Future Circular Electron Positron Collider Research Articles

Probing a light dark sector at future lepton colliders via invisible decays of the SM-like and dark Higgs bosons

A renormalizable UV model for axionlike particles or hidden photons, which may explain the dark matter, usually involves a dark Higgs field, which is a singlet under the standard model (SM) gauge group. The dark sector can couple to the SM particles via the portal coupling between the SM-like Higgs and dark Higgs fields. Through this coupling, the dark sector particles can be produced in either the early Universe or the collider experiments. Interestingly, not only the SM-like Higgs boson can decay into the light dark bosons, but also a light dark Higgs boson may be produced and decay into the dark bosons in a collider. In this paper, we perform the first collider search for invisible decays by taking both the Higgs bosons into account. We use a multivariate technique to best discriminate the signal from the background. We find that a large parameter region can be probed at the International Linear Collider operating at the center-of-mass energy of 250 GeV. In particular, even when the SM-like Higgs invisible decay is a few orders of magnitude below the planned sensitivity reaches of the International Linear Collider and the high luminosity LHC, the scenario can be probed by the invisible decay of the dark Higgs boson produced via a similar diagram. Measuring the dark Higgs boson decay into the dark sector will be a smoking gun signal of the light dark sector. A similar search of the dark sector would be expected in, e.g., the Cool Copper Collider, the Circular Electron Positron Collider, the Compact Linear Collider, and the Future Circular electron-positron Collider.

Production of the B_c meson at the CEPC

In this paper, we make a detailed study on the production of the B_c, B_c^*, B_c(2,^1S_0), and B^*_c(2,^3S_1) mesons via the three planned running modes (Z, W, and H) at the future Circular Electron–Positron Collider (CEPC). The fragmentation-function and full nonrelativistic QCD approaches are adopted to calculate the production cross sections. Considering the excited states shall decay to the ground B_c state (e.g. 1^1S_0-state) with almost 100% probability, our numerical results show that up to next-to-leading order QCD corrections, there are about 1.4times 10^8B_c events to be accumulated via the Z mode (sqrt{s}=m_{_Z}) of the CEPC, but only about 1.6times 10^4 and 1.1times 10^4B_c events to be accumulated via the W (sqrt{s}=160,mathrm{GeV}) and H (sqrt{s}=240,mathrm{GeV}) modes, respectively. Since the Z mode is the best mode among the three planned modes of the CEPC for studying the production of the B_c, B_c^*, B_c(2,^1S_0), and B^*_c(2,^3S_1) mesons and the differential distributions of these mesons may be measured precisely at this mode, we further present the differential cross sections dsigma /(dz,dmathrm{cos}theta ) and dsigma /dz via the Z mode of the CEPC.

Single top production via FCNC couplings at an $$e^{-}e^{+}$$ collider with center-of-mass energy $$\sqrt{s}=240$$ GeV

Aspects of top quark physics can be studied at a future Circular Electron-Positron Collider. In particular, flavor-changing neutral current interactions (FCNC) of the top quark can be studied even with a center-of-mass energy from \(\sqrt{s}\gtrsim m_t\) to \(\sqrt{s}=240\) GeV, through the production a single top quark in association with a charm quark, which can arise in several scenarios of new physics. We explore here this signal within the Left-Right Mirror Model, which is a Left-Right model that includes mirror fermions; this model is motivated as a possible solution to the strong CP problem and by its rich phenomenology. We find that for particular scenarios of the model parameters, which are in agreement with the most up-to-date experimental constraints, the signal coming from the process \(e^+e^-\rightarrow V \rightarrow tc\) (\(V=Z,\,Z^{\prime }\)) provides a detectable signal and thus, it can give evidence for FCNC interactions; in these scenarios, the mass of a new neutral gauge boson can be as high as \(m_{Z'}=7\) TeV.

Searching for axionlike particles at future electron-positron colliders

We investigate the prospects for discovering axion-like particles (ALPs) via a light-by-light (LBL) scattering at two colliders, the future circular collider (FCC-ee) and circular electron-positron collider (CEPC). The promising sensitivities to the effective ALP-photon coupling $g_{a\gamma\gamma}$ are obtained. Our numerical results show that the FCC-ee and CEPC might be more sensitive to the ALPs with mass 2 GeV $\sim$ 10 GeV than the LHC and CLIC.

