The electron-positron Future Circular Collider (FCC-ee) being developed at CERN will generate intense beamstrahlung radiation, thus requiring photon absorbers downstream of the interaction points. This work presents the conceptual design of flowing-liquid-lead absorbers capable of dissipating around 370 kW of photon power, with a mean photon energy of 62 MeV. Two configurations are investigated: an inclined-flow geometry, developed to increase the photon interaction length to maximize absorption, and a compact upstream slope with an additional pool, in which a free-surface lead flow intercepts the power peak before a downstream pool dissipates the remaining load. Photon-matter interactions are modeled using Monte Carlo simulations in fluka, while conjugate heat-transfer and free-surface dynamics are analyzed through computational-fluid-dynamics simulations using ansys fluent. Design refinements are introduced based on the simulated thermal and hydraulic performance and to mitigate secondary effects such as photon backscattering. Both configurations demonstrate stable operation within the 300 kg/s flow limit and maintain liquid-lead and structural temperatures within the operational range of 450–500 ^∘C. The results establish circulating liquid lead as a feasible and thermally robust baseline technology for beamstrahlung absorption in FCC-ee.

Relying on atomic scattering factors from evaluated databases, a new model for the reflectivity of x rays on solid surfaces has been developed for FLUKA v4-6.0. This model accounts for the variation of reflectivity as a function of the photon energy, its incidence angle, and linear polarisation; surface roughness effects are also taken into account. FLUKA reflectivities agree well with those obtained from state-of-the-art codes used for the characterization of optical devices, both for homogeneous solids and for multilayer mirrors. This new capability renders FLUKA a nearly one-stop shop for synchrotron radiation simulations: emission from bending magnets and wigglers, photon transport and interaction, electromagnetic (and hadronic when applicable) shower development in complex geometries, as well as x-ray reflection at designated solid surfaces can now be all accounted for in a single FLUKA run. This streamlined FLUKA simulation workflow greatly simplifies the plethora of simulation tools that Monte Carlo practitioners previously needed to rely on. Two application scenarios of this new reflectivity model are showcased: first, the use of a multilayer mirror to deflect x rays from an optical hutch onto an experimental hall at the MINERVA beamline of the ALBA synchrotron and, second, the assessment of the photon flux near the interaction point at the CERN's Future Circular Collider (in its electron-positron stage) as a result of upstream x-ray reflections.

The future circular electron-positron collider will provide collisions at four interaction points (IPs) along a 91-km ring, with beam energies ranging from 45.6 GeV (Z pole) to 182.5 GeV (tt & strns; threshold). Several processes, induced by both the collisions at the IPs and the circulating beams, contribute to the radiation environment of the experimental insertion regions, exposing the detector, machine equipment, and electronics. Quantifying these radiation fields and their effects is essential to guide design strategies. In this work, the Monte Carlo code FLUKA is employed to simulate the propagation of radiation showers from both IP-driven and beam-driven sources, including radiative Bhabha scattering, incoherent pair creation, beamstrahlung, synchrotron radiation, and beam-gas interactions. Power deposition, equivalent fluences, and total ionizing dose are estimated to assess the exposure of sensitive components and to characterize tunnel radiation levels at the Z-pole and tt & strns;-threshold modes. The results show that incoherent pairs dominate the detector environment, particularly in the tracker, while radiative Bhabha losses in the superconducting final-focus quadrupoles (FFQs) require shielding to ensure safe magnet operation. Tunnel radiation levels exceed the tolerance of standard equipment at both operational modes, being dominated by radiative Bhabha and collimation losses at the Z pole, and by synchrotron radiation at the tt & strns; threshold. While a tungsten layer is shown to effectively protect the FFQs, the high radiation levels elsewhere indicate the need for additional shielding in dipoles and drifts, as well as dedicated absorbers downstream of the IPs and collimators.

