Compact Linear Collider
The Compact Linear Collider (CLIC) is a proposed high-luminosity linear electron-positron collider designed to operate at multi-TeV energy scales, utilizing a novel two-beam acceleration scheme to achieve high accelerating gradients in normal-conducting radiofrequency structures.[1][2] Developed by an international collaboration involving over 70 institutes from more than 30 countries, CLIC aims to complement CERN's existing accelerator complex by providing a clean leptonic collision environment for precision studies beyond the capabilities of the Large Hadron Collider (LHC).[1] The project is envisioned in three stages, starting with an initial energy of 380 GeV over an 11 km length, scaling up to 1.5 TeV (29 km) and potentially 3 TeV (50 km), with integrated luminosities targeting approximately 4 ab⁻¹ at 380 GeV and 4 ab⁻¹ at 1.5 TeV to enable detailed investigations.[2][3][4] CLIC's physics programme focuses on high-precision measurements of the Higgs boson properties, including its self-coupling, and the top quark, alongside searches for physics beyond the Standard Model such as dark matter candidates, supersymmetric particles, and deviations in electroweak processes.[1][4] Unlike hadron colliders like the LHC, which produce complex event backgrounds, CLIC's electron-positron collisions offer polarized beams and a trigger-less readout system in highly granular detectors with approximately 110 million channels, facilitating the detection of rare signals with minimal pile-up.[3][4] Technologically, CLIC employs a 12 GHz X-band acceleration system powered by a high-current drive beam, achieving gradients of 72–100 MV/m, which allows for a more compact design compared to traditional linear colliders while maintaining power efficiency—improved threefold since 2018 optimizations.[2][5] The estimated cost for the first stage is approximately 7.17 billion CHF, with power consumption around 166 MW at 380 GeV, and recent advancements include a 50% luminosity increase and 100 Hz operation rates.[2] As of 2025, the CLIC design is mature, with ongoing validations through test facilities like CTF3, and construction of the initial stage could begin by 2033, aiming for first beams around 2041 if endorsed in future European Strategy updates.[2][4] This positions CLIC as a flexible, long-term facility capable of adapting to discoveries from the High-Luminosity LHC and future projects like the Future Circular Collider (FCC).[1][5]History and Development
Origins and Motivation
The Compact Linear Collider (CLIC) project originated at CERN in the mid-1980s, emerging as a proposed high-energy electron-positron (e⁺e⁻) collider to extend the capabilities of the Large Electron-Positron Collider (LEP), which was under construction at the time.[6] In 1985, a panel within the LEP Division, advised under Carlo Rubbia's Long Range Planning Committee, began assessing the feasibility of linear collider technologies to achieve multi-TeV collision energies, marking the inception of the CLIC study initially termed the "CERN Linear Collider."[7] By 1988, a dedicated group in CERN's Proton Synchrotron (PS) Division initiated design work on the first CLIC Test Facility (CTF1), involving collaboration across CERN divisions and external laboratories to explore novel acceleration concepts.[7] The primary motivation for CLIC stemmed from the need for a lepton collider that could provide precise measurements in electroweak physics and beyond the Standard Model, offering a cleaner collision environment compared to the proton-proton interactions at the planned Large Hadron Collider (LHC).[8] Unlike hadron colliders, where only a fraction of the center-of-mass energy is available due to parton distributions, e⁺e⁻ collisions in CLIC would deliver the full energy to the interacting particles, enabling direct production and detailed study of phenomena like the Higgs boson with minimal background interference.[6] This precision was seen as essential to complement and verify discoveries from the LHC, particularly in areas requiring high accuracy, such as Higgs couplings and electroweak symmetry breaking.[1] Early feasibility studies for CLIC, documented in the CLIC Note series starting with the first note in August 1985, focused on achieving high acceleration gradients to reach up to 3 TeV center-of-mass energy within a compact footprint of approximately 50 km.[9] A key innovation was the two-beam acceleration scheme, where a high-intensity drive beam generates radiofrequency power for the main beam, enabling gradients up to 100 MV/m and reducing the required length compared to traditional radiofrequency linacs.[10] The CTF1, operational from 1990 to 1995, validated this approach by demonstrating 30 GHz power generation from two-beam interactions, laying the groundwork for subsequent facilities.[10] CLIC developed as a European-led initiative in response to international linear collider proposals, such as Japan's JLC and Germany's TESLA, but distinguished itself by prioritizing higher energies through the efficient two-beam method rather than lower-frequency superconducting technologies.[9] This focus on compactness and scalability positioned CLIC as a potential post-LEP machine capable of TeV-scale physics without the spatial demands of circular accelerators.[8]Key Milestones and Updates