Searching for lepton flavor violating interactions at future electron-positron colliders

Lepton flavor violating interactions are absent in the standard model but are expected in various beyond standard models. In this work, the potential of the future circular electron-positron collider to probe the four fermion lepton flavor couplings via the $e^{+}e^{-}\rightarrow e^{\pm}\tau^{\mp}$ process is revisited by means of an effective field theory approach. We provide constraints at $95\%$ CL on the dimension-six Wilson coefficients including major sources of background processes and considering realistic detector effects at four expected operation energies $\sqrt{s}=157.5$, $162.5$, $240$ and $365$ GeV according to their corresponding integrated luminosities. We demonstrate that statistical combination of the results from four center-of-mass energies improves the sensitivity to the LFV couplings significantly. We compare the results with the prospects from Belle II with $50~ ab^{-1}$ and other studies at electron-positron colliders.

Ultra-Thin Solenoid and Cryostat Development for Novel Detector Magnets

In the scope of the Future Circular electron positron Collider study (FCC-ee), the IDEA detector is developed. It comprises a superconducting solenoid with free bore of 4 m, 6 m long and a central magnetic field of 2 T. The positioning of the magnet between the inner tracker and the electronic calorimeter heavily constrains the magnet design, as it is required to have the lowest possible radiation length, so minimum thickness and lowest density material. With respect to the classical solution of a solenoid enclosing the calorimeters, a cost reduction of about 50% is expected due to size reduction. An optimization of the different components of the magnet system has been carried out, resulting in the development of a new composite high-strength conductor that can be used to build a 30 mm thin solenoid. The quench analysis of the solenoid will be presented as it is of critical importance given the high energy density in the magnet of 21 kJ/kg. A cryostat made of concentric aluminium shells would account for about 50% of the radiation length of the magnet and most of this material is used in the outer vacuum shell of the cryostat to prevent buckling. In order to further reduce the radiation length, two fundamentally different approaches are being analysed. The first method focuses on reducing drastically the outer shell thickness. This leads to use honeycomb composites, reinforcing bars and corrugated shells for the outer shell of the cryostat. The second approach consists of supporting very thin cryostat shells directly on the solenoid cold mass using proper support. This can be achieved by replacing the thick walls and MLI insulation by a material that can sustain 1 atm while having low radiation length and low thermal conductivity. Cryogel Z has shown promising properties and its suitability for this project is being analysed. This novel approach has never been used so far for superconducting magnets.

Prompt J/psi photoproduction within the non-relativistic QCD framework at the CEPC

The prompt J/psi photoproduction within the non-relativistic QCD (NRQCD) framework at the future Circular Electron Positron Collider (CEPC) is studied, including the contributions from both direct and resolved photons. Employing different sets of long distance matrix elements, the total cross section is dominated by the color-octet channel. We present different kinematic distributions of J/psi production and the results show there will be about 50 J/psi events when the transverse momentum of J/psi is up to 20 GeV. It renders that the J/psi photoprodution at the CEPC is a well laboratory to test the NRQCD and further clarify the universality problem in NRQCD between electron positron collider and hadron collider.

Availability and critical systems of the Future Circular Electron–Positron Collider

The Future Circular Electron–Positron Collider (FCC-ee) is planned to be the world’s largest particle collider and a precision instrument to study the heaviest known particles. Achieving substantial physics results requires producing high amounts of integrated luminosity, which calls for sufficient machine availability. Although the operational availability of lepton colliders has been high, the increased complexity of the new infrastructure creates a challenge to maintain this. At the early stage of research, the main activity of an availability study should be to identify the causes that potentially have the most significant effect on downtime. This paper identifies critical systems of the FCC-ee based on available failure data. The paper further presents an operation model for the FCC-ee that can be used for assessing the effect of unavailability on overall performance. Special attention is given to systems with built-in redundancies as this design concept has been proven to increase accelerator availability.

