Circular Collider Research Articles

Alignment & stability challenges for FCC-ee

In order to achieve its ultra-low vertical emittance (1 pm) and high luminosity (of up to 230 times 10^{34}text{ cm}^{-2}text{ s}^{-1} per collision point), the e+e− Future Circular Collider (FCC-ee) requires a well-informed alignment strategy, powerful correction methods, and good understanding of the impact of vibrations. The large ring size, high natural chromaticity, small beta ^{*}, and the low coupling ratio make the FCC-ee design susceptible to misalignment and field errors, which if not properly addressed, threaten to increase the horizontal and vertical emittances and adversely affect the luminosity. Tight alignment tolerances around the 100 km ring would be a major cost driver and therefore alignment and stability need to be carefully studied. In this paper we present a status update, in which we apply analytical estimate methods, verified with simulation data, to determine the influence of the alignment of specific magnet types with the result informing the relative alignment tolerances. This is followed by simulations of a correction strategy that includes a large set of magnet misalignments and field errors. Finally, we also consider the tolerances on vibrations of quadrupoles through evaluating three cases: coherent vibration due to external seismic motion, vibrations resonant with the betatron frequency, and non-resonant, incoherent vibration.

Collinear approach for top quark production at ep colliders

An approximately analysis study of the collinear approach for $t\overline{t}$ production at the Future Circular Collider hadron-electron (FCC-he) and the Large Hadron electron Collider (LHeC) is performed. To study heavy quark production processes, the collinear generalized double asymptotic scaling (DAS) approach in a wide rang of $Q^{2}$ is used. The reduced cross sections for top quark pair production at the FCC-he (for $Q^{2}{\geq}m^{2}_{t}$) and LHeC (for $Q^{2}{\leq}m^{2}_{t}$) center-of-mass energies at the high inelasticity are derived. These results may be considering for both the future collider experiments at center-of-mass energy frontier and the improvement of the phenomenological models on kinematics of low values of the Bjorken variable $x$. The evolution of the gluon density for heavy-quark production in DIS is obtained at low $x$ to include effects of heavy-quark masses. A nonlinear modification of these results shown that the nonlinear corrections, with respect to the top quark mass, are very small for $Q^{2}{\propto}m^{2}_{t}$.

Development of High Strength and High RRR Aluminum-Stabilizer for Superconducting Cable in CEPC Detector Magnet

For the development of a large-aperture superconducting detector magnet in circular electron-positron collider (CEPC), super-conducting cables consisting of Nb-Ti/Cu wires and aluminum stabilizer are essential in both handling large electromagnetic forces during normal operation and redistributing the operation current of tens of kiloampere in case of a quench. High-purity aluminum stabilizer offers extremely high residual resistivity ratio (RRR) for quench protections, however, is too ductile to withstand huge Lorentz forces. Doping technology have been studied in detail in this paper to improve the yield strength of 4N8-aluminium (99.998%) while minimizing the reduction of RRR. All aluminum-alloy samples are characterized by a GM cryocooler for the measurement of RRR and by the standard tensile test facility for the measurement of 0.2% yield strength. It is found that by doping in Ni-0.025% Be-0.025%, annealing at 430 °C, cold-working hardening (∼21%), and curing at 130 °C for 15 h, the resultant aluminum alloy obtains an <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p0.2</sub> of 75 MPa at room temperature and a RRR of 417, meeting the basic design requirements of the superconducting detector magnet for CEPC.