Following CTF1, the CLIC project advanced with the second test facility (CTF2), operational from 1996 to 2000, which focused on high-gradient acceleration structures and beam loading effects. In 2001, the third CLIC Test Facility (CTF3) began operations, demonstrating the drive beam generation and power extraction crucial for the two-beam scheme, running until around 2011.[10] In 2012, the CLIC collaboration released its Conceptual Design Report (CDR), detailing a staged energy plan from an initial 500 GeV center-of-mass energy upgradable to 3 TeV, with comprehensive accelerator, detector, and physics studies.[11] Parameter updates in 2016 refined the baseline, confirming a maximum energy of 3 TeV while optimizing the first stage for 380 GeV operations to align with post-LHC physics goals, incorporating advancements in RF technology and beam dynamics. The project involves international collaborations, including CLICdp for detector and physics studies and the CLIC Accelerator collaboration, involving more than 70 institutes from over 30 countries.[1][12] In May 2025, the European Strategy Preparation and Update (ESPPU) report outlined a baseline CLIC configuration featuring two detectors sharing luminosity to enhance physics reach in a staged approach.[13] A September 2025 arXiv update refined the physics potential, incorporating recent accelerator improvements for better staging scenarios and sensitivity projections.[3] In February 2025, an American Physical Society (APS) study optimized the rings-to-main-linac transport for the 380 GeV stage, improving beam stability and luminosity delivery.[14] CLIC's integration into the European Strategy for Particle Physics (ESPP) updates continues, with a September 2025 CERN Courier article highlighting its strategic role as a potential post-LHC collider option amid ongoing feasibility assessments.[15]Design Overview
Acceleration Principle
The Compact Linear Collider (CLIC) utilizes a novel two-beam acceleration scheme to achieve high-energy electron-positron collisions in a compact footprint. In this method, a high-intensity drive beam with a peak current of 100 A and operating at a frequency of 12 GHz is generated separately and directed parallel to the low-intensity main beam of electrons and positrons. As the drive beam passes through power extraction and transfer structures (PETS), it decelerates, converting its kinetic energy into radiofrequency (RF) fields. These RF fields, operating at the same 12 GHz frequency, are coupled via waveguides to normal-conducting accelerating structures, where they impart energy to the main beam at an accelerating gradient of up to 100 MV/m. This energy transfer enables efficient acceleration without relying on traditional klystron-based RF sources, which are limited by power-handling constraints.[16] The energy gain per accelerating structure for the main beam arises from the portion of the drive beam's energy that is successfully converted and delivered. The overall transfer efficiency \eta is approximately 50%, accounting for losses in RF generation, waveguide transmission, and structure filling. \eta is determined by the design of the PETS and accelerating structures, with detailed simulations showing that up to 84% of the drive beam's kinetic energy can be extracted in the decelerators, though a portion is lost to heat and parasitic modes before reaching the main beam.[17][18] The achievable accelerating gradient is fundamentally limited by wakefield effects and RF breakdown phenomena inherent to the two-beam scheme. Wakefields are electromagnetic fields excited by the leading particles of a bunch, which interact with trailing particles, causing energy spread (longitudinal wakefields) and transverse deflections (transverse wakefields) that degrade beam quality. The transverse wakefield strength W_\perp scales with the structure's iris radius and frequency, and for CLIC's 12 GHz design, it imposes a limit on the gradient to prevent emittance growth exceeding acceptable levels (typically \Delta \epsilon / \epsilon < 10\%). The wake potential per unit length is given by W = \int E_z ds for longitudinal and analogous for transverse, but the limitation arises from the condition that the induced kick \Delta x' = (q / E) W_\perp < \delta x / L, where q is bunch charge, E is the accelerating field, and \delta x is the allowed misalignment tolerance. To derive the gradient limit, one equates the wake-induced voltage to the beam's tolerance: for short-range wakefields dominant in CLIC's short bunches (0.07 mm length), the maximum E satisfies e E L \approx (W_l / \lambda) \sigma_z^2, where W_l is the longitudinal wake, \lambda the wavelength, and \sigma_z the bunch length; this yields E \lesssim 100 MV/m before excessive energy spread (\Delta E / E > 1\%) occurs. RF breakdown further caps the gradient, manifesting as vacuum arcs when the surface field exceeds ~200 MV/m, with conditioning processes targeting a rate below $3 \times 10^{-7} m^{-1} to ensure operational reliability. These limits are unique to the high-current drive beam's interaction with structures, requiring damping manifolds and precise iris tapering for mitigation.