Production and constraints for a massive dark photon at electron-positron colliders

Dark sector may couple to the Standard Model via one or more mediator particles.We discuss two types of mediators:the dark photon A’and the dark scalar mediatorΦ.The total cross-sections and various differential distributions of the processes e+e-→qqA’and e+e→qqφ(q=u,d,c,s and b quarks)are discussed.We focus on the study of the invisible due to the cleaner background at future e+e-colliders.It is found that the kinematic distributions of the two-jet system could be used to identify(or exclude)the dark photon and the dark scalar mediator,as well as to distinguish between them.We further study the possibility of a search for dark photons at a future CEPC experiment with s1/2=91.2 GeV and 240 GeV.With CEPC running at s1/2=91.2 GeV,it would be possible to perform a decisive measurement of the dark photon(20 GeV相似文献   

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-kW 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 kV and current of 15.4 A, the klystron peak power reached 804 kW with output efficiency of about 65.3% at 40% duty cycle. The 1-dB 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.     

Prospects for chargino pair production at the CEPC

The proposed Circular Electron Positron Collider(CEPC)with a center-of-mass energy√s=240GeV will primarily serve as a Higgs factory.Additionally,it will offer good opportunities to search for new physics phenomena at low energies,which can be challenging with hadron colliders;however,these discoveries are highly motivated by theoretical models developed to explain,e.g.,the relic abundance of dark matter.This paper presents sensitivity studies for chargino pair production by considering scenarios for both a Bino-like and a Higgsino-like neutralino as the lightest supersymmetric particle and using a full Monte Carlo(MC)simulation.By assuming systematic uncertainties at the level of 5%,the CEPC has the ability to discover chargino pair production up to the kinematic limit of for√s/2 both scenarios.The results have a minor dependence on the reconstruction model and detector geometry.These results can also be considered as a reference and benchmark for similar searches at other proposed electron-positron colliders,such as the Future Circular Collider ee(FCC-ee)or the International Linear Collider(ILC),given the similar nature of the facilities,detectors,center-of-mass energies,and target luminosities.     

An optimal scheme for top quark mass measurement near the tt threshold at future e+e- colliders

A top quark mass measurement scheme near the tt production threshold in future e⁺e^- colliders, e.g.the Circular Electron Positron Collider (CEPC), is simulated. A χ² fitting method is adopted to determine the number of energy points to be taken and their locations. Our results show that the optimal energy point is located near the largest slope of the cross section v. beam energy plot, and the most efficient scheme is to concentrate all luminosity on this single energy point in the case of one-parameter top mass fitting. This suggests that the so-called data-driven method could be the best choice for future real experimental measurements. Conveniently, the top mass statistical uncertainty can also be calculated directly by the error matrix even without any sampling and fitting. The agreement of the above two optimization methods has been checked. Our conclusion is that by taking 50 fb^-1 total effective integrated luminosity data, the statistical uncertainty of the top potential subtracted mass can be suppressed to about 7 MeV and the total uncertainty is about 30 MeV. This precision will help to identify the stability of the electroweak vacuum at the Planck scale.     

Expected measurement precision of the branching ratio of the Higgs boson decaying to the di-photon at the CEPC

This paper presents the prospects of measuring \begin{document}$\sigma(e^{+} e^{-} \to ZH)\times {\rm Br}(H \to \gamma \gamma)$\end{document} in three Z decay channels \begin{document}$ Z \to q \bar{q}/ {\mu ^ + }{\mu ^ - }/ \nu \bar \nu $\end{document} using the baseline detector with \begin{document}$\sqrt{s} = 240$\end{document}GeV at the Circular Electron Positron Collider (CEPC). Simulated Monte Carlo events were generated and scaled to an integrated luminosity of 5.6 ab^–1 to mimic the data. Extrapolated results to 20 ab^–1 are also reported. The expected statistical precision of these measurements after combining three channels of Z boson decay was 7.7%. With some preliminary estimation on the systematical uncertainties, the total precision is 7.9%. The performance of the CEPC electro-magnetic calorimeter (ECAL) was studied by smearing the photon energy resolution in simulated events in the \begin{document}$e^{+} e^{-} \to ZH \to q\bar q\gamma \gamma $\end{document} channel. In the present ECAL design, the stochastic term in resolution plays the dominant role in the precision of Higgs measurements in the \begin{document}$H \to \gamma \gamma $\end{document} channel. The impact of the resolution on the measured precision of \begin{document}$\sigma(ZH)\times {\rm Br}(ZH \to q\bar q\gamma \gamma)$\end{document} as well as the optimization of the ECAL constant and stochastic terms were studied for further detector design.     