Probing the top quark flavor-changing couplings at CEPC * * Supported by CNRS via the LIA FCPPL, by the CEPC theory study grant, and by IHEP (Y7515540U1)

We propose to study the flavor properties of the top quark at the future Circular Electron Positron Collider (CEPC) in China. We systematically consider the full set of 56 real parameters that characterize the flavor-changing neutral interactions of the top quark, which can be tested at CEPC in the single top production channel. Compared with the current bounds from the LEP2 data and the projected limits at the high-luminosity LHC, we find that CEPC could improve the limits of the four-fermion flavor-changing coefficients by one to two orders of magnitude, and would also provide similar sensitivity for the two-fermion flavor-changing coefficients. Overall, CEPC could explore a large fraction of currently allowed parameter space that will not be covered by the LHC upgrade. We show that the c-jet tagging capacity at CEPC could further improve its sensitivity to top-charm flavor-changing couplings. If a signal is observed, the kinematic distribution as well as the c-jet tagging could be exploited to pinpoint the various flavor-changing couplings, providing valuable information about the flavor properties of the top quark.

Transient beam loading and rf power evaluation for future circular colliders

Interaction of the beam with the fundamental impedance of the accelerating cavities can limit the performance of high-current accelerators. It can result in a significant variation of bunch-by-bunch parameters (bunch length, synchronous phase, etc.) if filling patterns contain gaps that are not negligible compared with the cavity filling time. In the present work, this limitation is analyzed using the steady-state time-domain approach for the high-current option (Z) of the future circular electron-positron collider and for the future circular hadron-hadron collider. Mitigation of transient beam loading by direct rf feedback is addressed with evaluation of additional required generator power.

Initial state radiation correction and its effect on data-taking scheme for σB(e+e−→ ZH) measurement

The measurement of Born cross-section of [Formula: see text] process is one of the major goals of the future Circular Electron Positron Collider, which may reach a precision of 0.5% at 240 GeV. Such unprecedented precision must be guaranteed by both theoretical and experimental sides, such as the calculations of high order corrections, the knowledge of the [Formula: see text] line shape. Uncertainty of the radiative correction factor at 240 GeV caused by the [Formula: see text] line shape is evaluated in this work. Therefore, dedicated data-taking schemes are proposed in order to precisely calculate the ISR correction factor.

A potential scenario for Majorana neutrino detection at future lepton colliders

The existence of Majorana neutrinos must lead to lepton-number violating processes, and the Majorana nature of neutrinos can only be experimentally verified via lepton-number violating processes. We study an approach to search for Majorana neutrinos at future electron-positron colliders by exploiting this feature. We investigate ΔL = 2 like-sign dilepton production and find that lepton colliders with different center-of-mass (c.m.) energies have comparative advantages in resonant production of a Majorana neutrino in both light and heavy neutrino mass range. At the future Circular Electron-Positron Collider (CEPC), with 250 GeV c.m. energy and 5 ab−1 integrated luminosity, we find that there could be more significant sensitivity for resonant production of a Majorana neutrino in the mass range of 30-50 GeV than previous results at LEP2 or LHC. At the 1 TeV ILC with 1 ab−1 integrated luminosity, the sensitivity can be improved for neutrino mass in the range of 450-600 GeV.

High order mode power loss evaluation in future circular electron-positron collider cavities

The high order mode (HOM) power losses induced by a particle beam in accelerator components should be taken into account during their design and the choice of HOM couplers. In this work the power losses due to HOMs in the proposed 400.79 MHz radio frequency (rf) cavity designs were evaluated for various energy options (Z, W, H, and $\mathrm{t}\overline{\mathrm{t}}$) of the future circular electron-positron collider (FCC-ee). The constraints for the rf cavity design are dictated by the Z machine with the lowest beam energy, but the highest beam current. Still certain beam filling patterns (bunch and train spacings) are not feasible and should be avoided in beam operation. For other energy options the power loss are sufficiently low and can be extracted by conventional HOM couplers.

Monte Carlo study of particle identification at the CEPC using TPC dE\xa0/\xa0dx information

The kaon identification is crucial for the flavor physics, and also benefits the flavor and charge reconstruction of the jets. We explore the particle identification capability for tracks with momenta ranging from 2–20 GeV/c using the dE / dx measurements in the Time Projection Chamber at the future Circular Electron-Positron Collider. Based on Monte Carlo simulation, we anticipate that an average 3.2~sigma (2.6~sigma ) K/pi separation can be achieved based on dE / dx information for an optimistic (conservative) extrapolation of the simulated performance to the final system. Time-of-flight (TOF) information from the Electromagnetic Calorimeter can provide K/pi separation around 1 GeV/c and reduce the K / p mis-identification rate. By combining the dE / dx and TOF information, we estimate that in the optimistic scenario a kaon selection in inclusive hadronic Z decays with both the average efficiency and purity approaching 95% can be achieved.

