International Linear Collider Research Articles

Higgs physics at ILC

International Linear Collider (ILC) is a viable option for realization of a future Higgs factory. With proven accelerator technology and technically mature detector design, ILC offers well-understood physics program in the top, EW and Higgs sectors. Clean environment of electron–positron collisions, together with beam polarization of both beams and center-of-mass reach at a TeV scale, will make ILC a precision tool to probe realizations of new physics. The above will be illustrated on examples of fully simulated measurements in the Higgs sector, including Higgs couplings, Higgs self-coupling, CP violation measurements and Higgs exotic decay studies at ILC.

Searching for heavy neutral leptons through exotic Higgs decays at the ILC

In this study we investigate the feasibility of detecting heavy neutral leptons (Nd) through exotic Higgs decays at the proposed International Linear Collider (ILC), specifically in the channel of e+e−→qqH with H→νNd→νlW→νlqq. Analyses based on full detector simulations of the ILD are performed at the center-of-mass energy of 250 GeV for two different beam polarization schemes with a total integrated luminosity of 2 ab−1. A range of heavy neutral lepton masses between the Z boson and Higgs boson masses are studied. The 2σ significance reach for the joint branching ratio of BR(H→νNd)·BR(Nd→lW) is about 0.1%, nearly independent of the heavy neutral lepton masses, while the 5σ discovery is possible at a branching ratio of 0.3%. Interpreting these results in terms of constraints on the mixing parameters |ϵid|2 between SM neutrinos and the heavy neutral lepton, it is expected to have a factor of 10 improvement from current constraints. Published by the American Physical Society 2024

Search for the production of dark matter in the framework of a mono- Z′ portal at the ILC with simulated electron-positron collisions at s=500 GeV

In the present work, we study the possible production of the light neutral gauge boson (Z′) candidates, which originated from a simplified-model scenario based on the mono-Z′ model, in association with dark matter. This study has been performed by studying events with dimuon plus missing transverse energy produced in the simulated electron-positron collisions at the foreseen International Linear Collider (ILC), operating at 500 GeV center of mass energy and integrated luminosity of 1000 fb−1. In case no new physics has been discovered, we set upper limits at 95% confidence level on the masses of various particles in the model as, spin-1 (Z′), as well as the fermionic dark matter. Published by the American Physical Society 2024

One-loop contributions to WWZ from a seesaw variant with radiatively induced light neutrino masses

The experimental confirmation of neutrino mass and mixing disagrees with the picture of the Standard Model, and thus provides a good motivation to look for unknown physics. A seesaw variant in which neutrino masses are radiatively generated, with neutrinos characterized by Majorana fields and where a set of three hypothetical neutral heavy leptons are present, has been considered in the present work. We have calculated, estimated, and analyzed the contributions from virtual light and heavy neutrinos to the gauge vertex WWZ, in the context of a future electron-positron collider. We have found that contributions to both CP-even and CP-odd anomalous couplings are generated. Our estimations indicate that contributions as large as 10−3 can be achieved in both cases. This is particularly interesting for the CP-odd effects, which are expected to appear in the framework of the Standard Model since the three-loop order, thus being quite suppressed. By considering the expected sensitivity of the International Linear Collider to this gauge vertex, we conclude that these new-physics effects would be within its reach for e+e− collisions taking place at a center-of-mass energy of 800 GeV. Published by the American Physical Society 2024

Investigating higgsino dark matter in the semi-constrained NMSSM* *Supported by the National Natural Science Foundation of China (12275066, 11605123)