Sensitivity study of anomalous HZZ couplings at a future Higgs factory

We study the sensitivity of constraining the model independent HZZ coupling based on the effective theory up to dimension-6 operators at a future Higgs factory. Using the current conceptual design parameters of the Circular Electron Positron Collider, we give the experimental limits for the model independent operators given by the total Higgsstrahlung cross-section and the angular distribution of Z boson decays. In particular, we give the very small sensitivity limit for the CP violation parameter g, which will be a clear window to test the Standard Model and look for new physics signals     

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.     

Search for invisible decays of the Higgs boson produced at the CEPC

The Circular Electron Positron Collider (CEPC), proposed as a future Higgs boson factory, will operate at a center-of-mass energy of 240 GeV and will accumulate 5.6 ab^?1 of integrated luminosity in 7 years. In this study, we estimate the upper limit of BR(\begin{document}$H \rightarrow$\end{document} inv) for three independent channels, including two leptonic channels and one hadronic channel, at the CEPC. Based on the full simulation analysis, the upper limit of BR(\begin{document}$H \rightarrow$\end{document} inv) could reach 0.26% at the 95% confidence level. In the Stand Model (SM), the Higgs boson can only decay invisibly via \begin{document}$H\rightarrow ZZ^\ast\rightarrow\nu\overline{\nu}\nu\overline{\nu}$\end{document}, so any evidence of invisible Higgs decays that exceed BR(\begin{document}$H \rightarrow$\end{document} inv) of the SM will indicate a phenomenon that is beyond the SM (BSM). The invariant mass resolution of the visible hadronic decay system \begin{document}$ZH(Z \rightarrow qq$\end{document}, \begin{document}$ H \rightarrow$\end{document} inv) is simulated, and the physics requirement at the CEPC detector for reaching this is given.     

Measurement of H→μ~+μ~- production in association with a Z boson at the CEPC

The Circular Electron-Positron Collider(CEPC) is a future Higgs factory proposed by the Chinese high energy physics community. It is planned to operate at a center-of-mass energy of 240–250 Ge V and is expected to accumulate an integrated luminosity of 5 ab~(-1) over ten years of operation. At the CEPC, Higgs bosons will be dominantly produced from the ZH associated process. The vast number of Higgs events collected will enable precise studies of its properties, including Yukawa couplings to massive particles. With GEANT4-based simulation of detector effects, we study the feasibility of measuring the Higgs boson decaying into a pair of muons at the CEPC.The results with and without information from the Z boson decay products are provided, showing that a signal significance of over 10 standard deviations can be achieved and the H-μ-μ coupling can be measured within 10%accuracy.     

Optimizing Higgs factories by modifying the recoil mass

It is difficult to measure the WW-fusion Higgs production process(e~+e~-→νh)at a lepton collider with a center of mass energy of 240-250 Ge V due to its small rate and the large background from the Higgsstrahlung process with an invisible Z(e~+e~-→h Z,Z→ν).We construct a modified recoil mass variable,m~p_(recoil),defined using only the 3-momentum of the reconstructed Higgs particle,and show that it can separate the WW-fusion and Higgsstrahlung events better than the original recoil mass variable m_(recoil).Consequently,the m~p_(recoil)variable can be used to improve the overall precisions of the extracted Higgs couplings,in both the conventional framework and the effective-field-theory framework.We also explore the application of the m~p_(recoil)variable in the inclusive cross section measurements of the Higgsstrahlung process,while a quantitive analysis is left for future studies.