Probing a light dark sector at future lepton colliders via invisible decays of the SM-like and dark Higgs bosons

A renormalizable UV model for axionlike particles or hidden photons, which may explain the dark matter, usually involves a dark Higgs field, which is a singlet under the standard model (SM) gauge group. The dark sector can couple to the SM particles via the portal coupling between the SM-like Higgs and dark Higgs fields. Through this coupling, the dark sector particles can be produced in either the early Universe or the collider experiments. Interestingly, not only the SM-like Higgs boson can decay into the light dark bosons, but also a light dark Higgs boson may be produced and decay into the dark bosons in a collider. In this paper, we perform the first collider search for invisible decays by taking both the Higgs bosons into account. We use a multivariate technique to best discriminate the signal from the background. We find that a large parameter region can be probed at the International Linear Collider operating at the center-of-mass energy of 250 GeV. In particular, even when the SM-like Higgs invisible decay is a few orders of magnitude below the planned sensitivity reaches of the International Linear Collider and the high luminosity LHC, the scenario can be probed by the invisible decay of the dark Higgs boson produced via a similar diagram. Measuring the dark Higgs boson decay into the dark sector will be a smoking gun signal of the light dark sector. A similar search of the dark sector would be expected in, e.g., the Cool Copper Collider, the Circular Electron Positron Collider, the Compact Linear Collider, and the Future Circular electron-positron Collider.

Associated $$\phi $$ and $$J/\Psi $$ photoproduction in ultraperipheral PbPb collisions at the Large Hadron Collider and Future Circular Collider

In this paper we analyze the associated $\phi$ and $J/\Psi$ photoproduction in ultraperipheral $PbPb$ collisions through the double scattering mechanism for the energies of the Large Hadron Collider (LHC) and Future Circular Collider (FCC). Our results complement a previous analysis for the $\rho\rho$, $J/\Psi J/\Psi$ and $\rho J/\Psi$ production. We present our predictions for the total cross sections and rapidity distributions considering the rapidity ranges covered by the ALICE and LHCb detectors, which indicate that a future experimental analysis of $\phi J/\Psi$ final state is feasible. These results point out that the study of the double vector meson photoproduction in heavy ion collisions can be useful to constrain the double scattering mechanism and improve our understanding of the QCD dynamics at high energies.

Field quality and surface resistance studies of a superconducting REBa2Cu3O7−x —Cu hybrid coating for the FCC beam screen

To ensure beam stability in the future circular collider study of the European Organization for Nuclear Research, RE(=Y, Gd, Eu)Ba2Cu3O 7−x high-temperature superconductor was proposed as a low-surface impedance coating for its beam screen. Unfortunately, persistent currents in the superconductor will degrade the magnetic field homogeneity inside the beam chamber, endangering the stability of the beam trajectory. To counteract this effect, we have explored the possibility of using a highly conductive hybrid coating made of Cu and REBa2Cu3O 7−x . This decreases the surface impedance when compared to that of pure copper, while maintaining high magnetic field quality inside the beam screen chamber. This work formulates guidelines for hybrid coating geometries to comply with the field quality criterion of a circular accelerator during operation with dipole magnets by means of finite elements numerical analysis. We produced hybrid coating samples with compliant geometries via photolithography. Scanning Hall microscopy and radio-frequency characterization have given the first experimental confirmation that these hybrid coatings offer high field quality and present a surface resistance lower than that of copper for the beam screen of the future circular hadron collider. The excellent agreement shown between experimental results and simulations validates that the numerical analysis performed throughout this work can be used as a prediction tool for future proposed geometries.

APC Nb3Sn superconductors based on internal oxidation of Nb–Ta–Hf alloys

In the last few years, a new type of Nb3Sn superconducting composite, containing a high density of artificial pinning centers (APC) generated via an internal oxidation approach, has demonstrated a significantly superior performance relative to present, state-of-the-art commercial Nb3Sn conductors. This was achieved via the internal oxidation of Nb-4at.%Ta-1at.%Zr alloy. On the other hand, our recent studies have shown that internal oxidation of Nb–Ta–Hf alloys can also lead to dramatic improvements in Nb3Sn performance. In this work we follow up on this latter approach, fabricating a 61-stack APC wire based on the internal oxidation of Nb-4at.%Ta-1at.%Hf alloy, and compare its critical current density (J c) and irreversibility field with APC wires made using Nb-4at.%Ta-1at.%Zr. A second goal of this work was to improve the filamentary design of APC wires in order to improve their wire quality and electromagnetic stability. Our new modifications have led to significantly improved residual resistivity ratio and stability in the conductors, while still keeping non-Cu J c at or above the conductor J c specification required by the proposed Future Circular Collider. Further improvement via optimization of the wire recipe and design is ongoing. Finally, additional work needed to make APC conductors ready for applications in magnets is discussed.