In this study, we explored the characteristics of higgsino-dominated dark matter (DM) within the semi-constrained Next-to-Minimal Supersymmetric Standard Model (scNMSSM), covering a mass range from hundreds of GeV to several TeV. We carefully analyzed the parameter space under existing theoretical and experimental constraints to confirm the viability of higgsino-dominated lightest supersymmetric particles (LSPs) with masses between 100 GeV and 4 TeV. Our study examined various DM annihilation mechanisms, emphasizing the significant role of coannihilation with the next-to-lightest supersymmetric particle (NLSP), which includes other higgsino-dominated particles such as and . We categorize the annihilation processes into three main classes: coannihilation, Higgs funnel annihilation, and coannihilation. Each class combines interactions with . Our results indicate that achieving the correct relic density in heavier higgsino LSPs requires a combination of coannihilation and Higgs funnel mechanisms. We also assessed the potential of future experiments, such as XENONnT, LUX-ZEPLIN (LZ), PandaX-xT, and the Cherenkov Telescope Array (CTA), to probe these DM scenarios through direct and indirect detections. In particular, future spin-independent DM detections may cover all samples with the correct DM relic density for GeV. Furthermore, future colliders such as the International Linear Collider (ILC) and Compact Linear Collider (CLIC) are expected to exceed the detection capabilities of current hadron colliders, especially for higher mass NLSPs. Notably, CLIC, which will operate at 3000 GeV, is anticipated to enable thorough investigation of all samples with insufficient DM relic density for GeV.

ILC sensitivity for leptophilic scalar dark matter

We explore the viability of detecting a leptophilic scalar dark matter (DM) at the International Linear Collider (ILC) in a simplified model approach. We present the constraints on the couplings of scalar DM with the standard model fermions ensuing from the relic density bounds deduced from the 2018 Planck data. We, then, present the reach of the ILC in terms of Λ, the scale of the effective theory, by performing a detailed analysis for the Z associated DM pair production (i.e. e + e − → 2 jets + ET , e + e − → μ + μ − + ET and e + e − → e + e − + ET ). We present the results for various run scenarios as 3σ contours in the m ϕ − Λ plane. We also analyze the effect of beam polarization on the sensitivity of this search. We find that for the process with two hadronic jets in the final state, ILC can probe Λ up to 1.76 TeV for s=1 TeV that can further enhance to 1.99 TeV, after the inclusion of polarization effects.

Probing gauge-Higgs unification models at the ILC with quark–antiquark forward–backward asymmetry at center-of-mass energies above the Z mass

The International Linear Collider (ILC) will allow the precise study of e-e+→qq¯\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$e^{-}e^{+} \\rightarrow q\\bar{q}$$\\end{document} interactions at different center-of-mass energies from the Z\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$Z$$\\end{document}-pole to 1 TeV. In this paper, we discuss the experimental prospects for measuring differential observables in e-e+→bb¯\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$e^{-}e^{+} \\rightarrow b\\bar{b}$$\\end{document} and e-e+→cc¯\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$e^{-}e^{+} \\rightarrow c\\bar{c}$$\\end{document} at the ILC baseline energies, 250 and 500 GeV. The study is based on full simulation and reconstruction of the International Large Detector (ILD) concept. Two gauge-Higgs unification models predicting new high-mass resonances beyond the Standard Model are discussed. These models predict sizable deviations of the forward–backward observables at the ILC running above the Z mass and with longitudinally polarized electron and positron beams. The ability of the ILC to probe these models via high-precision measurements of the forward–backward asymmetry is discussed. Alternative scenarios at other energies and beam polarization schemes are also discussed, extrapolating the estimated uncertainties from the two baseline scenarios.

Effect of polarized colliding beam on Higgs boson production at the lepton collider

Different production processes involving the Higgs boson, such as annihilation and W/Z boson fusion, will be observed in the International Linear Collider (ILC). The ILC operates at a center-of-mass (CM) energy of [Formula: see text]–1000[Formula: see text]GeV. The study reveals that the production cross-section can either be enhanced or reduced depending on the CM energy and the specific combination used, which has implications for selecting appropriate production processes. Additionally, this investigation highlights that by polarizing beams, the number of measurable observables increases. These observables, such as left–right asymmetry, detailed effective polarization, and adequate effective luminosity, are crucial to ascertain contemporary physical parameters in physics models absurdly the Standard Model (SM).

