International Journal of Modern Physics A

The parameters of CEPC and FCC-ee are being fixed. Despite the same circumference and energy of the two colliders, their strategies of the target luminosity are different. The key issues for the target luminosity are the beam–beam and electron cloud effects. We present studies on the beam–beam and electron cloud effects for the two colliders and discuss the difference of the strategies of the two colliders.

With the discovery of the Higgs boson at around 125 GeV, a circular Higgs factory design with high luminosity $(L\sim 10^{34}{\mathrm{\text{cm}}}^{-2}{\mathrm{\text{s}}}^{-1})$ 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; it is one of the criteria to measure the accelerator and detector design performance. Because of the limited space available in the designed tunnel, many equipment including magnets, beam diagnostic instruments, masks, vacuum pumps, and components of the detector must coexist in a very small region. In this paper, some important MDI issues will be reported for the Interaction Region (IR) design, e.g. the final doublet quadrupoles physics design parameters, beam-stay-clear region and beam pipe, synchrotron radiation power and critical energy are also calculated.

A conceptual design study of 12-T two-in-one dipole magnets is ongoing with the iron-based superconducting (IBS) technology, as a candidate option for Super Proton Proton Collider (SPPC), which is designed with a circumference of 100 km and a center-of-mass energy of 70 TeV. Compared with Nb3Sn, the IBS design is competitive because of the potential much higher performance and lower cost of IBS conductors in the future. The design study is carried out with an expected Je level of IBS in 10 years. Besides, we also expect the IBS superconductor to have much better mechanical properties compared to stress-sensitive conductors like Nb3Sn and Bi-2212 wires. The 12-T dipole magnet is designed with common-coil configuration. We have optimized the field uniformity with two different layouts of the coil ends, including the soft-way bending and the hard-way bending. We also have explored the influence of a mid-plane gap on the field uniformity. The main parameters, the coil layouts, and the optimization of the field quality are presented.

The possibility to obtain the transversely polarized electron/positron beams at the $2\times 45$ GeV CEPC collider basing on the radiative self-polarization of particles in the main rings is under consideration. In addition, we consider one of the options for obtaining longitudinal polarization of electrons at CEPC.

The Circular Electron–Positron Collider (CEPC) is a 100-km ring $e^{+}{e}^{-}$ collider for a Higgs factory. The injector of CEPC is composed of Linac and Booster. The Linac is a normal conducting S-band Linac with a frequency of 2860 MHz; it provides electron and positron beams at an energy of up to 10 GeV with 100 Hz repetition frequency of 100 Hz. The Linac design and dynamic simulation results are discussed in detail in this paper, including electron bunching system, positron source, electron bypass transport line, damping ring and main Linacs.

CEPC is a 100-km double-ring circular electron–positron collider operating at 90–240 GeV center-of-mass energy of Z-pole, WW-pair production threshold and Higgs resonance. The conceptual design report (CDR) of CEPC has been published as an important step to move the project forward. The superconducting RF (SRF) system is one of the most important and challenging accelerator systems due to the wide range of beam energy and current. In this paper, the layout, parameters and configuration of the superconducting RF system for the CEPC collider ring will be introduced. Issues of beam cavity interactions including transient beam loading and coupled-bunch instabilities of accelerating mode are discussed.

The interaction region is designed to provide strong focusing at the interaction point. A local correction scheme is adopted to get a large momentum acceptance. The interaction region consists of several modular sections. This paper presents the optics design of each section and its optimization. As an example, the optics design for the CEPC single-ring scheme is presented.

The energy sawtooth will be significant in the future circular Higgs factory with beam energy as high as 120 GeV. For the partial double ring scheme, unlike the double ring scheme, the effects of energy sawtooth cannot be corrected by tapering the magnet strength with beam energy along the beamline. In this paper, the energy sawtooth effects in the partial double ring scheme and its mitigation methods are presented. As an example, the study of the energy sawtooth effects of the CEPC advanced partial double ring scheme is presented.

