The FCC-ee is designed to operate with exceptionally strong beam–beam interactions, making continuous injection a critical and non-trivial aspect of its operation. During the injection process, an unavoidable charge imbalance between the two colliding beams leads to asymmetric beam–beam forces, potentially compromising transverse stability. In this paper, we introduce a quasi-strong-strong (QSS) beam–beam scheme, implemented in the SAD simulation framework. The method preserves a self-consistent beam–beam lens by coupling paired weak–strong simulations, while avoiding the computational cost of full strong–strong tracking. The injection process is modeled as a gradual increase of the stored bunch population, allowing the isolated study of beam–beam–driven optics deformation under charge imbalance. Using the QSS approach, we investigate the feasibility of bootstrapping injection in the Z, W, and H operating modes of FCC-ee. Stable injection paths up to the nominal bunch population are identified in the W and H modes. In contrast, in the explored parameter region, the Z mode exhibits saturation of the stored population below the nominal value.

Luminosity leveling to limit the event pile up is a key ingredient of the LHC luminosity upgrade, the High-Luminosity LHC (HL-LHC). For a future circular hadron collider, such as the FCC-hh, operating at a centre-of-mass energy of 70-90 TeV, synchrotron radiation becomes significant, with radiation damping times of the order of one or a few hours. The rapid shrinkage of the emittance may call for a leveling of the beam-beam tune shift or of the event pile up, as previously explored. However, the strong synchrotron radiation emitted inside the cold superconducting magnets also represents a significant heat load and is likely to limit the total beam current. In this article, we discuss a new approach, namely synchrotron radiation power leveling, where the beam energy is adjusted during a physics store, either continually or in a few discrete steps, while the beam current decreases, so as to keep the synchrotron radiation power at or below a certain limiting value. In this way, both peak and integrated luminosity of the FCC-hh are increased, compared with operation at a fixed beam energy. The FCC-hh detectors, and in particular the physics event analysis, need to be prepared for this novel mode of operation. This article presents two example running scenarios for synchrotron radiation leveling at the FCC-hh. While not greatly reducing the integrated luminosity at highest collision energy, synchrotron-radiation leveling can significantly increase the number of events for key processes already occurring at lower energy. As an example, we show that it raises the number of di-Higgs production events by 60

Charged-particle colliders have proven key instruments of discovery in high-energy physics. Pushing the frontiers of our knowledge ever further has relied on, and still keeps calling for, ever better performance and novel techniques. During more than four decades, Swapan Chattopadhyay has made numerous essential contributions to this endeavour. Often far ahead of his time, he helped advance many areas of collider development, notably in the domains of stochastic cooling, the development of asymmetric B factories, the design of next- and next-next-generation of high-energy colliders, and the harnessing of energy-recovery for particle colliders.

To maintain optimal beam collision conditions and luminosity performance, SuperKEKB requires a fast orbit feedback dedicated to correcting offsets at the interaction point (IP). The 'iBump' feedback system calculates IP beam offset from Beam Position Monitor (BPM) measurements before and after collision and corrects by creating closed orbit bumps in the High Energy Ring (HER). This system has demonstrated robustness at stabilising IP offsets during operation. In this paper, we discuss operational aspects of the system and ongoing challenges, with a focus on the identification of vertical offset as the correction target of the iBump system. Dedicated studies on the current dependence of this feedback target as well as historical data are analysed.

To achieve physics performance at the Future Circular electron-positron Collider (FCC-ee), luminosity and beam lifetime must be maintained at close to design specifications. Alongside global feedbacks, a fast feedback system is proposed to mitigate beam offset errors at the interaction points (IP), caused by magnet vibrations or other time-varying errors. In this paper, the FCC-ee luminosity performance is simulated for beam-beam interactions including beam offsets, providing performance requirements for the fast feedback system.