The Contribution of Photon, Z Boson, Higgs, Radion and Unparticle to the Processes in Randall-Sundrum Model

The fundamentally well-known process of Bhabha is studied from the theoretical view of unparticle physics in the prominent Randall-Sundrum model. The cross-sections independently for photon (γ), Z boson (Z), vector unparticle (Uμ), Higgs (h), radion (), and scalar unparticle (U) exchange are calculated and evaluated. Numerical calculations showed that the contribution of unparticle exchange dominates in very high energy regions. While the standard model exchanges as γ and Z are predominant in the lower energy region, h and contribution is very small in comparison with the other exchanges. The results are plotted in the energy ranges available in the present designs of accelerators and near future energy upgrades of the International Linear Collider (ILC) and The Compact Linear Collider (CLIC).

Searches for axionlike particles via γγ fusion at future e+e− colliders

Opportunities for searches for axionlike particles (ALPs) coupling to photons in e+e− collisions at the Future Circular Collider (FCC-ee) and International Linear Collider (ILC) are investigated. We perform a study of the photon-fusion production of ALPs decaying into two photons, e+e−→γγe+a(γγ)e−, over the light-by-light continuum background, for the planned FCC-ee and ILC center-of-mass energies and integrated luminosities. An analysis of the feasibility measurements is presented using parametrized simulations for two types of detectors. Upper limits at 95% confidence level (C.L.) on the cross section for ALP production, σ(γγ→a→γγ), and on the ALP-photon coupling are obtained over the ma≈0.1–1000 GeV ALP mass range, and compared to current and future collider searches. Production cross sections down to σ(γγ→a→γγ)≈1 fb (1 ab) will be probed at ma≈1 (300) GeV, corresponding to constraints on the axion-photon coupling as low as gaγγ≈2×10−3 TeV−1. Published by the American Physical Society 2024

Searching for singlet vector-like leptons via pair production at ILC

Vector-like leptons (VLLs) as one kind of the most intriguing particles, have been widely concerned in several extensions of the Standard Model (SM). In this work, we explore the discovery potential of VLLs via pair production in the context of models that satisfy asymptotic safety at the International Linear Collider (ILC). The expected sensitivities of the ILC with the center of mass energy s=1 TeV and the integrated luminosity L=1 ab−1 to the parameter space of this kind of VLL models are derived. The results we obtained show that, for the VLL with mass MF in the region of MF∈[101 GeV,463 GeV], the Yukawa coupling κ can be as low as κ∈[0.032,0.098].

Sub-GeV dark matter search at ILC beam dumps

Light dark matter particles may be produced in electron and positron beam dumps of the International Linear Collider (ILC). We propose an experimental setup to search for such events, the Beam-Dump eXperiment at the ILC (ILC-BDX). The setup consists of a muon shield placed behind the beam dump, followed by a multi-layer tracker and an electromagnetic calorimeter. The calorimeter can detect electron recoils due to elastic scattering of dark matter particles produced in the dump, while the tracker is sensitive to decays of excited dark-sector states into the dark matter particle. We study the production, decay and scattering of sub-GeV dark matter particles in this setup in several models with a dark photon mediator. Taking into account beam-related backgrounds due to neutrinos produced in the beam dump as well as the cosmic-ray background, we evaluate the sensitivity reach of the ILC-BDX experiment. We find that the ILC-BDX will be able to probe interesting regions of the model parameter space and, in many cases, reach well below the relic target.