Abstract This work presents a proof of concept for MUSIC, a multi-purpose detector conceived for high-precision and ultra-high-energy physics studies in the challenging environment of $$\sqrt{s}=10$$ s = 10 TeV muon–antimuon collisions. The detector features a central tracking system, electromagnetic and hadronic calorimeters, and dedicated muon detectors. This paper outlines the main design elements of each subdetector, with an emphasis on the effects of machine-induced backgrounds and the reconstruction strategies employed for key physics objects. Performance results for electrons, photons, muons, and jets are reported, and studies of jet flavour identification are discussed.

The FCC-ee faces challenges in managing radiation from primary synchrotron photons, which can damage machine components and tunnel equipment due to cumulative exposure. Effective shielding is crucial to reduce equipment failure, prevent performance degradation, and limit reliance on costly radiation-hard materials. The proposed solution involves enclosing photon stoppers with shielding inserts and plates. With 2580 dipoles, each containing 10 photon stoppers, the machine requires shielding for 25800 stoppers. A preliminary lead-based design shows promise in dose reduction, but optimization is needed to control costs, meet integration constraints, and ensure manufacturing feasibility. Current estimates suggest each stopper will require 400 kg of shielding, totaling 10320 tons of lead. Optimization focuses on refining the shielding’s shape, size, and materials, while simplifying fabrication and installation to improve scalability. Goals include detailed cost estimates, spatial assessments, and a design addressing thermal management, mechanical integrity, and structural support, ensuring significant reduction of ionizing dose. This work is vital for proving the FCC’s feasibility.

Previous positron source designs have assumed that the converter target is significantly wider than the secondary beam size. This paper investigates targets with different transverse profiles to enhance positron production. Starting from the concept of wire-targets (James et al., 1991) proposed in 1991 for the former SLC positron source, we introduce conical-shaped targets to improve the yield by reducing positron reabsorption- an issue exacerbated by high-field solenoid lenses typically used for positron capture. Using Geant4 simulations, we propose new conical targets adapted for the future FCC-ee and its positron test facility P3 (PSI Positron Production experiment) at the Paul Scherrer Institute. Results indicate that conical targets can nearly double the positron production at the target and boost FCC-ee's positron yield by approximately 60%. Additionally, we present thermo-mechanical studies for the conical targets under FCC-ee's primary beam power parameters, and outline the mechanical implementation for a proof-of-principle demonstration at the P3 facility.

We review the 2024 Pb-Pb ion run at the Large Hadron Collider (LHC), in terms of the operational experience, the problems encountered and the main results. This run was the second heavy-ion physics period of LHC Run 3 at 6.8 Z TeV. With only 18 days scheduled for physics data-taking, the key objective was to address the problems encountered in the 2023 Pb-Pb run and establish stable and efficient operation. Thanks to several mitigation measures, the 2023 limitations were overcome, significantly improving the machine availability. Together with substantially higher intensity, thanks to the excellent performance of the Pb ion injectors, this paved the way for a record-high performance in terms of average daily integrated luminosity with ion beams at the LHC.

The Muon Collider is one of the most promising future collider facilities with the potential to reach multi-TeV center-of-mass energy and high luminosity. Due to the significant Higgs boson production cross section in muon-antimuon collisions at such high energies, the collider offers an excellent opportunity for in-depth exploration of Higgs boson properties. It holds the capability to significantly advance our understanding of the Higgs sector to a very high level of precision. However, the presence of beam-induced background resulting from the decay of the beam muons poses unique challenges for detector development and event reconstruction. In this paper, the prospects for measuring various Higgs boson properties at a center-of-mass energy of 3 TeV are presented, using a detailed detector simulation in a realistic environment. The study demonstrates the feasibility of achieving high precision results with the current state-of-the-art detector design. In addition, the paper discusses the detector requirements necessary to achieve this level of accuracy.

