Future Circular Collider (FCC)

• FCC is a large-scale collider complex featuring FCC-ee for high-luminosity electron–positron collisions and FCC-hh for 100 TeV proton–proton collisions.
• It employs advanced accelerator technologies like superconducting RF systems, high-field Nb₃Sn magnets, and crab-waist optics to achieve unprecedented luminosities and precision.
• The project targets key physics areas, including precision measurements of the Higgs and electroweak sectors and direct searches for new heavy particles beyond the Standard Model.

The Future Circular Collider (FCC) is a comprehensive research and infrastructure program initiated by CERN to establish the post-LHC energy and precision frontiers through a staged sequence of large-scale colliders. The FCC complex is designed around a new underground tunnel of approximately 80–100 km circumference in the Geneva region, initially dedicated to high-luminosity electron–positron collisions (FCC-ee), and later re-equipped as a 100 TeV proton–proton collider (FCC-hh), with substantial additional programmatic scope in heavy-ion, photon–photon, and electron–proton collision modes (Benedikt et al., 25 Apr 2025, Suarez, 2022, Janot, 2015, Dainese et al., 2019).

1. FCC Project Vision and Staged Implementation

The FCC is motivated by foundational questions not addressed by the Standard Model, such as the Higgs potential, matter–antimatter asymmetry, dark matter, and the flavor structure of fundamental particles. The FCC project advances the priorities of the European Strategy for Particle Physics, which calls for, as a first step, an ultra-high-luminosity electron–positron "Higgs and electroweak factory," followed by a 100 TeV energy-frontier hadron collider using the same infrastructure (Suarez, 2022, Benedikt et al., 25 Apr 2025, Bernardi et al., 2022).

Staging Plan:

• FCC-ee (years ~2045–2060): Circular e⁺e⁻ collider operating at √s = 91 GeV (Z), 160 GeV (W threshold), 240 GeV (ZH), and 365 GeV (t t̄ threshold), exploiting up to 4 interaction points (IPs). Physics deliverables include >10¹² Z bosons, ~10⁸ W pairs, ~10⁶ Higgs bosons, and precision top-threshold measurements with masses accessed to 10 MeV accuracy.
• FCC-hh (years ~2070–2095): Transformation of the tunnel for proton–proton collisions at √s = 100 TeV, targeting peak luminosities up to 3×10³⁵ cm⁻²s⁻¹ and integrated luminosities exceeding 20–30 ab⁻¹. The infrastructure is designed for readiness for heavy ions and e–p collisions as well (Suarez, 2022, Benedikt et al., 2022, Bernardi et al., 2022, Benedikt et al., 25 Apr 2025).

This integrated vision prioritizes both direct access to physics at the highest mass scales (tens of TeV) and per-mille-level indirect constraints via ultra-precise measurements (Janot, 2015, Suarez, 2022).

2. Collider Design and Accelerator Technologies

2.1 FCC-ee

Tunnel and Lattice: The FCC-ee will occupy an 80–100 km tunnel (baseline 91.2 km) with up to four collision points, two separate beam pipes for e⁺/e⁻ operation, and a full-energy top-up booster in the same tunnel for continuous injection. The ring is designed for multi-mode operation, with optics—standard FODO cells in the arcs and strong final-focus mini-beta telescopes—optimized for each energy stage (Koratzinos, 2014, Agapov et al., 2022, Janot, 2015).

Main Parameters of the Four Staging Energies:

Parameter	Z Pole	W Threshold	Higgs Factory	tt̄ Threshold
√s [GeV]	91.2	160	240	350–365
Circumference [km]	~100	~100	~100	~100
Peak L [cm⁻²s⁻¹]	2×10³⁶	1×10³⁵	5×10³⁴	1×10³⁴
Beam current [mA]	1500	60	35	6
Bunches/beam	16,000	400	240	80
β*_y [mm]	1.0	1.0	1.0	1.0
ε_x/ε_y [nm/pm]	0.3/0.001	0.1/0.001	0.05/0.001	0.02/0.001

The limiting factor is synchrotron radiation (SR), fixed to ≤50 MW per beam at every energy, determining allowable beam current and operating optics (Koratzinos, 2014, Janot, 2015, Agapov et al., 2022).

Luminosity Formulas:

• In the beam–beam-limited regime:

$$L = \frac{I\,n_b\,\gamma\,\xi_y}{2 e r_e\,\beta_y^*}$$

• Alternatively (per-bunch):

$$L = \frac{N^2\,f_{\rm rev}\,n_{\rm b}}{4\pi\,\sigma_x\,\sigma_y}\;H_D$$

with $H_D$ accounting for hourglass and crossing-angle effects.