The LEP experiment at CERN provided accurate measurements of the $Z$ neutral gauge boson properties. Although all measurements agree well with the SM predictions, the forward–backward asymmetry of the bottom-quark remains almost $3\sigma$ away from the SM value. We proposed that this anomaly may be explained by the existence of a new $U{(1)}_D$ gauge boson, which couples with opposite charges to the right-handed components of the bottom and charm quarks. Cancellation of gauge anomalies demands the presence of a vector-like singlet charged lepton as well as a neutral Dirac (or Majorana) particle that provides a dark matter candidate. Constraints from precision measurements imply that the mass of the new gauge boson should be around 115 GeV. We discuss the experimental constraints on this scenario, including the existence of a di-jet resonance excess at an invariant mass similar to the mass of this new gauge boson, observed in boosted topologies at the CMS experiment.

We investigate the prospects of searching dark sector models via exotic $Z$-boson decay at future $e^{+}{e}^{-}$ colliders with Giga $Z$ and Tera $Z$ options. Four general categories of dark sector models: Higgs portal dark matter, vector portal dark matter, inelastic dark matter and axion-like particles, are considered. Focusing on channels motivated by the dark sector models, we carry out a model independent study of the sensitivities of $Z$-factories in probing exotic decays. The limits on branching ratios of the exotic $Z$ decay are typically ${𝒪}(10^{-6}-10^{-8.5})$ for the Giga $Z$ and ${𝒪}(10^{-7.5}-10^{-11})$ for the Tera $Z$, and they are compared with the projection for the high luminosity LHC. We demonstrate that future $Z$-factories can provide its unique and leading sensitivity, and highlight the complementarity with other experiments, including the indirect and direct dark matter search limits, and the existing collider limits. Future $Z$ factories will play a leading role to uncover the hidden sector of the universe in the future.

We review the interplay between collider searches for vector-like quarks and dark matter direct detection experiments in composite Higgs models. With focus on a future 100 TeV collider, we also extend previous analyses to improve the reach for heavy quarks in the low mass region.

Studying the properties of the Higgs boson can be an important window to explore the physics beyond the Standard Model (SM). In this work, we present studies on the implications of the Higgs precision measurements at future Higgs Factories. We perform a global fit to various Higgs search channels to obtain the 95% C.L. constraints on the model parameter spaces of two-Higgs Doublet Model (2HDM) and Minimal Supersymmetric Standard Model (MSSM). In the 2HDM, we analyze tree-level effects as well as one-loop contributions from the heavy Higgs bosons. The strong constraints on $cos(\beta -\alpha )$, heavy Higgs masses and their mass splitting are complementary to direct search of the LHC as well as possible future $Z$ pole precision measurements. For the MSSM, we study both the Higgs couplings and mass precisions. The constraints on the CP-odd Higgs mass $m_A$ and stop mass scale $m_{\mathrm{\text{SUSY}}}$ can be complementary to the direct search of HL-LHC. We also compare the sensitivity of various future Higgs factories, namely, Circular Electron Positron Collider (CEPC), Future Circular Collider (FCC)-ee and International Linear Collider (ILC).

The Circular Electron–Positron Collider (CEPC) project aims to build a Circular Electron–Positron Collider capable of precision physics measurements at center-of-mass energies ranging from 90 GeV to 240 GeV. The CEPC will have a total circumference of at least one hundred kilometers and at least two interaction points. In its 10 years operation at 240 GeV, it will collect more than one million Higgs events. CEPC will also run at the $Z$ pole for two years, producing more than 300 billion $Z$ bosons in two years. It will also collect data around the $WW$ threshold for one year, in order to perform the $W$ boson mass measurement with high precision. These datasets will boost the precision of electroweak measurements by orders of magnitude. An overview is presented of the potential of CEPC to advance precision studies of electroweak physics with an emphasis on the opportunities in $W$ and $Z$ physics.