Muon colliders offer a compelling opportunity to explore the TeV scale and conduct precision tests of the Standard Model, all within a relatively compact geographical footprint. This paper introduces a new detector concept, MAIA (Muon Accelerator Instrumented Apparatus), optimized for √(s)=10 TeV μ^+ μ^- collisions. The detector features an all-silicon tracker immersed in a 5T solenoid field. High-granularity silicon-tungsten and iron-scintillator calorimeters surrounding the solenoid capture high-energy electronic and hadronic showers, respectively, and support particle-flow reconstruction. The outermost subsystem comprises an air-gap muon spectrometer, which contributes to muon identification. The performance of the MAIA detector is evaluated in terms of differential particle reconstruction efficiencies and resolutions. Beam-induced background and incoherent pair production simulations are overlaid to single particle gun samples to assess detector reconstruction capabilities under realistic experimental conditions. Even in the presence of backgrounds, reconstruction efficiencies exceed approximately 95% for energetic tracks, photons, and charged pions in the central region of the detector. This paper outlines promising avenues for future work, including forward region optimization, opportunities for enhanced flavor tagging and boosted object reconstruction, and technological developments needed to achieve the desired detector performance.

Off-momentum losses at the start of the LHC acceleration ramp in proton runs gave rise to multiple beam dumps by exceeding Beam Loss Monitor (BLM) thresholds in the momentum cleaning insertion (IR3). Accurately estimating the power deposition profiles in IR3 is necessary to determine where BLM thresholds can be optimized, thereby reducing unnecessary beam dumps and improving machine availability and performance. Understanding the loss limits in IR3 is crucial for future High-Luminosity LHC (HL-LHC) performance. In this study, we present FLUKA power deposition results and introduce a newly developed simulation model for BLM benchmarking in IR3. We provide a comprehensive overview of the power deposition in magnets and collimators, identifying potential bottlenecks in the system. Our simulations were benchmarked against multiple fills from 2023 and 2024 that led to beam dumps. The obtained results provide a deeper understanding of the IR3 collimation performance in view of HL-LHC operation in IR3.

The Future Circular Collider (FCC-ee) at CERN requires a betatron and momentum collimation system for reducing particle backgrounds in the detectors, and for protecting the machine in case of excessive beam losses. The system is composed of primary and secondary collimators, which will be housed in one of the technical insertions of the 91 km ring. In this paper, we present a first assessment of the beam-induced power deposition in the collimators using FLUKA Monte Carlo simulations. We show that dedicated shower absorbers are needed in the collimation insertion, which intercept secondary particles from the halo collimators and reduce the energy leakage to the environment. A first optimization of the shower absorber configuration is presented, considering different absorber positions and absorber lengths. We demonstrate that the power absorption of the betatron collimation system can be increased from about 50% to over 80% by adding two shower absorbers between primary and secondary collimators.

The use of liquid lead as a target material in particle accelerators is of significant interest due to its high density, high thermal power absorption capacity, and resistance to radiation damage. This makes it particularly well-suited for the high-intensity proton beams being studied for CERN’s Muon Collider proposal, with powers ranging up to 4 MW. To minimize shock propagation and manage the intense thermal and mechanical stresses induced by the high-power proton beam, a free-falling liquid lead curtain is explored as a promising concept. However, the target region requires strong magnetic fields, around 20 T, to re-focus the secondary particles generated at the target, introducing complex magnetohydrodynamic (MHD) effects in the liquid metal flow. These effects, particularly caused by Lorentz forces and MHD losses, present challenges to achieving stable and efficient high-power target systems. This work presents multiphase MHD simulations that reveal flow instabilities and highlight potential concerns within the free-falling curtain concept. The findings provide critical insights into the feasibility of liquid lead targets for high-intensity beams.

The lepton Future Circular Collider (FCC-ee) at CERN provides electron-positron collisions at four interaction points (IPs) along a 91 km ring, with beam energies spanning from 45.6 GeV (Z pole) to 182.5 GeV (ttbar threshold). The radiation showers produced by these collisions can reach sensitive components of the surrounding machine elements, possibly affecting their performance and lifetime. This contribution examines the case of radiative Bhabha scattering, which generates off-momentum beam particles that can be lost downstream. Some losses occur already at the superconducting final focusing quadrupoles (FFQs), where they can cause quenches and degradation of the coil materials. In this work, the Monte Carlo code FLUKA is used to study the impact of radiative Bhabha in the experimental insertion regions of FCC-ee. The radiation load in the FFQs and the radiation levels in the nearby tunnel and machine elements are simulated for the Z-pole and ttbar operational modes. For the FFQs, a tungsten shielding layer with optimized thickness is proposed to mitigate the radiation load in the magnet coils.