Studies of detector cells for a hadronic calorimeter based on plastic scintillators

In this paper, we study variations of a plastic scintillator tile coupled to silicon photomultiplier via a dome-shaped cavity originally developed in the CALICE collaboration. Four kinds of plastic scintillator detector cells with different sizes based on the structure were studied for applications in analog readout highly granular hadronic calorimeter (AHCAL) in future circular electron positron collider (CEPC) project. The responses to cosmic rays of the detector cells could reach 33.89 p.e./MIPs (minimum ionizing particles). The good results of both response uniformity and MIP detection efficiency (above 95%) show that the detector cells with larger sizes ( $$40\times 40\times 3$$ and $$50\times 50\times 3\,\hbox {mm}^{3}$$ ) could provide an option for AHCAL detector cells of CEPC detectors.

Lattice design for the CEPC double ring scheme

A future Circular Electron Positron Collider (CEPC) has been proposed by China with the main goal of studying the Higgs boson. Its baseline design, chosen on the basis of its performance, is a double ring scheme; an alternative design is a partial double ring scheme which reduces the budget while maintaining an adequate performance. This paper will present the collider ring lattice design for the double ring scheme. The CEPC will also work as a W and a Z factory. For the W and Z modes, except in the RF region, compatible lattices were obtained by scaling down the magnet strength with energy.

NLO QCD corrections forJ/ψ+c+c¯production in photon-photon collision

The $\ensuremath{\gamma}+\ensuremath{\gamma}\ensuremath{\rightarrow}J/\ensuremath{\psi}+c+\overline{c}$ inclusive process is an extremely important subprocess in $J/\ensuremath{\psi}$ production via photon-photon scattering, like at LEPII or various types of future electron-positron colliders. In this work we perform the next-to-leading order (NLO) QCD corrections to this process in the framework of nonrelativistic QCD (NRQCD) factorization formalism, the first NLO calculation for two projectiles to the 3-body quarkonium inclusive production process. By setting the center-of-mass energy at LEPII, the $\sqrt{s}=197\text{ }\text{ }\mathrm{GeV}$, we conduct analyses of the ${p}_{t}^{2}$ distribution and total cross section of this process at the NLO accuracy. It turns out that the total cross section is moderately enhanced by the NLO correction with a $K$ factor of about 1.46, and hence the predicted $J/\ensuremath{\psi}$ inclusive productivity is increased while the DELPHI data still overshoot the theoretical prediction. At the future Circular Electron-Positron Collider, the NLO corrections are found to be more significant, with a $K$ factor of about 1.76.

Design Consideration of 650-MHz Circular Electron–Positron Collider Klystron and Simulation of Its Beam Tester**Supported by the National Natural Science Foundation of China under Grant No 11475201, and the Innovation and Technology Fund of Institute of High Energy Physics.

We present the first phase R&D for the 800 kW cw, 650 MHz klystron for the future circular electron–positron collider (CEPC) project in China. The CEPC requires 192 klystrons and it is desired to be designed in the Institute of High Energy Physics, CAS, and manufactured domestically. Therefore, we present the manufacturing schedule of this project; the three-stage development from the beam test tube to the klystron having a high efficiency structure. Design of the beam test tube that comprises electron gun and collector is presented. First, gun simulation having a modulating anode is performed using DGUN software. The uniform beam trajectories with a beam perveance of 0.64 μA/V3/2 are simulated. We employ a Ba-dispenser cathode of radius 35 mm with ϕ10 hole at the center and obtain a current density on cathode less than 0.45 A/cm2. The beam trajectories are also simulated over beam test tube of length about 2 m with a magnetic field of 213 Gauss. Thermal analysis of the collector is performed using the ANSIS-CFX code and hence the cooling structure is determined. Mechanical design is almost carried out and it is in manufacturing stage.