Positron Collider Research Articles

Research and development of the high precision low field dipole magnet for CEPC Booster

In the Conceptual Design Report (CDR) stage of the Circular Electron and Positron Collider (CEPC), the injection energy of the Booster is 10 GeV and minimal working field of the dipole magnets is only 29 G, the magnets with iron cores could not meet the requirement of the field precision due to the remnant field. In the CEPC Technical Design Report (TDR) stage, the injection energy of the Booster is increased to 30 GeV and the minimal working field of the dipole magnets is raised to 95 G so that the influence of the remnant field becomes relatively small, the iron core dipole magnets can be used for the Booster. In this paper, the technical design of the CEPC Booster dipole magnets is described, the development and test of a full scale prototype pure dipole magnet are presented.

Problems and considerations about the injection philosophy and timing structure for CEPC

This paper describes the injection philosophy and the design of timing and filling scheme for the high luminosity Circular Electron and Positron Collider (CEPC) scheme under different energy modes. It is found that the RF frequency choice in Conceptual Design Report (CDR) cannot meet the injection requirements for the bunch number at the Z pole. A modified scheme was proposed to support the design luminosity, which basically meets our current design requirements and retains more flexibility for a future high-luminosity upgrade.

Mitigation of electron cloud effects in the FCC-ee collider

Electron clouds forming inside the beam vacuum chamber due to photoemission and secondary emission may limit the accelerator performance. Specifically, the electron clouds can blow up the vertical emittance of a positron beam, through a head-tail-type single-bunch instability, if the central electron density exceeds a certain threshold value, that can be estimated analytically. Using the codes PyECLOUD and VSim, we carried out detailed simulations of the electron-cloud build up for the main arcs and the damping ring of the FCC-ee collider, in order to identify the effective photoemission rate and secondary emission yield required for achieving and maintaining the design emittance. To this end, we present the simulated electron density at the centre of the beam pipe for various bunch spacings, secondary emission yields, and photoemission parameters, in the damping ring and in the arcs of the collider positron ring. To gain further insight into the underlying dynamics, the obtained spatial and energy distributions of the cloud electrons are illustrated as a function of time. In addition, we compare results obtained for two different secondary emission models (“Furman–Pivi” and “ECLOUD”), thereby indicating the uncertainty inherent in this type of study, without any prototype vacuum chambers yet available. We also point out a few situations where the two secondary-emission models yield similar density values. Finally, based on our simulation results for two different design variants, we conclude that the new parameter baseline of the FCC-ee will facilitate electron-cloud mitigation.

Beam-induced HOM power in CEPC collider ring cavity

The power loss of cavity high-order modes (HOMs) is a key issue in Circular Electron and Positron Collider (CEPC) RF system design. A large HOM power may cause a quench of SC cavity. The purpose of this article is to study the beam-induced HOM power for CEPC collider ring. The factors influencing the cavity HOM power are also investigated. Starting with the beam filling patterns, the beam spectrums of different beam time structures are deduced. Then, the longitudinal impedance is simulated for CEPC 2-cell 650 MHz cavity. Finally, the cavity HOM power is calculated for CEPC CDR design. The cavity HOM power is 459 W for Higgs, 506 W for W and 1026 W for Z-pole. These values are smaller than the average values. There is no overlap between beam spectral lines and cavity HOM frequency. The filling patterns of CEPC CDR Higgs, W and Z are safe. The dangerous filling patterns can be identified for CEPC Z-pole by scanning different parameters.

Readout Electronics for CEPC Semidigital Hadron Calorimeter Preprototype

Circular Electron and Positron Collider (CEPC) is a large experiment facility proposed by the Chinese particle physics community. One of the running options is being the Higgs factory. The calorimeter is the main part of this experiment to measure the jet energy. Semidigital hadron calorimeter (SDHCAL) is one of the options for the hadron measurement. Gas electron multiplier (GEM) detector with its high-position resolution and flexible configuration is one of the candidates for the active layer of the SDHCAL. The main purpose of this paper is to provide a feasible readout method for the GEM-based SDHCAL. A smallscale prototype is designed and implemented, including front-end board (FEB) and data interface board (DIF). The prototype electronics has been tested. The equivalent rms noise of all channels is below 0.35 fC. The dynamic range is up to 500 fC and the gain variation is less than 1%. The readout electronics is applied on a double-layer GEM detector with 1 cm × 1 cm readout pad. The result indicates that the electronics works well with the detector. The detection efficiency of minimum ionization particle (MIP) is over 95% with a 5-fC threshold.