A numerical study attitude of compatibility in scintillator detectors through matching pair scintillator-photodiode materials

In a group of scintillator detectors where photodiodes have replaced photomultiplier tubes, one way to increase the efficiency of the device is proper matching between the radiation spectrum of the scintillator and the spectral sensitivity of the photodiode. This study seeks to find the most compatible pair of scintillators with photodiodes using the simulation of photodiodes in COMSOL Multiphysics. In this research, three scintillators NaI(Tl), CsI(Tl), and Bi4Ge3O12 (BGO), along with three photodiodes silicon (Si) and gallium arsenide (GaAs) and indium phosphide (InP), have been studied to find the best compatible pair. Among the selected scintillators and photodiodes, the pair of Si with BGO produced the highest output current. Generating more current in a detector for constant input radiation means more sensitivity of that detector to the incident beam. In another part of this research, using the simulated photodiode in COMSOL Multiphysics, the current generated in the photodiode has been obtained for a fixed value of the ratio of the length of the photodiode to the junction depth of doping for various lengths. Then, using the Origin Pro software, the best curve corresponding to the data obtained from the simulation was determined. The results showed that these points coincide with an exponential curve with an excellent approximation. Using these types of detectors in the International Linear Collider (ILC) and in the Positron Emission Tomography (PET) field shows the unique applications of detectors.

Beam Loading Compensation of Traveling Wave Linac to a Multi-bunch Pulse with Gaps

In the electron driven ILC (International Linear Collider) positron source, the beam is generated and accelerated in a multi-bunch format with mini-trains. The macro-pulse contains 2 to 4 mini-trains with several gaps, because the pulse format is a copy of a part of the bunch storage pattern in DR (Damping Ring). This pulse format causes a variation of the accelerator field in the pulse due to the transient beam loading and an intensity fluctuation of captured positron. The beam loading is compensated by amplitude modulation on the input RF in the positron booster composed from L-band and S-band traveling wave RF cavity. In this article, we derive the exact solution for the compensation with the gaps. In addition, we evaluate the effect of the time constant (delay) of the input RF modulation due to klystron Q-value.

Design Study of a Superconducting Quadrupole Magnet System Sustainable Under Dark Current Heating in ILC Main Linac

The International Linear Collider (ILC) main linac comprises a series of 12 m-long cryomodules. The cryomodule contains eight 9-cell superconducting (SC) RF (SRF) cavities and an SC quadrupole magnet combined with dipole correctors to focus and steer electron and positron beams. The magnets are installed between the SRF cavity string and at the longitudinal center of a common cryomodule/cryostat. These magnets are conductively cooled by pure aluminum channels thermally connected to a 2 K two-phase helium pipe for cooling the SRF cavities. A recent study shows that field-emitted electrons, so-called “dark current” initiated in the SRF cavities, are transmitted through the SRF cavity string and reach the SC magnet. The energy is inevitably absorbed in the SC coil due to the magnetic field, resulting in risks of a quench caused by the coil heating. We are investigating alternate magnet designs by using Nb <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_{3}$</tex-math></inline-formula> Sn or <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\text{MgB}_{2}$</tex-math></inline-formula> SC and by adding the dark current absorber surrounding the beam-pipe to realize sustainable magnet operation under the dark current heating. We report the design study of the magnet system and interfaces to the cryomodule accommodating it with the SRF cavities.

Superconducting detector magnets for high energy physics

Various superconducting detector solenoids for particle physics have been developed in the world. The key technology is the aluminum-stabilized superconductor for almost all the detector magnets in particle physics experiments. The coil fabrication technology is also important and it has advanced along with the conductor technology, such as the inner coil winding technique, indirect cooling, transparent vacuum vessel, quench protection scheme using pure aluminum strips and so on. The detector solenoids design study is in progress for future big projects in Japan and Europe, that is, ILC (International Linear Collider), FCC (Future Circular Collider) and CLIC (Compact LInear Collider), based on the technologies established over many years. The combination of good mechanical properties and keeping a high RRR is a key point for the development of Al-stabilized conductor. The present concern for the detector solenoid development is to have been nearly losing the key technologies and experiences. Nowadays, there are no industrial companies having the capacity to manufacture such aluminum stabilized superconductor. Complementary efforts are seriously required to re-realize and validate the performance required in the future projects in collaboration with worldwide institutes and industries. Some mid-scale physics experiments required detector solenoids wound with not aluminum stabilized conductor but conventional copper stabilized conductor. The specific requirement is to control the magnetic field distribution precisely, and the efforts to realize the requirement are on going with regard to the magnetic field design technology with high precision simulation, coil fabrication technology and so on.

