Future Circular Collider
The largest scientific experiment ever built by humanity is in high-energy physics, and its planning began in the mid-1970s. The project was coined as the Large Electron Positron (LEP) Collider and since its start in 1989 it has been providing a trove of findings which eventually led to the Nobel Prize in physics (1999). The 27 km circular tunnel in which electrons and positrons were brought to collision lasted for 12 years, until 2001, and got a new life as a proton-proton collider under a new name, the Large Hadron Collider (LHC), which was proposed in the mid eighties and started operation in 2009. Already in its first run the CMS and ATLAS experiments simultaneously and independently discovered the Higgs boson in 2012 which lead to another Nobel Prize in physics (2013).
The Future Circular Collider (FCC) is a proposed next-generation particle collider at CERN that would be built in a new underground tunnel of about 100 km circumference, nearly four times larger than the existing LEP/LHC complex. The program foresees a staged approach, starting with FCC-ee, a high-precision electron–positron collider designed to study the Z and W bosons, the Higgs boson, and the top quark with unprecedented accuracy, followed by FCC-hh, a proton–proton collider reaching energies far beyond those of the LHC to explore new frontiers in particle physics. The scientific, technical, and implementation aspects of this program were studied in detail in the FCC Feasibility Study, launched in 2021 and completed in 2025.
Photo: Concept for the Future Circular Collider.
Source: https://home.cern/science/accelerators/future-circular-collider
The MIT group has been heavily involved in this effort, contributing to studies of detector performance, beam-induced backgrounds, and the overall physics potential of the FCC program. Our work includes investigations of beam backgrounds from the intense electron–positron collisions, as well as detector design and optimization, particularly for precision tracking and vertex reconstruction near the interaction point. In parallel, we study the precision electroweak and Higgs physics program enabled by FCC-ee. While the feasibility study represents an important milestone, further work is needed to refine detector concepts, improve background mitigation strategies, and fully develop the physics program for future FCC experiments. This project has been a great opportunity to get the next generation of physicists involved and excited about the future of the field, as you can read here or here!
Beam Induced Backgrounds
Beam-induced backgrounds are an important challenge for FCC-ee. When the intense electron and positron beams collide, strong electromagnetic fields can produce large numbers of secondary particles, known as incoherent electron–positron pairs. These particles can enter the detector and affect its performance, making it essential to understand and control them. Our group studies these backgrounds using detailed simulations of the beam–beam interaction, focusing on how these particles are produced and how they propagate toward the detector. By characterizing their energy, direction, and timing, we evaluate their impact on detector occupancies and provide input for the design of the interaction region and detector systems, helping to optimize FCC-ee detector concepts and develop strategies to mitigate beam-related backgrounds.
Photo: production kinematics of Incoherent Pair Creation: momentum as function of the polar angle
Vertex Detector Geometries
The vertex detector is the innermost tracking system of the FCC-ee detector and plays a crucial role in reconstructing particle trajectories close to the collision point, which is essential for identifying short-lived particles and enabling many key physics measurements. The MIT group studies different design options for the vertex detector of the IDEA detector concept to optimize its tracking performance. Using detailed simulations, we evaluate how changes in detector geometry and magnetic field affect the precision with which particle tracks can be reconstructed. In particular, designs with a shorter and lighter barrel structure and configurations where the first detector layer is placed closer to the beam pipe show significant improvements in impact parameter resolution. These studies provide important input for the ongoing optimization of the FCC-ee vertex detector while ensuring compatibility with realistic engineering and integration constraints.
Photo: improvement of the reconstructed vertex position resolution when chainging the first vertex silicon layer closer to the beampipe.
Precision electroweak and Higgs physics
The MIT group is also actively involved in studies of the electroweak and Higgs physics program at FCC-ee. In the electroweak sector, we contribute to precision studies of the Z boson lineshape, enabling extremely accurate measurements of the Z boson mass and width, as well as investigations of W boson properties. For the Higgs program, our group plays a major role in evaluating the physics potential of FCC-ee through detailed simulations, focusing on precision measurements of key Higgs properties such as its mass and couplings to quarks. The clean environment and large event samples expected at FCC-ee will allow these quantities to be measured with unprecedented precision, providing powerful tests of the Standard Model and sensitivity to possible new physics effects.
Table: Projected precision on Higgs measurements as obtained from FCC-ee simulations.
Source: Contribution to the European Strategy for Particle Physics Update 2025-2026
