International Journal of Modern Physics A

CEPC has two MDI locations. It is very complex in the mechanics design owing to extremely high precision and stability requirement as well as the limited space. This paper will describe two most complex mechanics issues and the progress. For the vacuum connection between detector and accelerator, two schemes including RVC and inflatable seal have been designed in detail. For the cryostat support system, the preliminary structure design and FEA have been done, and the alignment method is under study.

Since the first idea of a collider was patented by Rolf Wideröe (1902–1996) in 1943 and the first $e^{+}{e}^{-}$ circular collider, AdA, was successfully principle-proved in LAL, Orsay, France in 1963, different kinds of colliders have been built in the past and planned for the future. In this paper, we will make a rather complete and brief review of the history of different colliders and give a perspective for the future.

In this paper, we give preliminary designs of beam polarization manipulations by inserting three different types of insertions in the Circular Electron–Positron Collider (CEPC) at center-of-mass energies of 91 GeV ($Z$-pole). With the wigglers in the collider ring, we can obtain 5% transverse polarization in 1.1 h for the precise energy measurement. To overcome depolarization effects as the beam energy rises from 10 GeV to 45.5 GeV in the booster ring, Siberian snakes based on helical magnets are adopted. Finally, for longitudinally polarized beam collisions, a schematic design of spin rotators based on solenoids in the collider ring is studied.

Radio frequency (RF) parameters design is an important part in RF system design. In this paper we will introduce a general method of choosing appropriate RF parameters for a circular $e^{+}{e}^{-}$ collider. The RF parameters are determined with several analytical formulas. With this method, the RF parameters of the Circular Electron–Positron Collider (CEPC) for Conceptual Design Report (CDR) are verified. A new set of RF parameters for the CEPC with 1-cell LG Nb cavities is also designed. Besides, the RF parameters of the CEPC Advanced Partial Double Ring (APDR) and the CEPC Damping Ring (DR) are designed for the first time, and a preliminary study of the beam loading effects of APDR and DR is also carried out.

In November 2018, CEPC CDR was announced and since then, CEPC accelerator entered TDR phase towards its planned completion at the end of 2022. In this paper, we give status review of CEPC optimization design, hardware TDR R&D progresses, CEPC-SppC compatibility relation, civil engineering design, site selection, CIPC and international collaborations, etc. CEPC plan is also presented.

A damping ring system which includes a small 1.1 GeV ring and two transport lines is introduced in CEPC linac in order to reduce the transverse emittance of positron beam at the end of linac and hence reduce the beam loss in the booster. This paper introduces the parameter choice and optics study of damping ring. The corresponding instability effect and IBS effect are also checked to make sure the design current and design emittance can be realized. Except for damping ring, two transport lines are needed to match the parameters between linac and damping ring. Both designs for energy compressor and bunch compressor including beam simulations are discussed in this paper.

To obtain high luminosity, compact high gradient quadrupole magnets QD0 and QF1 are required on both sides of the interaction points of the proposed Circular Electron Positron Collider (CEPC). QD0 is a double aperture superconducting quadrupole closest to the interaction point with a crossing angle between two aperture centerlines of 33 mrad. Magnetic field crosstalk between two apertures of QD0 is negligible using iron yoke, and the 3D coil end is optimized by ROXIE. In the design study, both NbTi conductor and HTS conductor are taken into account. The first step of the R&D is to design and manufacture a QD0 short model magnet with a magnetic length of 0.5 m. In this paper, the R&D status of QD0 short model magnet is described, and the design study of quadrupole magnet including NbTi technology and HTS Bi-2212 technology is presented.

The CEPC booster will accelerate the $e^{-}{e}^{+}$ beam from 10 GeV to 120 GeV, so the field of the dipole magnets will change with the beam energy, of which the minimal working field is 29 Gs whereas the maximal field is 338 Gs. To reach the requirement of high precision at lower field level, three kinds of new dipole magnets are proposed and studied. To test and verify the designs of the magnets, two subscale prototype magnets are fabricated.

A Circular Electron Positron Collider (CEPC) with a circumference of about 100 km and a beam energy up to 120 GeV is proposed to be constructed in China. Over 80% of the collider ring is covered by conventional magnets. Most dipole and quadrupole magnets are twin aperture magnets with an inter-beam separation of 350 mm. Two 1-meter-long twin aperture prototype magnets are designed and manufactured. One is a combined dipole–sextupole magnet with aluminum bus bar coils, and the other one is a twin aperture quadrupole magnet with a DT4 compensation sheet to reduce the crosstalk effect between the two apertures. The design, fabrication and magnetic measurement of two prototype magnets are described in this paper.

An analytical magnetic field expression for a circular vacuum chamber with induced eddy on it is obtained. Two boundary conditions are discussed: with and without the iron poles; the former implies the use of the image current methods. After the current angular distribution in the vacuum chamber is calculated, a contour integration is applied to obtain the field distribution in the aperture. The application to the CEPC booster is also presented, which is one of the most important issues in the design stage.

The beam power of the CEPC Collider is about 60 MW, so an efficiency of an RF power source is very important for cost of project implementation. The most popular source for an accelerator is a klystron, which has the advantage that it can be operated at high power with a reasonable high efficiency. IHEP is developing 650 MHz klystron with 800 kW CW output power and 80% efficiency. To reach this goal, a couple of klystron prototypes will be manufactured in the near future. The first prototype is completely manufactured by Institute of Electronics (IE) and GLVAC Company and the first step of high-power conditioning and commissioning is also completed in IHEP. The design schemes of high-efficiency klystron are also in progress.