Muon g−2 in a type-X 2HDM assisted by inert scalars: A test at the ILC

A two-Higgs doublet model can predict the observed muon $g\ensuremath{-}2$ for an appropriately light pseudoscalar that now faces tight constraints. However, it was shown recently in the companion paper [N. Chakrabarty, Muon $g\ensuremath{-}2$ in a type-X 2HDM assisted by inert scalars: A test at the LHC, preceding paper, arXiv:2112.13126.] that augmenting the two-Higgs doublet model by an additional inert doublet can lead to an explanation to the muon $g\ensuremath{-}2$ anomaly for a much heavier pseudoscalar. In this study, we probe such a framework at the proposed International Linear Collider using beam polarization for $\sqrt{s}=1\text{ }\text{ }\mathrm{TeV}$. Using multivariate techniques, we analyze the signals ${e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}{\ensuremath{\tau}}^{+}{\ensuremath{\tau}}^{\ensuremath{-}}+$ missing transverse energy and ${e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}{\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}+$ missing transverse energy in the lepton- and muon-specific versions of the framework, respectively. Our analysis reveals that the ${e}^{+}{e}^{\ensuremath{-}}$ machine operating at a $3000\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$ luminosity predicts a $5\ensuremath{\sigma}$ discovery of a pseudoscalar as heavy as 400 GeV. Comparing with the companion paper, it is concluded that the International Linear Collider is a much more potent machine than the LHC in this regard.

Heavy neutral leptons at beam dump experiments of future lepton colliders

A new beam dump experiment that utilizes the beam of future high energy electron-positron colliders could be an excellent avenue to search for dark sector particles due to its unprecedented high energy and intensity. We consider heavy neutral leptons (HNLs) as a specific example to demonstrate the sensitivity of searches for dark sector particles at future electron-positron collider beam dump experiments. This includes the study of the reach at the International Linear Collider (ILC), the Cool Copper Collider (C3), and the Compact Linear Collider (CLIC). We comprehensively examine the HNL production and detector acceptance at these electron beam dump experiments. We show that these experiments will probe regions of HNL parameter space, not yet probed by past experiments, as well as by future approved experiments. Our study also motivates a more detailed analysis of heavy meson productions in high-energy electron-nucleon collisions in thick targets.

Description and stability of a RPC-based calorimeter in electromagnetic and hadronic shower environments

The CALICE Semi-Digital Hadron Calorimeter technological prototype completed in 2011 is a sampling calorimeter using Glass Resistive Plate Chamber (GRPC) detectors as the active medium. This technology is one of the two options proposed for the hadron calorimeter of the International Large Detector for the International Linear Collider. The prototype was exposed in 2015 to beams of muons, electrons, and pions of different energies at the CERN Super Proton Synchrotron. The use of this technology for future experiments requires a reliable simulation of its response that can predict its performance. GEANT4 combined with a digitization algorithm was used to simulate the prototype. It describes the full path of the signal: showering, gas avalanches, charge induction, and hit triggering. The simulation was tuned using muon tracks and electromagnetic showers for accounting for detector inhomogeneity and tested on hadronic showers collected in the test beam. This publication describes developments of the digitization algorithm. It is used to predict the stability of the detector performance against various changes in the data-taking conditions, including temperature, pressure, magnetic field, GRPC width variations, and gas mixture variations. These predictions are confronted with test beam data and provide an attempt to explain the detector properties. The data-taking conditions such as temperature and potential detector inhomogeneities affect energy density measurements but have small impact on detector efficiency.

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.