The FCC-ee offers the potential to measure the electron Yukawa coupling via direct s-channel Higgs production, e+e- -> H, at a centre-of-mass (CM) energy of -125 GeV. This measurement is significantly facilitated if the CM energy spread of e+e- collisions can be reduced to a level comparable to the natural width of the Higgs boson, Gamma H = 4.1 MeV, without substantial loss in luminosity. Achieving this reduction in collision-energy spread is possible through the "monochromatization" concept. The basic idea is to create opposite correlations between spatial position and energy deviation within the colliding beams, which can be accomplished in beam optics by introducing a nonzero dispersion function with opposite signs for the two beams at the interaction point. Since the first proposal in 2016, the implementation of monochromatization at the FCC-ee has been continuously improved, starting from preliminary parametric studies. In this paper, we present a detailed study of the interaction region optics design for this newly proposed collision mode, exploring different potential configurations and their implementation in the FCC-ee global lattice, along with beam dynamics simulations and performance evaluations including the impact of "beamstrahlung."

SuperKEKB, located at KEK, is a second generation B-factory, providing beam to the Belle-II experiment. Optics design and simulation of SuperKEKB were previously performed using the optics code SAD, developed at KEK. In this paper, we present a new model of SuperKEKB using the tracking code Xsuite, developed at CERN. An alternative strategy for modelling the interaction region, with controllable final focus quadrupoles, has been adopted. Optics comparisons between the new Xsuite model and existing SAD model, as well as tracking simulations including beam-beam modelling are presented.

The proposed Future Circular Collider (FCC) integrated programme consists of two stages: an electron–positron collider serving as a Higgs-boson, electroweak and top-quark factory,followed by a proton–proton collider operating at a collision energy around 100 TeV. In 2021, in response to the 2020 update of the European Strategy for Particle Physics, the CERN Council initiated the FCC Feasibility Study. This study covered, inter alia, physics objectives and potential, geology, civil engineering, technical infrastructure, territorial implementation, environmental aspects, R&D needs for the accelerators and detectors, socio-economic benefits, and cost. The FCC Feasibility Study was completed on 31 March 2025. We present a few key results along with accelerator R&D goals and discuss the next steps.

Determining Yukawa couplings of the Higgs boson is one of the most fundamental and outstanding measurements since its discovery. The FCC-ee, owing to its exceptionally high-integrated luminosity, offers the unique opportunity to measure the electron Yukawa coupling through s-channel Higgs production at 125 GeV centre-of-mass (CM) energy, provided that the CM energy spread can be reduced from 50 MeV to a level comparable to the Higgs bosons’ natural width of 4.1 MeV. To improve the energy resolution and reach the desired collision energy spread, the concept of a monochromatization mode has been proposed as a new operation mode at the FCC-ee, relying on the Interaction Region (IR) optics design with a nonzero dispersion function of opposite signs at the interaction point (IP). A first optics design and preliminary beam dynamics simulations have been carried out for V22 of the FCC-ee GHC lattice type. In response to the continuously evolving FCC-ee GHC optics, this paper presents the first updated monochromatization IR optics design based on V23 of the FCC-ee GHC optics.

In preparation for the 2026 Update of the European Strategy for Particle Physics, various options are being proposed for a future circular hadron collider, FCC-hh. Here, we discuss a few operational scenarios spanning c.m. energies from about 70-120 TeV, which correspond to the arc dipole field strengths ranging from 12 to 20 T. We present the respective integrated luminosity forecasts, considering a proton beam current similar to the one of the existing LHC (0.5 A) or the upcoming HL-LHC (1.1 A), and limiting the total synchrotron radiation power to at most 5 MW. Additional constraints are imposed on the beam-beam tune shift and the maximum event pile-up.on the maximum event pile up.

This paper presents a comprehensive overview of the Machine Detector Interface (MDI) design developed for the FCC-ee Feasibility Study. It highlights novel studies related to the lightweight interaction region, including a mechanical model of the vacuum chambers, integration of the vertex detector, the MDI alignment system, and assessments of machine-induced backgrounds. The small beam pipe radius and thickness, as well as the high power to be dissipated, require state-of-the-art mechanical design. The integration of all mechanical elements and detectors is challenging, necessitating careful studies to allow fulfilling conflicting requirements. The optimisation of the machine detector interface against formidable backgrounds is presented.