With the discovery of the Higgs boson at around 125 GeV, a circular Higgs factory design with high luminosity ($L\sim 10^{34} {\mathrm{\text{cm}}}^{-2} {\text{s}}^{-1}$) is becoming more popular in the accelerator world. The CEPC project in China is one of them. Machine Detector Interface (MDI) is the key research area in electron–positron colliders, especially in CEPC, since the synchrotron radiation (SR) photons can contribute to the heat load of the beam pipe and radiation dose may damage the components. And the heat load can cause the temperature rise in some part, and if the temperature rise is too high, the beryllium pipe in the interaction region will melt and the superconducting magnet may quench. Thus, the heat load distribution from synchrotron radiation and beam loss in the interaction region are analyzed carefully and results are given in this paper.

The CEPC includes a main ring and an injector. The injector consists of a booster and a linac. In order to meet the requirements of the booster, the baseline design of the linac is a 10 GeV electron and positron linac. Two alternative linac designs have also been introduced in this paper. For the linac baseline design, one-bunch-per-pulse is adopted. A 1.1 GeV damping ring is used to reduce the transverse emittance of positron beam. The main RF system of the linac adopts normal conducting S-band structure. Some key technologies of linac are development. The S-band structure and pulse compressor have been researched and studied. In the damping ring, two cavities used to provide 2 MV voltage. The preliminary cavity design has finished.

The CEPC booster needs to provide electron and positron beams to the collider at different energy with required injection efficiency. At Higgs energy, only the on-axis injection from booster to collider can be fulfilled in CDR. With a consideration of keeping the off-axis injection scheme for safety and reliability, a new booster design based on TME lattice is considered to reduce the emittance by three times after CDR. The new booster design has reached an emittance of 1.3 nm at 120 GeV and the DA without errors is even better than CDR. The geometry of new booster is designed carefully in order to share the same tunnel with collider. The design status of CEPC new booster including parameters, optics, dynamic aperture and geometry is discussed in this paper.

With the development of the circular collider, it is necessary to make accurate physics experimental measurements of particle properties at higher luminosity $Z$ pole. Micro-pattern gaseous detectors (MPGDs), which contain gaseous electron multiplier (GEM) and micro-mesh gaseous structures (Micromegas), have excellent potential for development as the readout devices of the time projection chamber (TPC) tracker detector. To meet the updated physics requirements of the high luminosity $Z$ from the preliminary concept design report (preCDR) to concept design report (CDR) at the circular electron–positron collider (CEPC). In this paper, the space charge distortion of the TPC detector is simulated with the CEPC beam structure. Using the multi-physics simulation software package, the distribution of ion estimated by Geant4 is used as the input for the differential equation, and the relationship between the ion density distribution and electric field in the detector chamber is simulated. These simulation results show that the maximum deviation for Higgs ${𝒪}(25\mu \mathrm{\text{m}})$ meets the performance requirements in CEPC TPC detector at the high luminosity $Z$ pole, while it is still a considerable challenge for $Z$ pole, with the maximum deviation ${𝒪}(>100\mu \mathrm{\text{m}})$. According to the previous developments, the cascaded structure of GEM and Micromegas detector has been measured. The new considerations of the detector’s requirements were given, the gain needs to be reached to about 2000 with IBF$\times$Gain under 0.1, and IBF means the ions back flow ratio of the detector. The pixel TPC is a potential option to replace the traditional MPGDs with the low gain, low occupancy, and outstanding pattern recognition. Finally, some update parameters and experiments results were compared.

We discuss the physics opportunities and challenges presented by high energy lepton colliders in the range of center-of-mass energy between few and several tens of TeV. The focus is on the progress attainable on the study of weak and Higgs interactions in connection with new physics scenarios motivated by the shortcomings of the Standard Model.

Dual-readout calorimetry is now a mature and well-known technology which guarantees excellent electromagnetic and hadronic resolution in the same detector. It has recently being proposed in the framework of IDEA (Innovative Detector for Electron–Positron Accelerators) for both Future Circular Collider (FCC-ee) and Circular Electron–Positron Collider (CEPC).

After being extensively tested on prototypes, the dual-readout calorimetry is now moving toward a technology design study in order to be realistically available for an experiment. In this context, a full simulation of the calorimeter has been developed and used to estimate the expected performance of the detector. At the same time, the development of a novel technique for mass production of the detector modules, at an effective cost, is ongoing. As a first step, an electromagnetic-size prototype is under construction for a testbeam data taking originally foreseen in November 2020 and now moved to spring 2021, due to the Covid-19 pandemic spread.

The CMS collaboration reported an intriguing $\sim 3\sigma$ (local) excess at 96 GeV in the light Higgs-boson search in the diphoton decay mode. This mass coincides with a $\sim 2\sigma$ (local) excess in the $b\bar{b}$ final state at LEP. We briefly review the proposed combined interpretations for the two excesses. In more detail, we review the interpretation of this possible signal as the lightest Higgs boson in the 2 Higgs Doublet Model with an additional real Higgs singlet (N2HDM). We show which channels have the best prospects for the discovery of additional Higgs bosons at the upcoming Run 3 of the LHC.