Volume 1 of the FCC Feasibility Report presents an overview of the physics case, experimental programme, and detector concepts for the Future Circular Collider (FCC). This volume outlines how FCC would address some of the most profound open questions in particle physics, from precision studies of the Higgs and EW bosons and of the top quark, to the exploration of physics beyond the Standard Model. The report reviews the experimental opportunities offered by the staged implementation of FCC, beginning with an electron-positron collider (FCC-ee), operating at several centre-of-mass energies, followed by a hadron collider (FCC-hh). Benchmark examples are given of the expected physics performance, in terms of precision and sensitivity to new phenomena, of each collider stage. Detector requirements and conceptual designs for FCC-ee experiments are discussed, as are the specific demands that the physics programme imposes on the accelerator in the domains of the calibration of the collision energy, and the interface region between the accelerator and the detector. The report also highlights advances in detector, software and computing technologies, as well as the theoretical tools/reconstruction techniques that will enable the precision measurements and discovery potential of the FCC experimental programme. The content and structure of this report are guided by the scope and priorities defined in the mandate of the FCC Feasibility Study. It is therefore not intended to serve as an exhaustive review of the full physics potential of FCC. Several topics, already covered in earlier reports such as the FCC CDR, are not reiterated here or are addressed only briefly, in alignment with the study’s focus. This volume reflects the outcome of a global collaborative effort involving hundreds of scientists and institutions, aided by a dedicated community-building coordination, and provides a targeted assessment of the scientific opportunities and experimental foundations of the FCC programme.

Linear accelerators (linacs) use alternating radiofrequency (RF) electromagnetic fields to accelerate charged particles in a straight line. Linacs were invented about 95 years ago and have seen many significant technical innovations since. A wide range of particle beams have been accelerated with linacs including beams of electrons, positrons, protons, antiprotons, and heavy ions. Linac parameter possibilities include pulsed versus continuous wave, low and high beam powers, low and high repetition rates, low transverse emittance beams, short bunches with small energy spreads, and accelerated multiple bunches in a single pulse. The number of linacs around the world has grown tremendously with thousands of linacs in present use, many for medical therapy, in industry, and for research and development in a broad spectrum of scientific fields. Researchers have developed accelerators for scientific tools in their own right, being awarded several Nobel prizes. Moreover, linacs and particle accelerators in general have enabled many discovery level science experiments in related fields, resulting in many Nobel prizes as well.

An inverse Compton scattering source based on the CompactLight injector and capable of producing MeV gamma-rays with a brilliance several orders of magnitude larger than existing sources is proposed. The CompactLight injector can operate at a bunch repetition rate of 1 kHz, with trains of 50 bunches and a bunch spacing of 5 ns, giving a maximum total flux of $8.62 \times {10}^{11}$ photons/s. For a normalised emittance of $0.3$ mm mrad, an average brilliance of $1.85 \times {10}^{14}$ photons/($\mathrm{s}$ ${\mathrm{mm}}^2$ ${\mathrm{mrad}}^2$ 0.1%BW) could be obtained. A 1 kW colliding laser was considered, corresponding to a laser pulse energy of 50 $\mathrm{m}$$\mathrm{J}$. Given the electron beam energy up to 300 MeV provided by the CompactLight photoinjector, a maximum photon energy of 2 MeV is obtained. Simulations of inverse Compton scattering were performed using the RF-Track particle tracking software. Parametric scans were used to derive the electron and laser spot sizes maximising the total flux. The accelerator optic components were also determined from the final focus design, which was optimised for a micrometer-level electron beam size at the interaction point. Given a maximum total flux in the order of ${10}^{12}$ photons/s and a maximum output photon energy in the MeV range, the proposed source could be used for various applications, including X-ray imaging.

The performance of future linear colliders will depend critically on beam-based alignment and feedback systems. In ILC and CLIC it is planned to perform dispersion-free steering in the main linacs. To this end the beams are accelerated with different gradients to evaluate the dispersion. The steering is performed by minimizing the average offset of the different beams in the beam position monitors and, at the same time, the difference between the beam trajectories. The experimental verification of the dispersion-free steering algorithm is essential to prove its effectiveness and to prepare the commissioning of such machines. The algorithm should take an orbit measurement at every cycle (train to train), estimate the correction from this information, and, from the system response matrices, apply the correction. We have successfully tested dispersion-free steering at FACET, including an adaptive system-identification algorithm, where the system response matrix is measured dynamically and automatically.

The CompactLight European consortium is designing a state-of-the-art X-ray free-electron laser driven by radiofrequency X-band technology. Rooted in experimental data on photo-injector performance in the recent literature, this study estimates analytically and numerically the performance of the CompactLight delivery system for bunch charges in the range 75–300 pC. Space-charge forces in the injector, linac transverse wakefield, and coherent synchrotron radiation in bunch compressors are all taken into account. The study confirms efficient lasing in the soft X-rays regime with pulse energies up to hundreds of microjoules at repetition rates as high as 1 kHz.