Cross-wake force and correlated head-tail instability in beam-beam collisions with a large crossing angle

This paper discusses novel coherent beam-beam instability in collisions with a large crossing angle. The instability appears in the correlated head-tail motion of two colliding beams. A cross-wake force, which is localized at the collision point, is introduced to represent the head-tail correlation between colliding beams. A mode-coupling theory based on this localized cross-wake force enables us to explain the correlated heal-tail instability. The use of a collision scheme with a large crossing angle is becoming popular in the design of electron--positron colliders. An example thereof is the SuperKEKB project, in which a collision with a large crossing angle is performed to boost the luminosity to $0.8\ifmmode\times\else\texttimes\fi{}{10}^{36}\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}2}\text{ }{\mathrm{s}}^{\ensuremath{-}1}$. Future circular colliders will also be designed with a large crossing angle. Strong-strong simulations, which have shown the first coherent head-tail instability, can limit the performance of proposed future colliders. The mechanism whereby this instability occurs is mode coupling due to the cross-wake force. This instability may affect all collider designs based on the crab waist scheme.

R&D of the CEPC scintillator-tungsten ECAL

The circular electron and positron collider (CEPC) was proposed as a future Higgs factory. To meet the physics requirements, a particle flow algorithm-oriented calorimeter system with high energy resolution and precise reconstruction is considered. A sampling calorimeter with scintillator-tungsten sandwich structure is selected as one of the electromagnetic calorimeter (ECAL) options due to its good performance and relatively low cost. We present the design, the test and the optimization of the scintillator module read out by silicon photomultiplier (SiPM), including the design and the development of the electronics. To estimate the performance of the scintillator and SiPM module for particles with different energy, the beam test of a mini detector prototype without tungsten shower material was performed at the E3 beams in Institute of High Energy Physics (IHEP). The results are consistent with the expectation. These studies provide a reference and promote the development of particle flow electromagnetic calorimeter for the CEPC.

Simulation of Crab Waist Collisions in DAΦNE with KLOE-2 Interaction Region

After the successful completion of the SIDDHARTA experiment run with crab waist collisions, the electron- positron collider DA{\Phi}NE has started routine operations for the KLOE-2 detector. The new interaction region also exploits the crab waist collision scheme, but features certain complications including the experimental detector solenoid, compensating anti-solenoids, and tilted quadrupole magnets. We have performed simulations of the beam-beam collisions in the collider taking into account the real DA{\Phi}NE nonlinear lattice. In particular, we have evaluated the effect of crab waist sextupoles and beam-beam interactions on the DA{\Phi}NE dynamical aperture and energy acceptance, and estimated the luminosity that can be potentially achieved with and without crab waist sextupoles in the present working conditions. A numerical analysis has been performed in order to propose possible steps for further luminosity increase in DA{\Phi}NE such as a better working point choice, crab sextupole strength optimization, correction of the phase advance between the sextupoles and the interaction region. The proposed change of the e- ring working point was implemented and resulted in a significant performance increase.

Physics Accomplishments and Future Prospects of the BES Experiments at the Beijing Electron--Positron Collider

The cornerstone of the Chinese experimental particle physics program is a series of experiments performed in the τ-charm energy region. China began building e+e− colliders at the Institute for High Energy Physics in Beijing more than three decades ago. Beijing Electron Spectrometer (BES) is the common root name for the particle physics detectors operated at these machines. We summarize the development of the BES program and highlight the physics results across several topical areas.

Physics cross sections and event generation of e+e− annihilations at the CEPC**Supported by CAS/SAFEA International Partnership Program for Creative Research Teams, and funding from CAS and IHEP for the Thousand Talent and Hundred Talent programs, as well as grants from the State Key Laboratory of Nuclear Electronics and Particle Detectors

The cross sections of the Higgs production and the corresponding backgrounds of e+e− annihilations at the CEPC (Circular Electron and Positron Collider) are calculated by a Monte-Carlo method, and the beamstrahlung effect at the CEPC is carefully investigated. The numerical results and the expected number of events for the CEPC are provided.

Light mesons at BESIII

BESIII had accumulated 1.3 × 10 9 J/ψ data samples. Selected results of the light mesons study based on part or full J/ψ samples are presented, including several observed structures around 1.85 GeV/c 2 , partial wave analysis of J/ψ → γηη, measurements of η and η � anomalous decays, and a0(980) in J/ψ → p ¯ pa0(980). Hadron spectroscopy would help us understand the substructure of observed hadrons and verify the theory. Quantum Chromodynamics (QCD) describes the strong interactions of colored quarks and gluons, it predicts the existence of new types of hadrons except the ordinary me- son and baryon in the quark model, such as glueballs, hy- birds and multiquark states. Experimental search of new hadrons and measure the new decay modes of known hadron would provide valuable information for the further understanding the strong interactions. The BESIII detector (1) at BEPC II (the upgraded Beijing Electron and Positron Collider), has collected the world's largest data samples at τ-charm energy region since 2009, including 1.3 billion J/ψ events, 0.5 billion ψ(3686) events, 2.9 fb −1 at the peak of the ψ(3770) reso- nance, and lots of data samples above 4.0 GeV/c 2 , which offers us a unique opportunity to study the hadron spec- troscopy and search for the new hadrons. The article reviews several structures around 1.85 GeV/c 2 found or confirmed by BESIII, the particle wave analysis results of J/ψ → γηη, the study of η and η � anomalous decays, and the analysis of J/ψ → p ¯ pa0(980).