Fast transient infrared detection for time-domain astronomy

Multi-messenger astronomy requires the use and development of telescopes and detectors for temporal observations across all the electromagnetic spectrum. The interest of our team is towards time-domain (or longitudinal, in the jargon of storage ring diagnostics) detectors able to make observations over the time rather than transversely by making photographs or spectrographs. The FAIRTEL (Fast InfraRed TELescope) experiment wants to build a very fast detector in the Mid InfraRed (MIR) to be used in time-domain astronomy. The experiment is oriented mainly toward the search of fast astronomical transients in the Galaxy, similarly to what has been observed in different electromagnetic ranges with the discovery of the FRBs (Fast Radio Burst) and the GRBs (Gamma-Ray Burst). In the last years, time-domain astronomy is growing in interest and intends to study events on time scales that can go from hundreds of milliseconds to microseconds and even shorter. The detector proposed by the FAIRTEL experiment, is based on HgCdTe semiconductors and on the experience done in the diagnostics for storage ring developed at DAFNE, the Italian e+/e− circular collider. The detection system design is in progress at the LNF (Laboratori Nazionali di Frascati) of INFN (Istituto Nazionale di Fisica Nucleare) while the telescope will be a standard reflecting type, like a Newtonian, a Cassegrain or a Ritchey–Chrétien telescope. The detection apparatus is under test at the IR synchrotron beamline SINBAD at DAFNE and it is foreseen to be implemented at the OPC (Osservatorio Polifunzionale del Chianti). It should be able to observe events with transients down to the nanosecond. A further development of the project involves the extension from ground-based observations to those by using balloons for scientific studies.

Requirement analysis for dE/dx measurement and PID performance at the CEPC baseline detector

The Circular Electron-Positron Collider (CEPC) can be operated not only as a Higgs factory but also as a Z-boson factory, offering great opportunities for flavor physics studies where Particle Identification (PID) is critical. The baseline detector of the CEPC could record TOF and dE/dx information that can be used to distinguish particles of different species. We quantify the physics requirements and detector performance using physics benchmark analyzes with full simulation. We conclude that at the benchmark TOF performance of 50ps, the dE/dx resolution should be better than 3% for incident particles in the barrel region with a relevant momentum larger than 2GeV/c. This performance leads to an efficiency/purity for K± identification of 97%/96%, for D0→π+K− reconstruction of 68.19%/89.05%, and for ϕ→K+K− reconstruction of 82.26%/77.70%, providing solid support for relevant CEPC flavor physics measurements.

Precise simulation of drift chamber in the CEPC experiment

The Circular Electron Positron Collider (CEPC) is one of the future experiments which aims to precisely study the properties of the Higgs boson and to search for new physics beyond the Standard Model. To achieve that, it puts strict requirements on the particle reconstruction and identification ability of the CEPC detector. In order to obtain better particle identification performance, the drift chamber is chosen for the outer tracking detector which can provide information used by the cluster counting method for getting excellent particle identification performance. To evaluate the performance of the cluster counting method, it is necessary to develop a simulation tool that can be used to precisely simulate the response of the gaseous particle detector. This work proposes a new way to combine Geant4 and Garfield++ at the Geant4 step level for the drift chamber simulation. Due to the extreme time consumption of simulating the avalanche process in Garfield++, a fast signal response simulation method based on a neural network has been developed to replace detailed simulation. It shows the fast simulation can give consistent results with Garfield++ simulation and achieve the speedup by a factor of 200.

