CEPC顶点探测器风振分析与减振优化

顶点探测器是中国科学院高能所提出的首次应用于环形正负电子对撞机(CEPC)中的重要探测设备,基于顶点探测器探测精度与工作可靠性要求,顶点探测器的最大振幅需小于1µm,不满足要求将导致探测失效。采用了理论与数值模拟相结合的分析方法,利用梁的挠度计算公式分析了影响探测器刚性的主要因素,通过Fluent与瞬态结构耦合模拟计算危险工况下,空气流体激励中断再恢复时结构关键部位的振动响应,并对计算结果使用Origin可视化处理,结果表明流体流速为2 m/s时结构的最大振幅为1.29µm。通过对结构进行尺寸、形状和拓扑优化,并结合敏感性分析法分析优化措施效果,计算数据表明结构的刚度对增加两侧支撑厚度不敏感,增加支架高度对结构的刚度影响较大,形状和材料分布对结构稳定性具有决定性影响。

Simulation and reconstruction of particle trajectories in the CEPC drift chamber

The circular electron-positron collider(CEPC)is designed to precisely measure the properties of the Higgs boson,study electroweak interactions at the Z-boson peak,and search for new physics beyond the Standard Model.As a component of the 4th conceptual CEPC detector,the drift chamber facilitates the measurement of charged particles.This study implemented a Geant4-based simulation and track reconstruction for the drift chamber.For the simulation,detector construction and response were implemented and added to the CEPC simulation chain.The development of track reconstruction involves track finding using the combinatorial Kalman filter method and track fitting using the tool of GenFit.Using the simulated data,the tracking performance was studied.The results showed that both the reconstruction resolution and tracking efficiency satisfied the requirements of the CEPC experiment.

Design and high-power test of 800-kW UHF klystron for CEPC

To reduce the energy demand and operation cost for circular electron positron collider(CEPC), the high efficiency klystrons are being developed at Institute of High Energy Physics, Chinese Academy of Sciences. A 800-k W continuous wave(CW) klystron operating at frequency of 650-MHz has been designed. The results of beam–wave interaction simulation with several different codes are presented. The efficiency is optimized to be 65% with a second harmonic cavity in three-dimensional(3D) particle-in-cell code CST. The effect of cavity frequency error and mismatch load on efficiency of klystron have been investigated. The design and cold test of reentrant cavities are described, which meet the requirements of RF section design. So far, the manufacturing and high-power test of the first klystron prototype have been completed.When the gun operated at DC voltage of 80 k V and current of 15.4 A, the klystron peak power reached 804 k W with output efficiency of about 65.3% at 40% duty cycle. The 1-d B bandwidth is ±0.8 MHZ. Due to the crack of ceramic window, the CW power achieved about 700 kW. The high-power test results are in good agreement with 3D simulation.

Development of short prototype of dual aperture quadrupole magnet for CEPC ring

Main quadrupole magnets are critical for the Circular Electron and Positron Collider(CEPC)and are specifically designed as dual aperture quadrupole(DAQ)magnets.However,the field crosstalk between the two apertures presents challenges.As the CEPC will work at four beam energies of Z,W,Higgs and ttbar mode,the DAQ magnets will operate at four field gradients spanning from 3.18 to 12.63 T/m.The first short quadrupole magnet prototype with the bore diameter of 76 mm and magnetic length of 1.0 m revealed the problems of large magnetic field harmonics and a magnetic center shift within the beam energy range.Accordingly,a compensation method was proposed in this work to solve the field crosstalk effect.By adjusting the gap height at the middle of the two apertures,the field harmonics and magnetic center shift are significantly reduced.After optimization,the short prototype was modified using a new scheme.The field simulations are validated from the magnetic measurement results.Further,the multipole field meets the requirements of the four beam energies.The detailed magnetic field optimization,field harmonics adjustment,and measurement results are presented herein.

System design and measurements of flux concentrator and its solid-state modulator for CEPC positron source

Positron sources are one of the most important components of the injector of a circular electron positron collector(CEPC).The CEPC is designed as an e^(+)e^(−)collider for a Higgs factory.Its accelerator system is composed of 100-km-long storage rings and an injector.The design goal of the positron source is to obtain positron beams with a bunch charge of 3 nC.The flux concentrator(FC)is one of the cores of the positron source.This paper reports the design,development,and measurements of an FC prototype system.The prototype includes an FC and an all-solid-state high-current pulse modulator.Preliminary tests show that the peak current on the FC can reach 15.5 kA,and the peak magnetic field can reach 6.2 T.The test results are consistent with the theoretical simulation.The FC system fulfills the requirements of the CEPC positron source as well as provides a reference for the development of similar devices both domestically and abroad.

