  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.