Probing the h cbar{c} coupling at a Future Circular Collider in the electron-hadron mode

We study the production of a neutral Higgs boson at a Future Circular Collider in the electron-hadron mode (FCC-eh) through the leading process e^- p rightarrow nu _e h q assuming the decay channel h rightarrow c bar{c} , where h is the Standard Model (SM)-like state discovered at the Large Hadron Collider (LHC). This process is studied in the context of a 2-Higgs Doublet Model Type III (2HDM-III) embedding a four-zero texture in the Yukawa matrices and a general Higgs potential, where both Higgs doublets are coupled with up- and down-type fermions. Flavour Changing Neutral Currents (FCNCs) are well controlled by this approach through the adoption of a suitable texture once flavour physics constraints are taken in account. Considering the parameter space where the signal is enhanced and in agreement with both experimental data and theoretical conditions, we analyse the aforementioned signal by taking into account the most important SM backgrounds, separating c-jets from light-flavour and gluon ones as well as b-jets by means of efficient flavour tagging. We find that the h c bar{c} coupling strength can be accessed with good significance after a luminosity of 1 ab^{-1} for a 50 TeV proton beam and a 60 GeV electron one, the latter with a 80% (longitudinal) polarisation.

Top cross section in the LHeC and FCC-he energy range

Predictions of the top-pair production cross section in electron-proton (ep) collisions at the future circular collider hadron-electron (FCC-he) and the large hadron electron collider (LHeC) in the leading order (LO) and the next-to-leading order (NLO) approximations in perturbative quantum chromodynamics (QCD) are presented. Numerical results for the top structure functions and the ratio of structure functions are computed at the renormalization scale μ2=Q2+4mt2 in the collinear generalized double asymptotic scaling (DAS) approach. These results are presented in terms of the effective parameters of the parameterization of F2(x,Q2), where relying on a Froissart-bounded. These quantitative results can be important for future collider experiments at the center-of-mass energy frontier and the improvement of the phenomenological models for the development of the cosmic ray cascades in ultra-high-energy domain. We obtained the uncertainties because of the power-law behavior x−Δ of the collinear parton distribution functions (PDFs), the factorization, and renormalization scales, on the top properties at high inelasticity for the FCC-he center-of-mass (CM) energy at the NLO approximation.

Beyond DGLAP improved saturation model

We present a modification of the DGLAP improved saturation model with respect to the nonlinear correction (NLC). The GLR-MQ improved saturation model is considered by employing the parametrization of proton structure function due to the Laplace transforms method, which preserves its behavior success in the low and high Q^{2} regions. We show that the geometric scaling holds for the GLR-MQ improved model in a wide kinematic region rQ_{s}. These results are comparable with other models in a wide kinematic region rQ_{s}. The behavior of the dipole cross sections, with respect to the GLR-MQ improved saturation model, are comparable with the Color Glass Condensate (CGC) model. The model describes the dipole cross sections in the inclusive and diffractive processes. We also compare the nonlinear corrections to the impact-parameter dependent saturation (IP-Sat) model with the impact-parameter dependent color glass condensate (b-CGC) dipole model. Finally, we consider the linear and nonlinear corrections to the IP Non-Sat model. These results provide a benchmark for further investigation of QCD at small x in future experiments such as the Large Hadron Collider and Future Circular Collider projects.

Exploring the potential of FCC-hh to search for particles from B mesons

The Future Circular hadron Collider (FCC-hh) is a proposed successor of the Large Hadron Collider (LHC). FCC-hh would push both the energy and intensity frontiers of searches for new physics particles. In particular, due to higher energy and luminosity than at the LHC, at FCC-hh there would be produced around ≃ 30 times larger amount of B mesons and ≃ 120 times of W bosons, which then may decay into feebly interacting particles. In this paper we demonstrate the potential of FCC-hh by studying its sensitivity to heavy neutral leptons (HNLs) with masses mN< mB. We consider various locations of a displaced decay volume embedded in the planned infrastructure of FCC-hh. We demonstrate that FCC-hh may substantially improve the reach of the parameter space of HNLs as compared to the searches proposed at the LHC.