The Future Circular Collider (FCC)-ee is a planned electron-positron collider under development. The future collider would be built in an about 91 km ring-shaped underground tunnel located beneath the French departments of Haute-Savoie and Ain, and the Swiss canton of Geneva. The FCC-ee may face challenges from electron cloud (e-cloud). The strongest effects are foreseen for the Z configuration, due to the highest number of bunches, which corresponds to the smallest bunch spacing, which is a key parameter for the e-cloud formation process. A high electron density in the beam pipe could limit the accelerator’s achievable performance through various mechanisms, such as transverse instabilities, transverse emittance growth, particle losses, vacuum degradation and additional heat loads on the inner surface of the vacuum chambers. In the design phase, the objective is to suppress the e-cloud effects in the FCC-ee. Therefore, effective e-cloud mitigation techniques, to avoid the e-cloud avalanche multiplication and its deleterious effects, are discussed in the paper.

In this study, we explore the application of laser-driven Compton backscattering (CBS) as a method to precisely adjust and regulate the intensity of colliding particle bunches in the Future Circular Collider (FCC-ee). Maintaining a tightly controlled charge balance between collision partner bunches within a 3–5% tolerance is critical for mitigating the impact of beamstrahlung on bunch length and preventing flip-flop instabilities. We present a realistic design for the CBS optical beamline and provide detailed simulation results that demonstrate its performance in the FCC-ee. Our analysis includes the distribution of scattered positrons, illustrating the feasibility of CBS for achieving the stringent intensity control requirements in this next-generation collider.

The Future Circular Collider, FCC-ee, is a proposed next generation electron-positron collider aiming to provide large luminosities at beam energies from 45.6 up to 182.5 GeV. This collider faces a major challenge to deliver the design performance in the presence of realistic lattice errors. A commissioning strategy has been developed including dedicated optics designs, efficient beam-based alignment and optics corrections based on refined optics measurements. First specifications on main magnets, corrector circuits, and instrumentation have also been investigated. A summary of all these aspects is presented in this paper.

SuperKEKB has achieved significantly higher specific luminosity than its predecessor KEKB, and it has proven a much more sustainable machine. It has successfully demonstrated several key design elements of FCC-ee. The design luminosity has not yet been reached, however. This observation is often (mistakenly) used to put into question the reliability of the FCC-ee design luminosity. In this note we review the accomplishments, challenges and obstacles of SuperKEKB, and compare these with the FCC-ee design.

FCC-ee aims at colliding electrons and positrons with vertical rms beam sizes between 30 and 50 nm with beam energies extending from about 40 GeV to 182.5 GeV. In this report, we collect experimental results of different high energy accelerator projects with comparable design vertical beam sizes.

This study explores the feasibility of generating high-energy photons, reaching up to 150 GeV, at the FCC-ee booster through inverse Compton scattering. The proposed scheme utilizes a laser within a Fabry-Perot cavity, enabling high repetition rates while minimizing recoil effects during individual collisions. This approach supports the potential use of the FCC-ee booster as a high-energy light source. The photon spectrum and energy distribution are analyzed, with simulation results presented for electron-laser interactions within the Fabry-Perot cavity.

FCC-ee luminosity optimization relies on measuring realistic signals from Bhabha scattering, beamstrahlung, and radiative Bhabha photons. Initial assessments of beamstrahlung signals examine the change in luminosity, beamstrahlung power and vertex detector hits in response to waist shifts, vertical dispersion and skew coupling at the collision point. These ongoing studies aim to extract IP-aberration-related signals from the energy spectrum, angular distribution, power of beamstrahlung photons, the vertex detector hits and the luminosity. Furthermore, the study integrates all these signals into a machine-learning-based approach for luminosity tuning and optimisation.

The measurement of electron Yukawa coupling ($y_{e}$) via direct *s*-channel Higgs production at $\sim$125 GeV centre-of-mass (CM) energy is significantly facilitated at the FCC-ee, provided that the CM energy spread can be reduced to a level comparable to the natural width of the Higgs boson. This reduction is possible through the “monochromatization” concept, which involves generating opposite correlations between spatial position and energy deviation in the colliding beams. Following initial parametric studies for this collision mode, three different interaction region optics designs, each featuring nonzero horizontal, vertical, or combined dispersion at the interaction point, have been proposed based on the Version 2022 of the FCC-ee Global Hybrid Correction optics. In this paper, we benchmark the upper limits contours on $y_{e}$ with simulated CM energy spread and luminosity using Guinea-Pig, in order to assess, optimize, and compare their physics performances.