Muons offer a unique opportunity to build a compact high-energy electroweak collider at the 10 TeV scale. A Muon Collider enables direct access to the underlying simplicity of the Standard Model and unparalleled reach beyond it. It will be a paradigm-shifting tool for particle physics representing the first collider to combine the high-energy reach of a proton collider and the high precision of an electron-positron collider, yielding a physics potential significantly greater than the sum of its individual parts. A high-energy muon collider is the natural next step in the exploration of fundamental physics after the HL-LHC and a natural complement to a future low-energy Higgs factory. Such a facility would significantly broaden the scope of particle colliders, engaging the many frontiers of the high energy community. The last European Strategy for Particle Physics Update and later the Particle Physics Project Prioritisation Panel in the US requested a study of the muon collider, which is being carried on by the International Muon Collider Collaboration. In this comprehensive document we present the physics case, the state of the work on accelerator design and technology, and propose an R&D project that can make the muon collider a reality.

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.

Interactions of dust grains with the LHC beams cause beam losses that can trigger premature beam aborts or even quenches of superconducting dipoles. While the simulated motion and ionisation of dust grains inside the proton beam are in good agreement with measured beam-loss data, a direct measurement of the dust movement is not available. A novel method was developed that reconstructs the trajectory of a dust grain based on the different beam loss profiles of transversely displaced bunches. A proof-of-principle experiment to validate the method using a thin wire to simulate the dust grain was performed in June 2024 at the LHC. This paper describes the beam experiment, compares the achieved displacements with simulations, and shows the reconstructed trajectories. Finally, it is discussed how the method can be applied for real dust events occurring during LHC operation.

During the 2023 ion run at the LHC, where crystal collimation was regularly adopted for the first time, strong background levels were observed at the Inner Tracking System (ITS) of the ALICE detector. Some of the readout chips became saturated, causing losses of angular acceptance. This background was mitigated using a dispersion knob in the beam optics, letting some residual background remain. Considering that the next upgrade of the ALICE ITS foresees a further reduction of the interaction chamber aperture, understanding the mechanisms leading to this background appears critical to envision appropriated mitigation solutions. Preliminary studies showed that this background was related to losses at the upstream tertiary collimator (TCT), impacted by 207Pb82+ ions issued from beam interaction with the crystals of the primary collimation stage. Based on FLUKA simulations, this paper investigates the propagation of the tertiary collimator showers towards the ALICE cavern.

The FCC-ee (Future Circular Collider) is a high-luminosity lepton collider study at CERN. Strong effects from quantum fluctuations, beamstrahlung, and Bhabha scattering limit the expected lifetime to well below one hour. Top-up injection continuously refills the colliding bunches to maximize the integrated luminosity. The current baseline aims at using conventional on-axis injection and a thin magnetic septum. However, the beam size at higher energy modes and the limited off-energy dynamic aperture at lower energy modes make the on-axis condition challenging to achieve. The conventional scheme also raises machine protection concerns, as the circulating beam is subject to a fast one-turn bump towards the thin septum during the injection process. This contribution presents the status of the top-up injection scheme for every energy mode of the FCC-ee collider. We then discuss the existing challenges and potential variations to the baseline scheme in view of mitigating operational challenges and machine protection risks.

This document is comprised of a collection of consolidated parameters for the key parts of the muon collider. These consolidated parameters follow on from the October 2024 Preliminary Parameters Report. Attention has been given to a high-level consistent set of baseline parameters throughout all systems of the complex, following a 10 TeV center-of-mass design. Additional details of the designs contributing to this baseline design are featured in the appendix. Likewise, explorative variations from this baseline set can be found in the appendix. The data is collected from a collaborative spreadsheet and transferred to overleaf.