基于波形采样的CEPC电磁量能器读出单元测试系统

随着环形正负电子对撞机(Circular Electron Positron Collider,CEPC)方案的提出,由硅光电倍增管(Silicon Photomultiplier,SiPM)和塑料闪烁体组成的读出单元作为电磁量能器的预研方案之一,逐渐成为研究热点。在探测器预研阶段需要对读出单元性能进行充分研究,因此我们设计了一套自动化测试系统用于对读出单元进行测试、分析并改善其性能。利用DT5751波形采样插件、SiPM驱动电路板、步进电机、低压电源、继电器等完成硬件搭建,以LabVIEW为开发平台完成了软件的串行协议接口之间的通信,实现了读出单元的自动化测试,编写了相应的数据处理程序。在此基础上进行了SiPM刻度、闪烁体发光均匀性以及反射材料对光输出影响的研究。测试结果表明,ESR反射膜包裹的塑闪条具有最高的光输出,塑闪条耦合10μm像素SiPM的光输出为耦合25μm像素SiPM的三分之一。

CEPC直线加速器束流垃圾桶初步设计

束流垃圾桶的材料、结构、尺寸的正确设计对于确保高能环形对撞机(Circular Electron Positron Collider,CEPC)直线加速器的安全运行有重要意义。采用蒙特卡罗程序FLUKA,研究了概念设计阶段的CEPC直线加速器束流垃圾桶的初步设计方案,给出了作为吸收体设计依据的射程的不同计算方法及验证过程,以及屏蔽体部分的材料选择与总体屏蔽厚度的关系;计算过程中引入减方差技巧提高了计算结果的精确程度。

Testing Bell inequality through h→ττ at CEPC

The decay of Higgs boson into two spin-1/2 particles provides an ideal system to reveal quantum entanglement and Bell-nonlocality.Future e^(+)e^(-) colliders can improve the measurement accuracy of the spin correlation of tau lepton pairs from Higgs boson decay.We show the testability of Bell inequality through h→ττ at Circular Electron Positron Collider(CEPC).Two realistic methods of testing Bell inequality are investigated,i.e.,Törnqvist's method and Clauser-Home-Shimony-Holt(CHSH)inequality.In the simulation,we consider the detector effects of CEPC including uncertainties for tracks and jets from Z boson in the production of e^(+)e^(-)→Zh.Necessary reconstruction approaches are described to measure quantum entanglement between τ^(+) and τ^(-).Finally,we show the sensitivity of CEPC to Bell inequality violation for the two methods.

用于CEPC探测器超导磁体的液氮虹吸实验研究

概括了环形正负电子对撞机(CEPC)探测器超导磁体拟采用的虹吸冷却低温系统的研制背景,重点介绍了液氮虹吸实验平台的设计、组装及检漏。分别以氮气液化后的液氮填充相分离器的1/5和1/3体积的要求进行了两次实验,其常温常压下的体积分别为0.32m^3和0.47m^3。实验发现当充装量为0.32m^3时,蒸发侧加热功率达到10W后发生了烧干现象;充装量为0.47m^3时,蒸发侧加热功率达到20W后液氮回路仍循环良好,管路最大温差不到1K,管路温度分布均匀,未发生烧干现象。实验结果表明,氮气的充装量以及蒸发侧的加热功率是影响虹吸冷却效果的两个重要因素。

The Higgs signatures at the CEPC CDR baseline

As a Higgs factory, the CEPC(Circular Electron-Positron Collider) project aims at precision measurements of the Higgs boson properties. A baseline detector concept, APODIS(A PFA Oriented Detector for the HIggS factory), has been proposed for the CEPC CDR(Conceptual Design Report) study. We explore the Higgs signatures for this baseline design with ■ Higgs events. The detector performance for reconstructing charged particles, photons and jets is quantified with H→μμ, γγ and jet final states, respectively. The resolutions of reconstructed Higgs boson mass are comparable for the different decay modes with jets in the final states. We also analyze the H→WW~* and ZZ* decay modes, where a clear separation between different decay cascades is observed.