Almost all experimental apparatuses at existing colliders employ large muon systems located after all other subdetectors. Given the large size of most of the experimental detectors, the existing muon system has to cover areas of a few thousand square meters. It can be anticipated that future detectors at future colliders will be even larger in size. Therefore, for a practical reason of cost, the most suited detectors to realize these large muon systems are gas detectors. In particular, in recent years, Micro-Pattern Gas Detectors (MPGDs) have enjoyed very interesting developments, providing several new types of detectors with very good spacial and time resolution, high-rate capability and high radiation tolerance. MPGDs also have the distinct advantage of being, at least for some detectors and some parts of them, mass produceable by industry, since they employ materials and manufacturing procedures that are extensively used for Printed Circuit Boards (PCBs) production. A particularly innovative MPGD, the $\mu$RWell, is described as a possible candidate to build large muon systems for future colliders. The results obtained so far with this new technology are reported.

The Belle II experiment at SuperKEKB $e^{+}/e^{-}$ collider, Japan, is aiming at exploring flavor physics in high intensity frontier. The target instantaneous luminosity of SuperKEKB is at $8\times 10^{35}{\mathrm{\text{s}}}^{-1}{\mathrm{\text{cm}}}^{-2}$, which is 40 times higher than that at KEKB. The renovated Belle II detectors around the interaction point are expected to collect $50{\mathrm{\text{ab}}}^{-1}$ data within 7 years. This gives high data statistics for precision measurements and probing new physics in rare decay studies. The Belle II experiment has successfully recorded the first collision events at SuperKEKB in April 2018, and is ready for physics data taking. This paper will cover the current status of Belle II and highlight its prospects on flavor physics.

The Circular Electron Positron Collider (CEPC) has been proposed as a Higgs/$Z_0$ factory, which would allow precision measurements of the Higgs boson properties, as well as of $W^{\pm }$ and $Z_0$ bosons. The baseline design of CEPC tracking system consists of a vertex detector with three concentric double-sided pixel layers, a high precision (about 100 $\mu$m) large volume time projection chamber (TPC) and a silicon tracker on both barrel and end-cap regions. The tracking system has similar performance requirements to the ILD detector in ILC detectors but without power-pulsing, which leads to significantly additional constrains on detector specifications, especially for the case of machine operating at $Z$-pole energy region with high luminosity. In this paper, we will give an overview of the CEPC TPC detector, the requirements and challenges for the detector with possible technologies. The on-going R&D activities of the TPC detector module and prototype will also be reported.

The Belle II experiment integrates two dedicated particle identification (PID) subdetectors for pion/kaon separation. The Aerogel Ring Image Cherenkov (ARICH) detector is a forward region PID detector based on two layers of aerogel read out by hybrid avalanche photo detectors. The Time of Propagation (TOP) detector is a novel Cherenkov barrel PID system built for the Belle II detector upgrade based on quartz radiator bars read out by Micro-Channel Plate PMTs. The readout electronics of the TOP system are built around a switched capacitor array waveform sampling ASIC operating at 2.7 GSa/s. Acquired waveforms are processed in real time in the front-end electronics, extracting the individual timing of detected photons to better than 100 ps. This contribution presents the concept of the Belle II particle identification systems with a focus on the current status of commissioning, calibration and operation of the Belle II TOP detector.

We have summarized the results about the precision measurements with the di-boson processes $WV$, $Wh$, $Zh$ at the HL-LHC by using the effective field theory approach. We have focused on the semi-leptonically decaying channels and make projections on the reach on the mass scale for different new physics scenarios.

We summarize the fundamental aspects of dual-readout calorimetry, a calorimetric technique able to overcome the non-compensation problem by means of two independent scintillation and Cherenkov light signals detection. The expected ultimate energy resolution for single-hadron detection, together with the excellent particle identification capability, makes a dual-readout fiber calorimeter one of the most promising options for future leptonic colliders. In this paper, we include the main benefits of a new silicon photomultiplier-based readout system that allows to sample showers with an unprecedented spatial resolution.