“Progress in knowledge has no price” with these words, Alain Blondel concluded the FCC Physics workshop at CERN. Over two days, the meeting reviewed the results of the FCC Conceptual Design Report and discussed a reliable implementation plan to meet the ambitious physics targets of this project.

The FCC study proposes a versatile research infrastructure that will expand in all different fronts our exploration of tiniest scales of matter and experimentally test a wide range of theories in searches for new physics. The FCC study envisages an intensity-frontier lepton collider (FCC-ee) as the first step followed by an energy-frontier hadron machine (FCC-hh) while other options include a proton-lepton (FCC-ep) interaction point and experiments at the injectors of FCC. This staged approach can offer a broad physics programme for the next 70 years representing a visionary and technically feasible answer to the challenges and questions posed by the current high-energy physics landscape.

The novel situation in particle physics calls for very broad search machines improving knowledge simultaneously in all three directions of sensitivity, precision and energy. The FCC Conceptual Design Report (CDR) demonstrates how this can be done both in a feasible and sustainable way, by taking advantage of synergies and complementarities between the successive colliders. In his opening talk, Michael Benedikt, the FCC study project leader, reviewed the key findings of the CDR and discussed the physics goals, the technological challenges for both accelerators and detectors and implementation scenarios. Key topics identified in the CDR include civil-engineering considerations about the tunnel and surface building, the FCC conductor development programme, prototyping of high-field magnet and superconducting RF cavities.

Following the Higgs discovery at the LHC physics is at a special moment of its history. On one hand, understanding the properties of the Higgs particle defines a clear goal for future collider physics. In this perspective, nothing about the Higgs boson should be taken for granted, and even seemingly naïve questions like whether the Higgs gives mass to the first and second generation of fermions or simply its total decay width, should be experimentally verified. On the other hand, a number of experimental facts and theoretical questions remain that the now completed Standard Model does not explain, which require the existence of new particles or phenomena. In his talk, Michelangelo Mangano emphasised the importance of studying in depth the dynamics that generate the potential of the Higgs boson and take a close look even at the most basic assumptions about its properties. Moreover, he discussed both the experimental observations, dark matter, baryon asymmetry and neutrino masses as well as the theoretical questions that drive future exploration in particle physics. This is an extraordinary endeavour in which particle physics progresses hand-in-hand with astrophysics, cosmology and many other sciences. Because the energy scale at which these new particles or phenomena will show up is presently largely unknown, more sensitivity, more precision and more energy are called for.

The first step of the FCC, an intensity-frontier lepton collider (FCC-ee) demonstrably has a high level of technological readiness. Construction could start by 2028 delivering the first physics beams in 2038 thus ensuring a smooth continuation at the end of the HL-LHC programme.  This extremely high-luminosity collider (much higher luminosity than the linear machines!) will start its life by  recording in very clean conditions more than 200,000 times the Z boson decay sample of LEP, allowing exploration of rare phenomena (symmetry violations and rare decays) with a huge step in sensitivity. Among the highlights of the FCC-ee program figures the sensitivity to new particles produced in Z decays with couplings as small as 10^-11  those of the Standard Model (heavy neutrinos, axion-like particles etc…). With 10⁸ W’s, millions of Higgs bosons and top quarks following soon after, FCC-ee could perform a most comprehensive set of precision measurements probing the presence of new physics (with SM couplings) up to energies of 70 TeV. Importantly it will uniquely provide a per-mil level model-independent measurement of the Higgs boson coupling to the Z, which will serve as “fixed-candle” for further measurements of the Higgs, thus playing a similar role to LEP revealing the Standard Model’s amazing predictive power that became most useful at the LHC. 

The second step, that could be operational around 2060 is a >100 TeV proton collider, with 7 times the energy of the LHC and 30 times higher luminosity. This energy-frontier collider will explore an uncharted energy regime but also produce copious top, W and Higgs particles allowing to search for a complementary set of rare phenomena. The physics programme of the hadron machine is almost perfectly complementary to that of the lepton collider. One of the highlights of the FCC-hh program will be the ability to precisely (5%) measure the Higgs self-coupling, the key element of electroweak symmetry breaking to elucidate the nature of the electroweak phase transition. Finally, it will allow to test WIMPs as thermal dark matter candidates while also opening a broader perspective for dark sector searches as discussed in depth by Matthew McCullough in his talk. Another highlight of FCC-hh, using the 10 billion Higgs produced, is to be able to search for invisible Higgs decays (excellent dark matter candidates) down to a few in 10,000. Together with a heavy ion operation programme and the possibility of integrating a lepton-hadron interaction point (FCC-eh), it provides the amplest perspectives for research at the energy frontier. A duration of 25 years is projected for the subsequent operation of the FCC-hh facility to complete the currently envisaged physics programme.

