN. The FCC study covered an energy-frontier hadron collider (FCC-hh), a highest-luminosity high-energy lepton collider (FCC-ee), the corresponding 100 km tunnel infrastructure, as well as the physics opportunities of these two colliders, and a high-energy LHC, based on FCC-hh technology. This document constitutes the third volume of the FCC Conceptual Design Report, devoted to the hadron collider FCC-hh. It summarizes the FCC-hh physics discovery opportunities, presents the FCC-hh accelerator design, performance reach, and staged operation plan, discusses the underlying technologies, the civil engineering and technical infrastructure, and also sketches a possible implementation. Combining ingredients from the Large Hadron Collider (LHC), the high-luminosity LHC upgrade and adding novel technologies and approaches, the FCC-hh design aims at significantly extending the energy frontier to 100 TeV. Its unprecedented centre-of-mass collision energy will make the FCC-hh a unique instrument to explore physics beyond the Standard Model, offering great direct sensitivity to new physics and discoveries.