Investigation into electron cloud effects in the International Linear Collider positron damping ring

We report modeling results for electron cloud buildup and instability in the International Linear Collider positron damping ring. Updated optics, wiggler magnets, and vacuum chamber designs have recently been developed for the 5 GeV, 3.2-km racetrack layout. An analysis of the synchrotron radiation profile around the ring has been performed, including the effects of diffuse and specular photon scattering on the interior surfaces of the vacuum chamber. The results provide input to the cloud buildup simulations for the various magnetic field regions of the ring. The modeled cloud densities thus obtained are used in the instability threshold calculations. We conclude that the mitigation techniques employed in this model will suffice to allow operation of the damping ring at the design operational specifications.

Temperature fields across the BES III beam pipe

During the operations of the new generation Beijing Electron and Positron Collider (BEPC II) and the Beijing Spectrometer (BES III), the inner surface of the beam pipe is subjected to high radiation heat loads. Continuous high temperatures or large temperature fluctuations can cause the main drift chamber (MDC) to detect particles abnormally. Hence, a beam pipe with a cooling system was designed, and temperature fields across the beam pipe and the MDC inner cylinder were studied experimentally and by finite-element simulations. When the inlet temperatures of the cooling oil and cooling water for the central and extended beam pipes were set to 291.4K and 291.6K, respectively, results showed that the inner surface temperature of the MDC inner cylinder can be controlled within the range of 292.8–293.9K for various powers of radiation heat. This range satisfies the operating requirements of 293±2K.

DAΦNE operation with electron-cloud-clearing electrodes.

The effects of an electron cloud (e-cloud) on beam dynamics are one of the major factors limiting performances of high intensity positron, proton, and ion storage rings. In the electron-positron collider DAΦNE, namely, a horizontal beam instability due to the electron-cloud effect has been identified as one of the main limitations on the maximum stored positron beam current and as a source of beam quality deterioration. During the last machine shutdown in order to mitigate such instability, special electrodes have been inserted in all dipole and wiggler magnets of the positron ring. It has been the first installation all over the world of this type since long metallic electrodes have been installed in all arcs of the collider positron ring and are currently used during the machine operation in collision. This has allowed a number of unprecedented measurements (e-cloud instabilities growth rate, transverse beam size variation, tune shifts along the bunch train) where the e-cloud contribution is clearly evidenced by turning the electrodes on and off. In this Letter we briefly describe a novel design of the electrodes, while the main focus is on experimental measurements. Here we report all results that clearly indicate the effectiveness of the electrodes for e-cloud suppression.

Demonstration of a High-Field Short-Period Superconducting Helical Undulator Suitable for Future TeV-Scale Linear Collider Positron Sources

The first demonstration of a full-scale working undulator module suitable for future TeV-scale positron-electron linear collider positron sources is presented. Generating sufficient positrons is an important challenge for these colliders, and using polarized e(+) would enhance the machine's capabilities. In an undulator-based source polarized positrons are generated in a metallic target via pair production initiated by circularly polarized photons produced in a helical undulator. We show how the undulator design is developed by considering impedance effects on the electron beam, modeling and constructing short prototypes before the successful fabrication, and testing of a final module.

Design and thermal behavior of vapor cooled current leads for the detector and IR magnets in BEPC II

A superconducting detector magnet and two sets of interaction region (IR) magnets are adopted in the upgrade project of Beijing Electron and Positron Collider (BEPC II). The operating currents of these magnets differ from 3.6 kA to 24 A. Totally 13 pairs of vapor cooled current leads (VCCL) were designed and fabricated for these magnets. After the successful test experiments, all the VCCL have been installed to BEPC II system and operate properly. This paper presents the detail of the design and the experimental behavior of these VCCL.