[19][20][21] This two-beam approach yields significant advantages in compactness by enabling gradients an order of magnitude higher than conventional RF linacs (typically 20-30 MV/m). For instance, the main linac for CLIC's 380 GeV stage spans 11 km, in contrast to over 30 km required for equivalent energy in lower-gradient normal-conducting systems. Scaling to 3 TeV, the full collider fits within a 50 km site length, including beam delivery systems, far shorter than the 200+ km projected for conventional designs at similar energies.[16][22] Technical challenges in implementing this principle include optimizing deceleration efficiency in the drive beam to maximize usable RF power, controlling RF breakdown rates through material conditioning and pulse shaping, and suppressing transverse wakefields via structure geometry to avoid beam blow-up. These issues demand sub-micrometer manufacturing tolerances and active stabilization systems, distinguishing the two-beam method from standard single-beam acceleration.[17][23]Energy Staging Plan
The Compact Linear Collider (CLIC) employs a three-stage energy plan to progressively access a wide range of center-of-mass energies, enabling comprehensive studies from precision Higgs physics to explorations of new particles at the TeV scale. This staged approach, updated in 2025, begins with Stage 1 at 380 GeV—lowered from the previous 500 GeV baseline to better align with Z-boson and Higgs threshold measurements—followed by Stage 2 at 1.5 TeV for enhanced Higgs and top-quark investigations, and Stage 3 at 3 TeV to probe heavy particles and beyond-Standard-Model phenomena.[13][24] Luminosity targets have been refined in the 2025 update to reflect improved beam emittance simulations and the adoption of two-detector operations, which share the luminosity between interaction points. At 380 GeV, the instantaneous luminosity reaches 4.5 × 10^{34} cm^{-2} s^{-1} with a 100 Hz repetition rate, yielding an integrated luminosity of approximately 0.43 ab^{-1} per year and 4.3 ab^{-1} over 10 years of operation. This scales to 3.7 × 10^{34} cm^{-2} s^{-1} at 1.5 TeV (50 Hz, 0.4 ab^{-1}/year, 4 ab^{-1} total) and 5.9 × 10^{34} cm^{-2} s^{-1} at 3 TeV (50 Hz, 0.625 ab^{-1}/year, 5 ab^{-1} over 8 years), achieved through beam current increases without altering the fundamental repetition rates beyond Stage 1.[13][24] The staging rationale leverages CLIC's modular design, allowing energy upgrades without a complete rebuild, thus minimizing downtime and costs while maximizing physics output over an estimated 20-30 years of total operation. Each stage is planned for about 10 years of runtime (185 days/year at 75% availability), with a 2-year transition interval between stages. Upgrades involve gradual lengthening of the main linacs—from 11.4 km at 380 GeV to 29 km at 1.5 TeV and beyond—along with additions of klystrons and higher-gradient accelerating modules to the drive-beam complex. Luminosity in this scheme follows the formula L = \frac{N^2 f}{4 \pi \sigma_x \sigma_y}, where N is the number of particles per bunch, f the repetition rate, and \sigma_x, \sigma_y the horizontal and vertical beam sizes at the interaction point, enabling scalable performance as parameters are optimized per stage.[13][24]Physics Objectives
Higgs Boson Studies
The Compact Linear Collider (CLIC) enables high-precision studies of the Higgs boson primarily through electron-positron annihilation at its initial center-of-mass energy of 380 GeV, where the Higgs-strahlung process e^+ e^- \to Z H dominates production with a cross-section of approximately 160 fb, yielding around 160,000 Higgs events for 1 ab^{-1} of integrated luminosity. This clean environment facilitates direct reconstruction of the Higgs via the recoil mass technique against the Z boson, providing model-independent access to the Higgs mass and couplings. At higher energy stages of 1.5 TeV and 3 TeV, vector boson fusion (e^+ e^- \to H \nu_e \bar{\nu}_e) contributes significantly, with cross-sections growing quadratically with energy, allowing sensitivity to electroweak couplings without reliance on associated particles.[25][26] CLIC's measurements target the Higgs total width \Gamma_H with 1.4% precision from the first two energy stages, corresponding to an uncertainty well below 10 MeV given the Standard Model value of approximately 4.1 MeV. The trilinear self-coupling \lambda_{HHH} is projected to be determined to [-8\%, +11\%] accuracy over a 10-year program, leveraging double Higgs production in vector boson fusion and Higgs-strahlung at higher energies. Branching ratios to dominant modes, such as H \to b\bar{b} (Standard Model prediction: 58%) and H \to W W^* (21%), can be measured at the percent level, testing the Higgs sector's consistency with the Standard Model. Sensitivity to invisible decays reaches BR(H \to invisible) < 0.69\% at 90% confidence level, probing potential dark matter couplings or hidden sectors.[2][26] Relative to the LHC's hadron collisions, CLIC's point-like leptons enable s-channel access for model-independent coupling extractions, avoiding uncertainties from parton distribution functions and QCD backgrounds. This yields superior precision on absolute couplings and enhanced detection of invisible decays through missing energy signatures. CLIC can exclude exotic Higgs sectors, such as singlet extensions with mixing angle \sin^2 \gamma > 0.0024 (0.24%), up to scales of 1.5 TeV via direct production and decay analyses.[26][1] In 2025 updates, CLIC's baseline incorporates a two-detector setup at 380 GeV with dual beam delivery systems, sharing luminosity to boost statistics by roughly 50% over prior single-detector designs and enhancing Higgs measurement sensitivities through parallel data accumulation. This configuration aligns with the energy staging plan, optimizing the 380 GeV threshold for maximal Higgs-strahlung rates.[27][2]Top Quark Investigations