Time of flight detector for charged particle identification based on circular electron-positron collider

The circular electron-positron collider (CEPC) requires a 3% precision in the measurement of d&lt;i&gt;E&lt;/i&gt;/d&lt;i&gt;x&lt;/i&gt; to identify long-lived charged particles. However, the measurement of d&lt;i&gt;E&lt;/i&gt;/d&lt;i&gt;x&lt;/i&gt; has a blind area for each of charged particles of &lt;inline-formula&gt;&lt;tex-math id="M8"&gt;\begin{document}$\pi / \rm{K}$\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20222271_M8.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20222271_M8.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt;, &lt;inline-formula&gt;&lt;tex-math id="M9"&gt;\begin{document}$\pi / \rm{P}$\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20222271_M9.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20222271_M9.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt;, and &lt;inline-formula&gt;&lt;tex-math id="M10"&gt;\begin{document}$\rm{K} / \rm{P}$\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20222271_M10.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20222271_M10.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt;, having transverse momenta of 1 GeV/&lt;i&gt;c&lt;/i&gt;, 1.6 GeV/&lt;i&gt;c&lt;/i&gt;, and 2 GeV/&lt;i&gt;c&lt;/i&gt; respectively. One potential solution is to use a high-precision time-of-flight (TOF) detector with a time resolution of less than 50 ps to fill in the blind area. To address this, we propose a small particle TOF detector that uses small plastic scintillators (&lt;inline-formula&gt;&lt;tex-math id="M11"&gt;\begin{document}$1 \;\; \mathrm{cm} \times 1 \;\; \mathrm{cm} \times 0.3 \;\; \mathrm{cm}$\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20222271_M11.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20222271_M11.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt;) silicon photomultipliers for readout. In this work, we introduce the construction of the detector and calibrate its performance by using &lt;inline-formula&gt;&lt;tex-math id="M12"&gt;\begin{document}${ }^{90} \mathrm{Sr} $\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20222271_M12.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20222271_M12.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt; electron collimators and high-speed waveform acquisition electronics. Using a constant fraction timing method, we find that the time resolution of the detector is about 48 ps, satisfying the CEPC’s requirements for TOF detection.

Accelerated long-term aging study of new antioxidant plastic scintillators

In this paper, the smaller plastic scintillator (polystyrene as solvent) tiles are polymerized and shaped by two methods, i.e. injection molding technology and thermal pressing technology by domestic manufacturer, to be operated for more than 10 years in the hadron calorimeter of the Circular Electron Position Collider (CEPC) or other purposes. Other benefits include, researching aging problems, by adding 0.1% material antioxidants of 2,6-Di-tert-butyl-4-methylphenol (BHT) and without BHT, and according to Van't Hoff rule of equivalent chemical reaction rate of high temperature, the 10 years aging study is simulated in short term. A strobed coincidence setup is arranged to test the light yield of the tile to be measured with silicon photomultipliers (SiPMs) with ADC read out and a simple trigger system with small solid angle acceptance is adopted. The light yields of measured tiles without BHT decrease by about 26.6% (injection molding technology) and 43.3% (thermal polymerization technology) after ten-year equivalent aging time. Meanwhile, the light yields of the tiles with BHT almost did not change after ten-year equivalent aging time.