Key Technologies:

• RF System: Superconducting cavities (400–800 MHz), up to 12 GV total voltage, with tailored high-power couplers for large beam currents at Z pole (Janot, 2015, Koratzinos, 2014).
• Crab-Waist Collision Optics: The crab-waist scheme with a 30 mrad total crossing angle allows high beam–beam tune-shift parameters ($\xi_y\sim0.1$) while controlling parasitic collisions and nonlinear resonances (Boscolo et al., 2019, Koratzinos et al., 2015, Koratzinos, 2014).
• Magnet Lattice: FODO cells in arcs, mini-beta telescopes for low $\beta^*_y$, final-focus quadrupoles at $L^*\sim2$ m, local/global chromaticity correction, and stringent alignment tolerances (tens of μm) (Koratzinos et al., 2015, Janot, 2015).
• Energy Calibration: Resonant depolarization with non-colliding pilot bunches, insertion wigglers, and continuous spin-tune control yield energy measurement uncertainties below 100 keV (Koratzinos et al., 2015).
• Synchrotron Radiation and Beamstrahlung Management: Large dynamic and momentum acceptance ($\delta p/p\sim 2\%$), low-emittance optics, SR-masking in interaction region, final-focus magnet integration with compensation coils (Boscolo et al., 2019, Koratzinos, 2014).

2.2 FCC-hh

Proton Collider Parameters:

Parameter	Value
Circumference	91.2–97.8 km
Beam energy	50 TeV/beam
c.m. energy	100 TeV
Dipole field	16 T (Nb₃Sn)
Bunch spacing	25 ns
Bunches per beam	~10,400
Peak luminosity (Phase 2)	3×10³⁵ cm⁻²s⁻¹
Normalized emittance	2.2 μm (nominal)

Enabling technologies include large-scale high-field Nb₃Sn dipoles, new cryogenics to intercept 2–5 MW per sector at 50 K (SR power), advanced collimation, and combined-function arc magnets (Benedikt et al., 2022, Benedikt et al., 2018).

Technical Challenges:

• Superconducting Magnets: R&D on cost-effective 16 T Nb₃Sn conductors and industrial-scale coil fabrication. Prototype fields >14.5 T demonstrated (Benedikt et al., 2018, Benedikt et al., 2022).
• Cryogenics and Power: Refrigeration at 2 K and 50 K beam screens with total site power up to ~600 MW (Benedikt et al., 2022, Benedikt et al., 25 Apr 2025).
• Synchrotron Radiation: Unlike previous hadron colliders, strong radiation damping effects impact beam dynamics, requiring active control of emittance and tune shift (Benedikt et al., 2018).

3. Physics Objectives and Performance Reach

3.1 Precision Electroweak, Higgs, and Top Physics (FCC-ee)

With per-mille-level luminosities and energy calibration, FCC-ee enables

• Z mass and width to ~0.1 MeV,
• W mass to 0.5 MeV,
• Higgs boson couplings to sub-per-mille (e.g., $g_{HZZ}$ to 0.15%, $g_{Hbb}$ to 0.4%),
• top mass to 10–20 MeV and EW/Yukawa couplings at sub-percent levels (Benedikt et al., 25 Apr 2025, Janot, 2015, Novotny, 2022, d'Enterria, 2017).

This constrains Standard Model parameters and New Physics scales to $O(10)$–$O(100)$ TeV via global fits, including SMEFT analyses (Janot, 2015, Benedikt et al., 25 Apr 2025).

3.2 Higgs Sector and Beyond

Key benchmark achievements:

• Higgs self-coupling ($\lambda_3$): Determined to 5–8% via double Higgs production at 100 TeV in FCC-hh (d'Enterria, 2017, Suarez, 2022).
• Invisible Higgs width: Sensitivity to BR$(H\to{\rm inv})<0.3–0.5\%$ (d'Enterria, 2017, Benedikt et al., 25 Apr 2025).
• First-generation Yukawa couplings: Direct access to $y_e$, $y_u$, $y_d$ via rare exclusive decays or resonant production at $\sqrt{s}=125$ GeV with beam monochromatization (d'Enterria, 2017).
• Precision top Yukawa measurement: Via cross-section ratios $σ({\rm t\bar{t}H})/σ({\rm t\bar{t}Z})$ at FCC-hh, theoretical uncertainty $<$2%, yielding $\Delta y_t/y_t\sim1.5\%$ in combination with FCC-ee (Janot, 2015).

3.3 Discovery Reach (FCC-hh)

FCC-hh directly extends the mass reach for new states (e.g., heavy vector bosons, colored resonances, supersymmetric partners) up to 30–50 TeV, exceeds HL-LHC by factors of 7 (energy) and 10–60 (luminosity), and can exclude or detect WIMP dark matter up to $m_\chi\sim 2.5$ TeV (Bernardi et al., 2022, Suarez, 2022, Benedikt et al., 2022, Benedikt et al., 25 Apr 2025).