The Compact Linear Collider (CLIC) enables precision studies of top quark production and properties starting from its initial energy stage at 380 GeV, where the top-antitop pair (ttbar) production threshold is reached around 350 GeV. Single top quark production occurs via electroweak processes, such as W boson exchange, providing complementary channels for investigation. At 380 GeV, the ttbar cross-section is approximately 500 fb, decreasing to about 25 fb at 3 TeV, allowing for the collection of large samples of top quarks with integrated luminosities up to several ab^{-1} across stages.[28] CLIC's high luminosity and clean lepton collider environment facilitate top quark mass measurements with a statistical precision of 30 MeV using 1 ab^{-1} at 380 GeV, through techniques like threshold scans around 350 GeV with 100 fb^{-1} integrated luminosity spaced at 1 GeV intervals. The top Yukawa coupling |y_t| can be determined to 3% precision at 1.5 TeV with 2.5 ab^{-1}, leveraging associated production modes. Rare decays, such as t → cH, are sensitive to branching ratios below 10^{-4}, with limits on flavor-changing neutral currents like t → cγ reaching BR < 2.6 × 10^{-5} at 95% confidence level using 1 ab^{-1} at 380 GeV. The top quark width is given by \Gamma_t = \frac{G_F m_t^3}{8\pi\sqrt{2}} |V_{tb}|^2 \left(1 + \frac{16}{3}\alpha_s(m_t) + \cdots \right), where radiative corrections beyond leading order are included for accuracy in threshold fits.[28] Unique to CLIC, the low background and precise beam polarization enable measurements of top quark spin correlations via clean jet reconstruction and forward-backward asymmetries, with sensitivities enhanced at higher energies. The ttH coupling can be probed to 10% accuracy through associated ttH production, which constitutes a significant fraction of events at TeV scales. Stage 2 operations at 1.5 TeV are particularly optimal for detailed threshold scans, angular asymmetries, and Yukawa determinations, building on initial stage data for improved systematic control. These precision measurements also provide indirect sensitivity to new phenomena involving tops, such as deviations in couplings signaling beyond-Standard-Model physics.[28]Searches for New Physics
The Compact Linear Collider (CLIC) is designed to probe beyond-Standard-Model (BSM) physics at TeV-scale energies through direct production of new particles and indirect effects on precision observables.[29] At its highest energy stage of 3 TeV, CLIC's clean lepton collisions enable high sensitivity to weakly interacting particles that may evade detection at hadron colliders like the LHC. This capability arises from the collider's high luminosity—up to 5 ab⁻¹—and precise vertex reconstruction, allowing for the identification of subtle signatures such as displaced vertices and missing transverse energy.[29] Key signatures for new physics at CLIC include missing energy events from supersymmetric (SUSY) processes, such as e^+ e^- \to \tilde{\chi} \tilde{\chi} + jets, where \tilde{\chi} denotes the lightest neutralino or chargino, often serving as a dark matter candidate.[29] These events feature large missing transverse momentum from undetected stable particles, complemented by jets or leptons from cascade decays. Contact interactions, indicative of fermion compositeness, manifest as deviations in four-fermion scattering processes, with enhanced sensitivity due to CLIC's polarized beams.[29] Additionally, dark matter candidates are probed via mono-jet or mono-photon events, where an initial-state radiation jet or photon recoils against invisible particles produced in association with the Higgs or Z boson.[29] Reach projections for CLIC demonstrate substantial extensions beyond current limits. In extra dimension models, Kaluza-Klein (KK) modes of gravitons or gauge bosons can be discovered up to masses of 10 TeV through resonant production and decays to dileptons or dijets.[29] Leptoquarks, hypothetical particles mediating baryon-number-violating processes, are accessible up to 5 TeV via single or pair production, with clean signatures in final states containing leptons and jets.[29] At the 3 TeV stage with 3 ab⁻¹ integrated luminosity, exclusion limits for contact interaction scales \Lambda exceed 40 TeV for flavor-universal operators, assuming standard model-like couplings.[29]| Model | Discovery Reach (TeV) | Exclusion Limit (TeV) | Energy Stage | Luminosity (ab⁻¹) |
|---|---|---|---|---|
| Extra Dimensions (KK modes) | 10 | N/A | 3 TeV | 3 |
| Leptoquarks | 5 | N/A | 3 TeV | 3 |
| Contact Interactions | N/A | >40 (\Lambda) | 3 TeV | 3 |