The Future Circular Collider: a Summary for the US 2021 Snowmass Process

TL;DR

The paper advocates a two‑stage Future Circular Collider program (FCC‑ee followed by FCC‑hh) housed in a ≈90 km tunnel near CERN, targeting transformative precision in Higgs, electroweak, QCD, and flavor physics, plus expansive direct searches for new physics. It emphasizes the synergistic hardware, with FCC‑ee enabling ultra‑high‑precision measurements and SMEFT sensitivity, and FCC‑hh extending the energy frontier to ≈50 TeV scales for broad discovery potential and detailed Higgs studies. The document also highlights dark sector opportunities (ALPs, HNLs), heavy neutral lepton searches, and extensive QCD and flavor programs, all supported by a robust detector and software strategy and substantial US involvement. Together, FCC‑ee and FCC‑hh offer a comprehensive path to deepening our understanding of the Standard Model and probing physics beyond it, with strong complementarity across energy scales and experimental modalities.

Abstract

In this white paper for the 2021 Snowmass process, we give a description of the proposed Future Circular Collider (FCC) project and its physics program. The paper summarizes and updates the discussion submitted to the European Strategy on Particle Physics. After construction of an approximately 90 km tunnel, an electron-positron collider based on established technologies allows world-record instantaneous luminosities at center-of-mass energies from the Z resonance up to tt thresholds, enabling a rich set of fundamental measurements including Higgs couplings determinations at the sub percent level, precision tests of the weak and strong forces, and searches for new particles, including dark matter, both directly and via virtual corrections or mixing. Among other possibilities, the FCC-ee will be able to (i) indirectly discover new particles coupling to the Higgs and/or electroweak bosons up to scales around 7 and 50 TeV, respectively; (ii) perform competitive SUSY tests at the loop level in regions not accessible at the LHC; (iii) study heavy-flavor and tau physics in ultra-rare decays beyond the LHC reach, and (iv) achieve the best potential in direct collider searches for dark matter, sterile neutrinos, and axion-like particles with masses up to around 90 GeV. The tunnel can then be reused for a proton-proton collider, establishing record center-of-mass collision energy, allowing unprecedented reach for direct searches for new particles up to the around 50 TeV scale, and a diverse program of measurements of the Standard Model and Higgs boson, including a precision measurement of the Higgs self-coupling, and conclusively testing weakly-interacting massive particle scenarios of thermal relic dark matter.

Figures (17)

• Figure 1: Potential instantaneous luminosity versus center-of-mass energy for FCC-ee.
• Figure 2: Cross sections for various processes in $\mathrm{e^+e^-}$ collisions versus center-of-mass energy.
• Figure 3: The new FCC layout referred to as PA31-1.0; four possible experiments could be located at PA, PD, PG, and PJ while RF stations would be located PH and PL and injection/extraction and collimation could be located in PB and PF straights.
• Figure 4: Left: W$^+$W$^-$ production cross section as a function of the $\mathrm{e^+e^-}$ collision energy Abada:2019lih. The central curve corresponds to the predictions obtained with $m_\mathrm{W}=80.385$ GeV and $\Gamma_\mathrm{W}=2.085$ GeV. Purple and green bands show the cross section curves obtained varying the W mass and width by $\pm 1$ GeV. Right: Expected uncertainty contour for the $S$ and $T$ parameters for various colliders in their first energy stage eps_strategydeBlas:2019rxi.
• Figure 5: The 68% probability reach for $c_i/\Lambda^2$ from a fit to the EFT Lagrangian in Eq. (3.19) of deBlas:2019rxi. The right axis shows the corresponding bound on the new physics interaction scale.