Comparative evaluation of future collider options

The universe is built from fundamental particles, and for decades, scientists have used massive machines called particle colliders to smash these particles together at incredible speeds. By studying the debris from these collisions, researchers can uncover the rules that govern matter and energy, searching for answers to questions that have puzzled humanity for centuries. The most famous of these machines is the Large Hadron Collider, located on the border of France and Switzerland, which has already discovered the Higgs boson, a particle that gives mass to other particles. However, as our current machine reaches the limits of its power, the global scientific community is looking ahead to what comes next. The goal is to build a new, even more powerful facility that can peer deeper into the fabric of reality, potentially revealing new physics that our current tools cannot see.

A new report from CERN, the European organization that runs the Large Hadron Collider, offers a clear and detailed comparison of the different paths forward. The authors, a group of experts who have spent years evaluating the options, have gathered data on several proposed future colliders to determine which one offers the best balance of scientific promise, technical feasibility, cost, and environmental impact. Their work is not just a list of ideas but a rigorous assessment designed to help decision-makers choose the next major project for European particle physics. The report considers machines that could start operating in the mid-2040s, as well as more ambitious concepts that might come online later in the century. It weighs the pros and cons of circular machines, which steer particles around a giant loop, against linear machines, which shoot particles in a straight line, and even explores exotic concepts involving muons, a heavier cousin of the electron.

The report begins by focusing on the most immediate priority: an electron-positron "Higgs factory." This type of machine is designed to produce vast numbers of Higgs bosons so scientists can study them with extreme precision. The leading candidate for this role is a circular collider called the Future Circular Collider, or FCC-ee. This machine would be housed in a new tunnel nearly 100 kilometers long, circling beneath the French and Swiss countryside. The FCC-ee is designed to collide particles at different energy levels, allowing it to study the Higgs boson and other particles with unparalleled accuracy. The report highlights that this circular design is highly efficient; because the particles travel in a loop, they can be reused many times, generating a huge amount of data while consuming less electricity than a straight-line machine of similar power. The study estimates that building this machine would require a significant investment, with the largest portion of the cost going toward the massive civil engineering work needed to dig the tunnel and build the surface facilities. However, the team notes that the design is mature, and the technology required to build it is largely available or close to being ready.

Another option for a Higgs factory is a linear collider, which would shoot electrons and positrons in a straight line before they collide. Two specific proposals are examined: the Compact Linear Collider, or CLIC, and a Linear Collider Facility, or LCF, which would be built at CERN based on designs from a project in Japan. Unlike the circular machine, a linear collider fires particles only once, which means it needs to be much longer to achieve the same energy levels. The report finds that while these machines can produce polarized beams, a feature that helps scientists understand the properties of particles, they are generally less energy-efficient than their circular counterparts for the lower energies needed for a Higgs factory. The CLIC proposal uses a unique method where a second beam of particles provides the power to accelerate the main beam, while the LCF relies on superconducting technology that has already been proven in other large projects. The authors conclude that while these linear options are technically sound, the circular FCC-ee offers a better path for the immediate future due to its higher data output and lower energy consumption per unit of information gathered.

Looking further into the future, beyond the 2050s, the report evaluates the possibility of building a hadron collider, a machine that smashes protons together at energies far higher than anything currently possible. The FCC-hh is the primary candidate for this role. It would be installed in the same 100-kilometer tunnel as the FCC-ee, but after the electron-positron machine has finished its work, the tunnel would be repurposed to house a proton collider. This machine would use powerful magnets to steer protons at energies of up to 85 tera-electronvolts, a scale that could allow scientists to discover entirely new particles or forces. The report indicates that the technology for the magnets needed for this machine is advancing rapidly, though it will require significant research and development to reach the necessary performance levels. The timeline for this project is long, with construction likely to begin in the 2060s and operation starting in the 2070s, but it represents the ultimate goal of pushing the energy frontier to its limits.

The study also considers more speculative ideas, such as a muon collider. Muons are unstable particles that decay quickly, making them difficult to use in a collider, but they offer the potential to reach extremely high energies in a much smaller space than a proton collider. The report is clear that while the physics potential of a muon collider is exciting, the technology is not yet ready. Critical challenges, such as cooling the muon beams to a tight focus and managing the intense radiation produced by their decay, have not been solved. The authors state that a muon collider cannot be considered a realistic option for the next major facility because it would require a decade or more of dedicated research just to prove that it can be built and operated safely. Similarly, the report examines the possibility of reusing the existing Large Hadron Collider tunnel for a new machine. While this might seem like a cost-saving measure, the study finds that the space inside the current tunnel is too small to accommodate the advanced magnets needed for a high-energy machine, and any such project would likely delay the construction of a truly next-generation facility by decades.

Environmental sustainability is a major theme throughout the evaluation. The authors calculate the carbon footprint of building and operating these massive machines, taking into account the energy required to dig the tunnels, manufacture the components, and run the experiments. They find that the construction phase, particularly the excavation of tunnels and the production of concrete, accounts for the majority of the greenhouse gas emissions. However, the report also notes that the electricity used to run these machines in the future will likely come from cleaner sources, as the energy grid in Europe is expected to become much greener over the coming decades. The circular collider, with its ability to reuse particles and generate less waste heat per unit of data, emerges as the most environmentally friendly option for the near future. The team also explores ways to reduce the environmental impact, such as reusing the heat generated by the machines to warm nearby buildings, a strategy that has already been tested successfully at CERN.

In terms of cost and resources, the report provides a detailed breakdown of what each project would require. The circular collider is estimated to cost around 15 billion Swiss francs for the initial phase, a figure that includes the tunnel, the accelerator, and the detectors. The linear colliders are projected to be somewhat cheaper to build initially, but they may require more expensive upgrades later to reach higher energies. The human resources needed to construct these machines are also substantial, with estimates ranging from 10,000 to 15,000 full-time workers over the course of the construction period. The authors emphasize that the decision on which path to take must be made soon, with key choices expected between 2028 and 2030, to ensure that the new machine can be ready to operate as soon as the current Large Hadron Collider finishes its run in 2041.

Ultimately, the report concludes that the Future Circular Collider, starting with its electron-positron phase and evolving into a proton collider, offers the most robust and realistic path forward for European particle physics. It balances the need for high-precision measurements of the Higgs boson with the long-term ambition of exploring the highest energy frontiers. While other options like linear colliders or muon machines have their own merits, the report suggests they are either less efficient for the immediate goals or too far from technical maturity to be the primary focus. The study serves as a comprehensive guide, laying out the facts, the challenges, and the opportunities, so that the scientific community and its partners can make an informed decision about how to continue the quest to understand the universe.