The Future Circular Collider (FCC) project

The UZH particle physics groups are deeply engaged in both theoretical and experimental physics at the Large Hadron Collider (LHC). Currently, our researchers are actively involved ion theCMS, LHCb, and SND@LHC experiments.  Tith the LHC expected to operate until around 2041, the particle physics community is intensively studying plans for the next particle collider at CERN.

The Future Circular Collider (FCC) is a proposed particle accelerator aiming to advance our understanding of nature. The FCC envisions a new 91 km-long circular collider ring, poised to succeed the LHC as the next multi-generational international facility for particle physics research.

In 2012, the ATLAS and CMS experiments at the LHC discovered the Higgs boson – the quantum excitation of the Higgs field that permeates the universe. Through the interaction of other particles with this field, they get their mass. The Higgs boson was the last missing piece to the Standard Model (SM) of particle physics, that describes all known elementary particles and their interactions, except for gravity. Additionally, the SM does not provide answers to many fundamental questions of nature such as:

• What is dark matter?
• How can the observed matter-antimatter asymmetry be observed?
• Where does the structure of the SM come from?

The Higgs boson is at the centre of most of these open questions in particle physics, as 15 out of the 19 free parameters of the SM are related to it. The study the Higgs boson is of monumental importance to deepen our understanding of the fundamental aspects of our Universe. One of the goals of the FCC project is to produce millions of Higgs events allowing us to study it with superb precision. 

The Future Circular Collider project foresees two stages that span almost the rest of the 21st century. FCC-ee will produce intense collisions of electrons and their positive partners, the positrons. In the second stage, the FCC ring would be equipped with ~16 Tesla strong magnets to collide hadrons with energies of about 100 TeV (FCC-hh), a more than seven-fold increase compared to the LHC. 

The electrons and positrons colliding in FCC-ee are leptons. In contrast to the hadrons that collide at the LHC,  leptons are elementary particles. The sum of the momenta of the particles produced in lepton collisions is, therefore, always equal to the center-of-mass energy (√s) of the collisions, and the kinematics of collision can be fully reconstructed (except for undetectable particles like neutrinos).

As a circular collider, the FCC offers multiple interaction points where experiments can capture collisions. Housing four complementary experiments, the FCC-ee focuses on various aspects of the physics program while cross-validating findings. Due to the recirculation of non-interacting particles from collisions, they can be utilized again in subsequent collisions. Thanks to the recycling of non-interacting particles from collisions, they can be reused in subsequent collisions, resulting in a high event rate (luminosity), especially at lower collision energies (√s). Moreover, the FCC-ee is specifically tailored as a Higgs factory, where electrons and positrons collide at √s = 240 GeV (equivalent to the combined mass of the Z boson and the Higgs boson). Additionally, it serves as a hub for top quark and electroweak boson research, with collisions occurring at 350–365 GeV for the former and at 91 GeV (Z boson mass) and 160 GeV (mass of two W bosons) for the latter.

The FCC-ee is projected to produce approximately two million top quarks, 1.5 million Higgs bosons, and 2.4·10⁸ W bosons at collision energies of 350-365 GeV, 240 GeV, and 160 GeV, respectively. However, the most demanding run occurs at the lowest energy, where around 6·10¹² Z bosons will be collected at the Z pole, produced at √s = 91 GeV. To fully leverage the resulting minimal statistical uncertainties, it's imperative to maintain experimental systematic uncertainties at an O(10^-4) level—a formidable challenge for FCC-ee detectors.

In high-energy particle physics experiments, various detectors measure the type, momentum, and energy of particles produced in collisions. At the core of these experiments lies the vertexing and tracking system, crucial for determining the trajectories of charged particles. This system enables the precise reconstruction of particle interactions and decays, known as vertices. Understanding vertices forms the foundation for measuring particle lifetimes, flavor tagging, and accurately tracing complex decay chains in flavor physics processes. At FCC-ee, the measurement of vertices is entrusted to a specialized detector known as the vertex detector, a focal point of investigation for the FCC group at UZH.

At the forefront of detector research, our focus is exploring Monolithic Active Pixel Sensors (MAPS) for integration into FCC-ee vertex detectors. MAPS technology combines particle signal generation, amplification, and readout on a single silicon die, minimizing power consumption and facilitating the use of lightweight sensors—an essential requirement for FCC-ee. Additionally, we conduct simulations of the current vertex detector design within the IDEA detector concept using DD4hep. This enables precise evaluation of vertex detector performance at FCC-ee. We also delve into novel vertex detector concepts utilizing ultra-light MAPS technology. For more information, see the dedicated detector page.

Our group is dedicated to identifying key measurements achievable at FCC-ee, particularly emphasizing processes where vertexing capabilities are paramount. Furthermore, we have developed a cutting-edge flavor tagging algorithm for FCC-ee deployment, leveraging advanced machine-learning techniques. The DeepJet Transformer, grounded in attention mechanisms, accelerates tagger training time, aiding in future collider detector optimization. Our tagger incorporates various input features, including secondary vertices and vertices of neutral particles (V0s) reconstructed by the vertex detector, enhancing flavor tagging performance, notably in identifying strange jets. Improved strange tagging could unveil its coupling to the Higgs boson at FCC-ee. For further insights, refer to our dedicated physics page. For more information, see our dedicated physics page.

In 2020, the European particle physics community identified the development of an electron-positron Higgs factory as the top priority for the next collider after the LHC (see here). Following up on this, the CERN Council endorsed the FCC Feasibility Study to investigate the viability of the FCC colliders, experiments, and related infrastructure between 2021 and 2025. Subsequently, the CERN Council endorsed the FCC Feasibility Study, tasked with examining the viability of the FCC colliders, experiments, and associated infrastructure from 2021 to 2025.

The FCC community, inclusive of our group at UZH, is actively engaged in evaluating the feasibility of the FCC program, with progress thus far detailed in the mid-term report of the feasibility study.

The Feasibility Study is slated to conclude in 2025, followed by the next iteration of the European Strategy for Particle Physics (ESPP), expected to be finalized by June 2026. This process aims to reach a consensus on future directions, ensuring Europe's continued leadership in scientific research and fostering opportunities for groundbreaking discoveries. Regarding project approval by the CERN Council, current estimates suggest a projected timeframe between 2027 and 2028.