Performance of the plugged-in 22Na based slow positron beam facility**Supported by NSFC (10475096) and Special Fund for Equipment of CAS (U-37)

The Beijing intense slow positron beam facility is based on the 1.3 GeV linac of Beijing Electron- Positron Collider (BEPC) aiming to produce mono-energetic intense slow positron beam for material science investigation. The plugged-in 22Na based slow positron beam section has been newly constructed to supply continuous beam time for the debugging of positron annihilation measurement stations and improve the Beijing intense slow positron beam time using efficiency. Performance testing result of the plugged-in 22Na based slow positron beam facility are reviewed in this paper, with the measurement of the beam transport efficiency, the view of beam spot, the adjustment of beam position, the measurement of beam intensity and energy spread etc. included.

STRUCTURE DESIGN OF THE BEIJING SPECTROMETER Ⅲ PIPE

The Beijing spectrometer III (BESIII) beam pipe is in the center of the BESIII, which is the detector of the upgrade project of Beijing electron and positron collider (BEPC II). Electrons and positrons collide in the BESIII beam pipe. According to the demands of the BEPC II, a key program of Chinese Academy of Sciences, the BESIII beam pipe is designed based on the finite elements analysis, The BESIII beam pipe is installed in the inner cylinder of the BESIII drift chamber. As a vacuum tube, the BESIII beam pipe is designed as 1 000 mm in length, 63 mm in inner diameter and 114 mm in outer diameter, respectively. The BESIII beam pipe consists of a central beryllium pipe cooled by EDM-1, the oil No.1 for electric discharge machining, and two extended copper pipes cooled by deionized water (DW). The three parts are jointed by vacuum welding. Factors taken into account in the design are as follows. (1) The wall thickness of the central beryllium pipe should be designed as small as possible to reduce the multi-scattering and improve the particle momentum resolution. And the wall thickness of the extended copper pipe should be designed as large as possible to protect the detectors from the backgrounds. (2) The BESIII beam pipe must be sufficiently cooled to avoid the damage and prevents its influence to the BESIII drift chamber (DC) operation. The inner surface temperature of the DC inner cylinder must be maintained at 293 +/- 2 K. (3) The magnetic permeability of the materials used in the BESIII beam pipe must be less than 1.05 H/m to avoid large magnetic field distortions. (4) The static pressure of the vacuum chamber of the BESIII beam pipe must be less than 800 mu Pa. The simulating results show that the designed structure of the BESIII beam pipe satisfies the requirements mentioned above. The structure design scheme is evaluated and adopted by the headquarters of BEPC II.

Numerical studies of International Linear Collider positron target and optical matching device field effects on beam

For an International Linear Collider (ILC) undulator-based positron source target configuration, a strong optical matching device (OMD) field is needed inside the target to increase the positron yield (by more than 40%) [Y. K. Batygin, Proceedings of the 2005 ALCPG and ILC Workshops, Snowmas, CO, 14–27 August 2005 (unpublished)] It is also required that the positron target be constantly rotated to reduce thermal and radiation damages. Eddy currents, produced by an OMD field in turn, interact with the magnetic field and produce a drag (stopping) force. This force not only produces heat in the disk but also creates a dipole deflecting field, which affects the beam. Therefore it is important to simulate such a system in detail to design the motor and cooling system and also a correction magnet system. In order to guide the ILC target design, an exact simulation of the spinning disk in a magnetic field is required. In this paper we present a simulation method implemented using COMSOL and compare it with the experimental results recently obtained at Stanford Linear Accelerator Center and Lawrence Livermore National Laboratory. Good agreement between the simulation and the experiment gives confidence in the validity of the method. We give detailed results on the proposed ILC target system, such as parametric studies for reduction of the power required to keep the target spinning. We present simulation results of the induced deflection field and of the reduction of the OMD field effect.

CPphases in correlated production and decay of neutralinos in the minimal supersymmetric standard model

We investigate the associated production of neutralinos $e^+e^-\to\tilde{\chi}^0_1\tilde{\chi}^0_2$ accompanied by the neutralino leptonic decay $\tilde{\chi}^0_2\to\tilde{\chi}^0_1 \ell^+\ell^-$, taking into account initial beam polarization and production-decay spin correlations in the minimal supersymmetric standard model with general CP phases but without generational mixing in the slepton sector. The stringent constraints from the electron EDM on the CP phases are also included in the discussion. Initial beam polarizations lead to three CP--even distributions and one CP--odd distribution, which can be studied independently of the details of the neutralino decays. We find that the production cross section and the branching fractions of the leptonic neutralino decays are very sensitive to the CP phases. In addition, the production--decay spin correlations lead to several CP--even observables such as lepton invariant mass distribution, and lepton angular distribution, and one interesting T--odd (CP--odd) triple product of the initial electron momentum and two final lepton momenta, the size of which might be large enough to be measured at the high--luminosity future electron--positron collider or can play a complementary role in constraining the CP phases with the EDM constraints.