Status and powering test results of HTS undulator coils at 77 K for compact FEL designs

The production of low emittance positron beams for future linear and circular lepton colliders, like CLIC or FCC-ee, requires high-field damping wigglers. Just as compact free-electron lasers (FELs) require high-field but as well short-period undulators to emit high energetic, coherent photons. Using high-temperature superconductors (HTS) in the form of coated ReBCO tape superconductors allows higher magnetic field amplitudes at 4 K and larger operating margins as compared to low-temperature superconductors, like Nb-Ti.This contribution discusses the development work on superconducting vertical racetrack (VR) undulator coils, wound from coated ReBCO tape superconductors. The presented VR coils were modularly designed with a period length of 13 mm. Powering tests in liquid nitrogen of multiple vertical racetrack coils were performed at CERN. The results from the measurements are presented for three VR coils and compared with electromagnetic simulations.

Particle identification with the cluster counting technique for the IDEA drift chamber

IDEA (Innovative Detector for an Electron–positron Accelerator) is a general-purpose detector concept, designed to study electron–positron collisions in a wide energy range in a very large circular leptonic collider. Its drift chamber is designed to provide an efficient tracking, a high precision momentum measurement and an excellent particle identification by exploiting the application of the cluster counting technique. To investigate the potential of the cluster counting techniques on physics events, a simulation of the ionization cluster generation is needed, therefore we developed an algorithm which can use the energy deposit information provided by the Geant4 toolkit to reproduce, in a fast and convenient way, the cluster number and cluster size distributions. The results obtained confirm that the cluster counting technique allows to reach a resolution two times better than the traditional dE/dx method. A beam test has been performed during November 2021 at CERN on the H8 beam line to validate the simulations results, to define the limiting effects for a fully efficient cluster counting and to count the number of electron clusters released by an ionizing track at a fixed βγ as a function of the track angle. The simulation and the beam test results will be described briefly in this issue.

Design and characterisation of the JadePix-3 CMOS pixel sensor

The JadePix-3 CMOS pixel sensors have been developed and studied with a design optimised for a high position resolution of 3μm, which is one of the key requirements for the physics programmes at the Circular Electron-Positron Collider (CEPC) experiments. The sensors are orientated to the vertex detector for the experiments at the proposed future CEPC. The sensors with high-resistance substrates were produced using Tower Semiconductor’s 180 nm CMOS Imaging Sensor (CIS) process, which feature small sensing diodes, low power analogue front-ends with an in-pixel discriminator, and a rolling shutter readout scheme to minimise the pixel pitch. In the sensors, the pixel matrix of 512 rows × 192 columns is divided into four sectors with varied pixel size (16μm×26μm and 16μm×23.11μm) and circuit design. Electrical pulse tests reveal that for various sectors, the minimum thresholds range from 90 to 140 e, with a noise hit rate below an upper limit of 1 × 10−10/frame/pixel. An infra-red laser beam and a radioactive source of 90Sr have been used to characterise the sensors. The tests show that with the equivalent threshold charge fixed to 220 e−, a position resolution between pitch/212 and pitch/12 can be achieved by tuning the laser power for a signal charge between 440 e− and 880 e−. The measured power consumption is 127 mW for the JadePix-3 sensor and 87 mW/cm 2 when extrapolated to a large sensor of 1 cm × 2.5 cm.

Expected measurement precision of the branching ratio of the Higgs boson decaying to the di-photon at the CEPC* *Supported by the IHEP Innovative Project on Sciences and Technologies (E2545AU210)

This paper presents the prospects of measuring in three Z decay channels using the baseline detector with GeV at the Circular Electron Positron Collider (CEPC). Simulated Monte Carlo events were generated and scaled to an integrated luminosity of 5.6 ab–1 to mimic the data. Extrapolated results to 20 ab–1 are also reported. The expected statistical precision of these measurements after combining three channels of Z boson decay was 7.7%. With some preliminary estimation on the systematical uncertainties, the total precision is 7.9%. The performance of the CEPC electro-magnetic calorimeter (ECAL) was studied by smearing the photon energy resolution in simulated events in the channel. In the present ECAL design, the stochastic term in resolution plays the dominant role in the precision of Higgs measurements in the channel. The impact of the resolution on the measured precision of as well as the optimization of the ECAL constant and stochastic terms were studied for further detector design.