3.4 Heavy-Ion Program

FCC-hh heavy-ion running enables:

• Pb–Pb at $\sqrt{s_{NN}} = 39$ TeV, integrated luminosities up to $110$ nb$^{-1}$/month,
• unprecedented QGP energy densities ($\varepsilon\sim40$ GeV/fm$^3$),
• access to rare hard probes such as Higgs, top and multi-TeV jets, and
• unique small-$x$ QCD and gluon saturation studies, with measured $x$ down to $10^{-7}$ (Dainese et al., 2019, Dainese et al., 2016, Armesto et al., 2014, Benedikt et al., 25 Apr 2025).

4. Detector, Software, and Machine–Detector Interface

Both FCC-ee and FCC-hh require

• sub-3 μm-resolution silicon pixel vertex detectors for heavy-flavor tagging (X/X₀ ~ 0.1%/layer),
• full-coverage TPC or silicon trackers with $\sigma_p/p\leq2\times10^{-5}$ @ 100 GeV/c,
• granular electromagnetic and hadronic calorimetry ($\sigma_E/E\sim1–2\%/\sqrt{E}$ for e/γ; $50–60\%/\sqrt{E}$ for jets),
• muon detectors with coverage to $|\eta|\sim4$,
• low-mass, radiation-tolerant electronics, and
• pure software triggers at FCC-ee, multi-level DAQ at FCC-hh (Benedikt et al., 25 Apr 2025).

The machine–detector interface is optimized for minimal synchrotron radiation backgrounds, precise solenoid compensation for low vertical emittance blow-up, and integration of final-focus quadrupoles within the detector solenoid (Boscolo et al., 2019, Koratzinos et al., 2015).

5. Project Timeline, Challenges, and Feasibility

Timeline and Construction:

• Feasibility paper: 2021–2026; technical design: 2026–2032.
• Civil engineering (tunnel, caverns, shafts): 2033–2038.
• FCC-ee installation/commissioning: 2039–2043; operation: 2044–2059.
• FCC-hh installation: 2060–2065; operation through 2095 (Benedikt et al., 25 Apr 2025, Suarez, 2022, Benedikt et al., 2022).

Infrastructure Reuse: The tunnel and most surface/subsurface infrastructure are designed for both FCC-ee and FCC-hh, with pre-sized detector caverns, and staged installation of RF, cryogenics, and high-field magnets (Suarez, 2022, Benedikt et al., 25 Apr 2025).

Key R&D and Engineering Challenges:

• Industrialization and cost reduction for >16 T Nb₃Sn dipole magnets,
• Efficient SRF systems and multi-MW power couplers,
• Compact, high-strength, low-alignment-tolerance final-focus magnets,
• Robust, low-impedance (NEG-coated) vacuum chambers to accommodate >50 MW SR loads,
• Grid power and refrigeration architecture (~600 MW site-wide),
• Ultra-high-precision alignment, surveying, and beam monitoring systems.

Technological advances are anticipated in both accelerator and detector domains, including AI-driven event reconstruction, exascale distributed computing, and modular, radiation-hard integrated electronics (Benedikt et al., 25 Apr 2025).

6. Broader Impact and Global Context

The FCC program stands as a globally coordinated, long-term infrastructure for high-energy physics, leveraging international partnerships and synergies with ep and eA options, maintaining complementarity with US, Asian, and other European projects (e.g., ILC, CLIC, CEPC, LHeC, EIC). It is designed for flexibility (beam particles, collision types), cost-optimized infrastructure reuse, and a science platform lasting into the late 21st century. The anticipated reach into physics beyond the Standard Model is both via direct access to multi-TeV states and indirect sensitivity to scales up to O(100) TeV from per-mille-level precision measurements (Bernardi et al., 2022, Benedikt et al., 25 Apr 2025, Suarez, 2022, Koratzinos et al., 2015, Janot, 2015, Agapov et al., 2022).

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References

• (Benedikt et al., 25 Apr 2025) Future Circular Collider Feasibility Study Report: Volume 1, Physics, Experiments, Detectors
• (Suarez, 2022) The Future Circular Collider (FCC) at CERN
• (Koratzinos et al., 2015) The FCC-ee paper: Progress and challenges
• (Janot, 2015) Precision measurements of the top quark couplings at the FCC
• (Dainese et al., 2019) Future heavy-ion facilities: FCC-AA
• (Benedikt et al., 2018) Proton Colliders at the Energy Frontier
• (Benedikt et al., 2022) Future Circular Hadron Collider FCC-hh: Overview and Status
• (Armesto et al., 2014) Heavy-ion physics studies for the Future Circular Collider
• (d'Enterria, 2017) Higgs physics at the Future Circular Collider
• (Koratzinos, 2014) FCC-ee accelerator parameters, performance and limitations
• (Boscolo et al., 2019) Machine detector interface for the $e^+e^-$ future circular collider
• (Bernardi et al., 2022) The Future Circular Collider: a Summary for the US 2021 Snowmass Process
• (Novotny, 2022) FCC-ee, an Accelerator for New Physics Searches in the Heavy Quark Sector
• (Agapov et al., 2022) Future Circular Lepton Collider FCC-ee: Overview and Status
• (Dainese et al., 2016) Heavy ions at the Future Circular Collider