Future Circular Collider
The Future Circular Collider is a proposed particle accelerator with an energy significantly above that of previous circular colliders, such as the Super Proton Synchrotron, the Tevatron, and the Large Hadron Collider. The FCC project is considering three scenarios for collision types: FCC-hh, for hadron-hadron collisions, including proton-proton and heavy ion collisions, FCC-ee, for electron-positron collisions, and FCC-eh, for electron-hadron collisions.
In FCC-hh, each beam would have a total energy of 560 MJ. With a centre-of-mass collision energy of 100 TeV the total energy value increases to 16.7 GJ. These total energy values exceed the present LHC by nearly a factor of 30.
CERN hosted an FCC study exploring the feasibility of different particle collider scenarios with the aim of significantly increasing the energy and luminosity compared to existing colliders. It aims to complement existing technical designs for proposed linear electron/positron colliders such as the International Linear Collider and the Compact Linear Collider.
The study explores the potential of hadron and lepton circular colliders, performing an analysis of infrastructure and operation concepts and considering the technology research and development programmes that are required to build and operate a future circular collider. A conceptual design report was published in early 2019, in time for a scheduled update of the European Strategy for Particle Physics.
Background
The CERN study was initiated as a direct response to the high-priority recommendation of the updated European Strategy for Particle Physics, published in 2013 which asked that "CERN should undertake design studies for accelerator projects in a global context, with emphasis on proton-proton and electron-positron high-energy frontier machines. These design studies should be coupled to a vigorous accelerator R&D programme, including high-field magnets and high-gradient accelerating structures, in collaboration with national institutes, laboratories and universities worldwide". The goal was to inform the next Update of the European Strategy for Particle Physics and the wider physics community for the feasibility of circular colliders complementing previous studies for linear colliders as well as other proposal for particle physics experiments.The launch of the FCC study was also in line with the recommendations of the United States' Particle Physics Project Prioritization Panel and of the International Committee for Future Accelerators, a working group of the International Union of Pure and Applied Physics.
The discovery of the Higgs boson at the LHC, together with the absence so far of any phenomena beyond the Standard Model in collisions at centre of mass energies up to 8 TeV, has triggered an interest in future circular colliders to push the energy and precision frontiers complementing studies for future linear machines. The discovery of a "light" Higgs boson with a mass of 125 GeV revamped the discussion for a circular lepton collider that would allow detailed studies and precise measurement of this new particle. With the study of a new 80–100 km circumference tunnel, that would fit in the Geneva region, it was realized that a future circular lepton collider could offer collision energies up to 400 GeV at unprecedented luminosities. The design of FCC-ee was combining the experience gained by LEP2 and the latest B-factories.
Two main limitations to circular-accelerator performance are energy loss due to synchrotron radiation, and the maximum value of magnetic fields that can be obtained in bending magnets to keep the energetic beams in a circular trajectory. Synchrotron radiation is of particular importance in the design and optimization of a circular lepton collider and limits the maximum energy that can be reached as the phenomenon depends on the mass of the accelerated particle. To address these issues a sophisticated machine design along with the advancement of technologies like accelerating cavities and high-field magnets are needed.
Future "intensity and luminosity frontier" lepton colliders like those considered by the FCC study would enable the study with very high precision of the properties of the Higgs boson, the W and Z bosons and the top quark, pinning down their interactions with an accuracy at least an order of magnitude better than today. The FCC-ee could collect 1012 Z bosons, 108 W pairs, 106 Higgs bosons and 4 · 105 top-quark pairs per year. As a second step, an "energy frontier" collider at 100 TeV could be a "discovery machine" offering an eightfold increase compared to the current energy reach of the LHC.
The FCC integrated project, combining FCC-ee and FCC-hh, would rely on a shared and cost effective technical and organizational infrastructure, as was the case with LEP followed by LHC. This approach improves by several orders the sensitivity to elusive phenomena at low mass and by an order of magnitude the discovery reach for new particles at the highest masses. This will allow to uniquely map the properties of the Higgs boson and Electroweak sector and broaden the exploration for different Dark Matter candidate particles complementing other approaches with neutrino beams, non-collider experiments and astrophysics experiments.
Motivation
The LHC has advanced the science of matter and the Standard Model. The discovery of the Higgs boson completed the particle-related component of the Standard Model of Particle Physics, the theory that describes the laws governing most of the known Universe. Yet the Standard Model cannot explain several observations, such as:- evidence for dark matter,
- prevalence of matter over antimatter,
- the neutrino masses.
The Future Circular Collider study develops options for potential high-energy frontier circular colliders at CERN for the post-LHC era. Among other things, it plans to look for dark matter particles, which account for approximately 25% of the energy in the observable universe. Though no experiment at colliders can probe the full range of dark matter masses allowed by astrophysical observations, there is a very broad class of models for weakly interacting massive particles in the GeV – tens of TeV mass scale, and which could be in the range of the FCC.
FCC could also lead the progress in precision measurements of Electroweak precision observables. The measurements played a key role in the consolidation of the Standard Model and can guide future theoretical developments. Moreover, results from these measurements can inform data from astrophysical/cosmological observations. The improved precision offered by the FCC integrated programme increases the discovery potential for new physics.
Moreover, FCC-hh will enable the continuation of the research programme in ultrarelativistic heavy-ion collisions from RHIC and LHC. The higher energies and luminosities offered by FCC-hh when operating with heavy-ions will open new avenues in the study of the collective properties of quarks and gluons.
The FCC study also foresees an interaction point for electrons with protons. These deep inelastic scattering measurements will resolve the parton structure with very high accuracy providing a per mille accurate measurement of the strong coupling constant. These results are essential for a programme of precision measurements and will further improve the sensitivity of search for new phenomena particularly at higher masses.
Scope
The FCC study originally put an emphasis on proton-proton high-energy collider that could also house an electron/positron high-intensity frontier collider as a first step. However after assessing the readiness of the different technologies and the physics motivation the FCC collaboration came up with the so-called FCC integrated programme foreseen as a first step FCC-ee with an operation time of about 10 years at different energy ranges from 90 GeV to 350 GeV, followed by FCC-hh with an operation time of about 15 years.The FCC collaboration has identified the technological advancements required for reaching the planned energy and intensity and performs technology feasibility assessments for critical elements of future circular colliders. The project needs to advance these technologies to meet the requirements of a post-LHC machine but also to ensure the large-scale applicability of these technologies that could lead to their further industrialization. The study also provides an analysis of the infrastructure and operation cost that could ensure the efficient and reliable operation of a future large-scale research infrastructure. Strategic R&D has been identified in the CDR over the coming years will concentrate on minimising construction costs and energy consumption, whilst maximising the socio-economic impact with a focus on benefits for industry and training.
Scientists and engineers are also working on the detector concepts needed to address the physics questions in each of the scenarios. The work programme includes experiment and detector concept studies to allow new physics to be explored. Detector technologies will be based on experiment concepts, the projected collider performances and the physics cases. New technologies have to be developed in diverse fields such as cryogenics, superconductivity, material science, and computer science, including new data processing and data management concepts.