Cross section and Higgs mass measurement with Higgsstrahlung at the CEPC

The Circular Electron Positron Collider（CEPC） is a future Higgs factory proposed by the Chinese high energy physics community. It will operate at a center-of-mass energy of 240–250 Ge V. The CEPC will accumulate an integrated luminosity of 5 ab-1over ten years of operation, producing one million Higgs bosons via the Higgsstrahlung and vector boson fusion processes. This sample allows a percent or even sub-percent level determination of the Higgs boson couplings. With GEANT4-based full simulation and a dedicated fast simulation tool, we have evaluated the statistical precisions of the Higgstrahlung cross section σZH and the Higgs mass m H measurement at the CEPC in the Z →μ~＋μ^-channel. The statistical precision of σZH（m_H） measurement could reach 0.97%（6.9 MeV） in the model-independent analysis which uses only the information from Z boson decays. For the standard model Higgs boson, the m H precision could be improved to 5.4 Me V by including the information from Higgs decays. The impact of the TPC size on these measurements is investigated. In addition, we studied the prospect of measuring the Higgs boson decaying into invisible final states at the CEPC. With the Standard Model ZH production rate, the upper limit of B（H → inv.） could reach 1.2% at 95% confidence level.

Precision Higgs physics at the CEPC

The discovery of the Higgs boson with its mass around 125 GeV by the ATLAS and CMS Collaborations marked the beginning of a new era in high energy physics.The Higgs boson will be the subject of extensive studies of the ongoing LHC program.At the same time,lepton collider based Higgs factories have been proposed as a possible next step beyond the LHC,with its main goal to precisely measure the properties of the Higgs boson and probe potential new physics associated with the Higgs boson.The Circular Electron Positron Collider(CEPC)is one of such proposed Higgs factories.The CEPC is an e^+e^- circular collider proposed by and to be hosted in China.Located in a tunnel of approximately 100 km in circumference,it will operate at a center-of-mass energy of 240 GeV as the Higgs factory.In this paper,we present the first estimates on the precision of the Higgs boson property measurements achievable at the CEPC and discuss implications of these measurements.

Characterization of the first prototype CMOS pixel sensor developed for the CEPC vertex detector

Purpose CMOS pixel sensor has become extremely attractive for future high-performance tracking devices.It has been proposed for the vertex detector at the Circular Electron Positron Collider,which will allow precision measurements of the properties of the Higgs boson.To meet the stringent requirements for low power consumption,it is necessary to optimize the pixel sensor diode geometry to reach a high charge-over-capacitance ratio that allows reduction in analog power consumption.Methods Collection electrode size and footprint are two critical elements in sensor diode geometry and have deciding impacts on the charge collection performance.A prototype CMOS pixel sensor,named JadePix-1,has been developed with pixel sectors implemented with different electrode sizes and footprints,and its charge collection performance has been characterized with radioactive sources.Results Charge-to-voltage conversion gains for pixel sectors under test have been calibrated with low-energy X-rays.Characterization results have been obtained for equivalent noise charge(below 10e−),charge collection efficiency(around 40%),charge-over-capacitance ratio(above 0.015 V)and signal-to-noise ratio(higher than 55).Conclusion Small collection electrode size and large footprint are preferred to achieve high charge-over-capacitance ratio that promises low analog power consumption.Ongoing studies on sensor performance before and after irradiation,combined with this work,will conclude the diode geometry optimization.

Cavity fundamental mode and beam interaction in CEPC main ring

Background CEPC is a 100-kilometer-long electron-position collider,aiming to produce Higgs,W and Z-pole.Double ring(DR)is the baseline design of its main ring,and advanced partial double ring(APDR)is the alternative one.Purpose The purpose of this paper is to study the beam loading effects and the corresponding longitudinal beam dynamics of CEPC DR and APDR.Methods The phase shift of the bunches modulated by the bunch gap is calculated with the approximation formulas and simulated with the program.All these methods are compared,and the application conditions of them are also elaborated.In addition,the longitudinal coupled-bunch instability in CEPC main ring is calculated by analytical formulas.Results In CEPC DR high-lumi Z,the phase shift can be reduced to 0.51 deg(°).Besides,the total number of unstable longitudinal modes is 15 in CEPC DR high-lumi Z when no feedback system is added.Conclusion The phase shift of the bunches in CEPC can be reduced to an acceptable value if the optimal filling pattern is chosen.For CEPC APDR,the RF parameters are calculated and the beam loading effects are tolerable.

Physics design of collimators to reduce lost particle background at the CEPC

At the CEPC （Circular Electron Positron Collider）, which is proposed by the Chinese high energy physics community, the dominant background comes from radiative Bhabha scattering and the beamstrahlung effect according to preliminary research. Therefore, it is necessary to incorporate a collimator system to intercept particles that may be lost near the interaction region （IR）. In this paper, we introduce some limitations in choosing the position and width of the collimators. A certain parameter range is determined which is confined by the β function and the width of the collimators. A suitable choice of the half width is made by exploring this parameter range. A simulation of the particle loss rate in the IR and the hit density in the vertex detector with and without the collimators shows that the set of parameters of the collimators we designed is appropriate and effective.