The FCC integrated programme provides the most complete and model-independent studies of the Higgs boson. On one hand it will extend the range of measurable Higgs properties, its total width, and its self-coupling, allowing more incisive and model-independent determinations of its couplings. On the other hand, the combination of superior precision and energy reach provides a framework in which indirect and direct probes of new physics complement each other, and cooperate to characterise the nature of possible discoveries. For example with FCC-ee we could measure the Higgs couplings to the Z boson with accuracy better than 0.17%, using which FCC-pp will be able to make a model-independent ttH coupling determination to <1%. Moreover, a combination of FCC-hh and FCC-ee will allow measuring the Higgs self-coupling with prevision better than 5%, much higher than any other proposed collider programme.

Together, FCC-ee, hh and eh can provide detailed measurements on the Higgs properties. The figure shows indicative precision in the determination of couplings to gauge bosons, quarks and leptons, as well as of the Higgs self-coupling, of its total width and of the invisible decay rate.

Patrick Janot investigated the advantage of longitudinally polarised beams in FCC-ee in Higgs couplings precision. Such polarised beams at FCC-ee would give a precision 20% better at the expense of much greater complexity and lower luminosity and thus longitudinal polarisation is not considered a worthwhile option for FCC-ee, which focuses on the high luminosity provided by the circular set-up. For comparison, that one year at the FCC-ee with two interaction points corresponds to eight years of luminosity at the ILC that has polarised beams.  Moreover, the synergies between FCC (ee + hh) will offer unbeatable precision in the study of the Higgs and its interactions with other particles and allow to study in depth the properties of this unique particle.

However, this is only one of the possibilities to answer the most fundamental questions in particle physics. FCC offers a unique programme of precision measurements reducing current experimental limits and searching for deviations from the Standard Model precisions that could point to new physics. The research programme envisioned for FCC will enable searches for tiny violations of the Standard Model symmetries in the Z, W, b, τ and Higgs decays. Moreover, the complementarity of the two machines will allow direct searches for dark matter candidates and heavy neutrinos that can either couple very weakly with the SM particles or have higher masses.

Another important aspect emphasized in a number of talks is the need for further theoretical improvements in the predictions of SM phenomena at levels where higher-order contributions become significant. This stands as a huge challenge for the theoretical community to match the huge step in statistical precision with the FCC. A group of enthusiastic theorists has already started mapping the considerable work ahead.  

The timely implementation of a staged programme can only be ensured with an early start of the project preparatory phase. Planning now for a 70-year long programme may sound a remote goal; However as Blondel observed in the concluding talk of the conference, the first report on LEP dates back to 1976 and certainly the authors couldn’t envision at that moment that 60 years later a High-Luminosity LHC would be using the same tunnel! 

The impressive amount of work documented in the four volumes of the FCC CDR will inform the update of the European Strategy for Particle Physics. The discussions during the two-day workshop demonstrated that the FCC is uniquely placed among other similar designed machines as it provides the broader physics programme to attack the open problems from different fronts. Moreover, CERN’s previous history in the management and completion of large-scale projects and the existing accelerator infrastructure testify to the reliability of the FCC design report. To move forward with a technical design report, additional resources will be needed and a coordinated international effort to advance the enabling technologies and optimize the machine and detector design.

The FCC project is a large and ambitious facility but the accelerators that are being considered through their synergies and complementarities offer an extraordinary tool for investigating the outstanding questions in particle physics. “The FCC offers the broadest capabilities of sensitivity, precision and high energy reach that is proposed today” notes Blondel “it sets ambitious but feasible goals for the global community resembling previous leaps in the long history of our field”.

You can find more information and the full programme of this event here: https://indico.cern.ch/event/789349/timetable/

The FCC Conceptual Design Report (CDR) is available here: fcc-cdr.web.cern.ch