CEPC positron source design

Purpose Circular Electron Positron Collider(CEPC)is a 100-km-ring e+e−collider for a Higgs factory.The injector is composed of a 10-GeV Linac and a 120-GeV Booster.The CEPC Linac is a normal conducting S-band Linac with frequency of 2860 MHz providing electron and positron beams at an energy up to 10 GeV with 100 Hz repetition frequency.The positron source is the most important part of the CEPC Linac,and the bunch charge of positron beam within some cut-off condition should be larger than 3 nC.Methods The positron source of CEPC is composed of a target,a flux concentrator,a capture section,a pre-accelerating section and a beam separation system.Higher positron yield and capture efficiency are the design goal.Based on simulation,the design will be presented and discussed in detail.Result The positron yield at the PSPAS exit is larger than 0.55,and the normalized rms emittance is 2370 mm-mrad with a strict cut-off condition.The detailed simulation results will be presented.Conclusions The design of the positron source including positron production and capture will be presented and can meet the requirements.

Beam-induced HOM power in CEPC collider ring cavity

Background 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.Purpose 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.Methods 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.Results 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.Conclusion 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.

Suppression of longitudinal coupled-bunch instabilities with RF feedback systems in CEPC main ring

Background Circular electron positron collider(CEPC)is a 100-km electron positron collider proposed by IHEP.The longitudinal coupled-bunch instability(LCBI)of CEPC main ring operating to study the Z particle(Z machine)may be a limiting factor of CEPC and needs to be considered seriously.Purpose The purposes of this paper are to calculate the LCBI caused by the fundamental mode of superconducting RF cavities in CEPC main ring,which is the most critical impedance,and to complete the design of the RF feedback systems suitable for CEPC,whose specifications can suppress the LCBI to a manageable level.Methods The LCBI growth rate in the CEPC main ring is calculated in the frequency domain.Two kinds of RF feedback,i.e.,direct feedback and one-turn delay feedback,are simulated with the program to suppress the LCBI.And according to the suppression effect of LCBI growth rate after adding RF feedback,the required design parameters are given.Results Two operation conditions of Z machine have severe LCBI without suppression,and dozens of longitudinal modes are unstable.Only the direct RF feedback is needed to suppress LCBI in the case of Z-30 MW,while both the direct RF feedback with maximum gain and one-turn feedback are needed in the case of Z-50 MW.The LCBI growth rates can be reduced to the order of half frequency of the synchronous oscillation.Conclusion The LCBI of CEPC Z machine has been studied.Selecting appropriate feedback RF feedback can reduce the LCBI to an acceptable value that bunch by bunch feedback can suppress.

基于DIRAC的高能物理分布式计算系统在CEPC实验上的应用

2 0 1 2年L H C实验发现希格斯粒子后,我国粒子物理学界规划了高能环形正负电子对撞机(CEPC),以及它升级后的超级质子对撞机(Spp C),用于研究希格斯物理,并在高能量前沿探索超越标准模型的新物理。CEPC实验的计算量非常大,即使是在预研阶段,一次探测器全模拟就需要833万CPU小时的计算量和1PB存储空间。当本地计算资源不足时,分布式计算是一种快速整合合作单位计算资源来满足实验大量计算需求的有效方法。2012年,高能所基于DIRAC中间件建立了一套约2000 CPU核规模的分布式计算系统,服务于BESIII实验。本文介绍了将此分布式计算系统扩展到支持CEPC实验的改进方案,并介绍了此系统在CEPC实验的实际应用。

Study of beamstrahlung effects at CEPC

The discovery of a 125 GeV Higgs boson at the LHC marked a breakthrough in particle physics. The relative lightness of the new particle has inspired consideration of a high-luminosity Circular Electron positron Collider （CEPC） as a Higgs Factory to study the particle＇s properties in an extremely clean environment. Given the high luminosity and high energy of the CEPC, beamstrahlung is one of the most important sources of beam- induced background that might degrade the detector performance. It can introduce even more background to the detector through the consequent electron-positron pair production and hadronic event generation. In this paper, beamstrahlung-induced backgrounds are estimated with both analytical methods and Monte Carlo simulation. Hit density due to detector backgrounds at the first vertex detector layer is found to be -0.2 hits/cm2 per bunch crossing, resulting in a low detector occupancy below 0.5%. Non-ionizing energy loss （NIEL） and total ionizing dose （TID）, representing the radiation damage effects, are estimated to be -1011 1 MeV neq/cm2/yr and --300 kRad/yr, respectively.