International Linear Collider Research Articles

Probing a degenerate-scalar scenario in a pseudoscalar dark-matter model

We study a pseudoscalar dark-matter model arising from a complex singlet extension of the standard model (SM), and show that the dark-matter--nucleon scattering is suppressed when two CP-even scalars are degenerate. In such a degenerate-scalar scenario we explore the model parameter space which satisfies constraints from the direct detection experiments and the relic density of dark matter. In addition, we discuss a possibility to verify such a scenario by using the recoil mass technique at the International Linear Collider. We find that a pair of states separated by 0.2 GeV can be distinguished from the single SM-like Higgs state at 5$\sigma$ with integrated luminosity of 2 ab$^{-1}$.

Model-independent study on the anomalous [formula omitted] couplings at the ILC

The potential of the process γγ→ττ¯γ is examined in a model-independent way using the effective Lagrangian approach for the International Linear Collider, a proposed electron-positron machine which is designed with standard configurations of 0.25 TeV/2000 fb−1, 0.35 TeV/200 fb−1 and 0.5 TeV/4000 fb−1. The limits obtained for the tau lepton's anomalous magnetic dipole moment a˜τ and the tau lepton's anomalous electric dipole moment d˜τ defining the anomalous ττ¯γ couplings at 95% confidence level and systematic uncertainties of δsys=0,5,10% are compared with the experimental results. The best limits obtained with δsys=0% on the anomalous couplings are −0.00082<a˜τ<0.00050 and |dτ˜|<3.59×10−18ecm, respectively. Also, the limits on the anomalous couplings do not improve proportionately to the luminosity due to the systematic error considered here. The reason of this situation is the systematic error which is much bigger than the statistical error. If the systematic error is improved, we expect better limits on the couplings. However, the best limits on the anomalous couplings for the process γγ→ττ¯γ with s=500 GeV, Lint=4000 fb−1 and δsys=0% can be improved up to 4 times for a˜τ and 9 times for d˜τ according to case with δsys=10%. Our results show that γγ collisions at the ILC give results up to one order of magnitude for a˜τ and two orders of magnitude for d˜τ better than the current experimental limits.

Search for the anomalous WWγ couplings through the process [formula omitted] at ILC with unpolarized and polarized beams

We investigate the anomalous W+W−γ couplings through the process e−e+→νeν‾eγ for unpolarized and polarized electron (positron) beams at the International Linear Collider. We give the 95% Confidence Level limits on the anomalous couplings with and without the systematic uncertainties for various values of center-of-mass energies and the integrated luminosities. We show that the obtained limits on the anomalous couplings through the process e−e+→νeν‾eγ can highly improve the current experimental limits.

Kinematic edge detection using finite impulse response filters

Various physics observables can be determined from the localization of distinct edge-like features in distributions of measurement values. In this paper, we address the observation that neither differentiating nor fitting the measured distributions is robust against significant fluctuations in the experimental data. We propose the application of Finite Impulse Response (FIR) filters instead. To demonstrate the method, we consider the typical case in particle physics in which the precise localization of kinematic edges, often blurred by e.g. background contributions and detector effects, is crucial for determining particle masses. We show that even for binned data, typical for high energy physics, the optimal FIR filter kernel can be approximated by the first derivative of a Gaussian (FDOG). We study two highly complementary supersymmetric scenarios that, if realized in nature, could be observed at a future high-energy e+e− collider such as the International Linear Collider (ILC) or the Compact Linear Collider (CLIC). The first scenario considers the production of ẽ±−pairs while the second focuses on the χ̃1± and χ̃20-pair production. We demonstrate that the FIR filter method for edge extraction is superior to previously employed methods in terms of robustness and precision.

CP -violating Higgs boson ditau decays: Baryogenesis and Higgs factories

We demonstrate how probes of $CP$-violating observables in Higgs boson ditau decays at prospective future lepton colliders could provide a test of weak scale baryogenesis with significant discovery potential. Measurements at the Circular Electron Positron Collider, for example, could exclude a $CP$ phase larger than 2.9\ifmmode^\circ\else\textdegree\fi{} (5.6\ifmmode^\circ\else\textdegree\fi{}) at 68% (95%) C.L. assuming the Standard Model value for magnitude of the tau lepton Yukawa coupling. Conversely, this sensitivity would allow for a $5\ensuremath{\sigma}$ discovery for 82% of the $CP$ phase range $[0,2\ensuremath{\pi})$. The reaches of the Future Circular Collider for electrons and positrons (FCC-ee) and International Linear Collider are comparable. As a consequence, future lepton colliders could establish the presence of $CP$ violation required by lepton flavored electroweak baryogenesis with at least $3\ensuremath{\sigma}$ sensitivity. Our results illustrate that Higgs factories are not just precision machines, but can also make $\mathcal{O}(1)$ measurement of the new physics beyond the Standard Model.

The International Linear Collider Project—Its Physics and Status

The discovery of Higgs particle has ushered in a new era of particle physics. Even though the list of members of the standard theory of particle physics is now complete, the shortcomings of the theory became ever more acute. It is generally considered that the best solution to the problems is an electron–positron collider that can study Higgs particle with high precision and high sensitivity; namely, a Higgs factory. Among a few candidates for Higgs factory, the International Linear Collider (ILC) is currently the most advanced in its program. In this article, we review the physics and the project status of the ILC including its energy expandability.

Testing aligned CP-violating Higgs sector at future lepton colliders

We discuss the testability of CP-violating phases at future lepton colliders for the scenario which satisfies electric dipole moment data by destructive interferences among several phases. We consider the general but aligned two Higgs doublet model which has the CP-violating phases in the Higgs potential and the Yukawa interaction. The Yukawa interaction terms are aligned to avoid flavor changing neutral currents at tree level. The Higgs potential is also aligned such that the coupling constants of the lightest Higgs boson with the mass of 125 GeV to the Standard Model (SM) particles are the same as those of the SM at tree level. We investigate the azimuthal angle distribution of the hadronic decay of tau leptons arising from production and decay of the extra Higgs bosons, which contains information of the CP-violating phases. From the signal and background simulation, we find that the scenario with finite CP-violating phases can be distinguished from CP conserving one at future lepton colliders like the International Linear Collider.

Probing extended Higgs sectors by the synergy between direct searches at the LHC and precision tests at future lepton colliders

We discuss a possibility that the parameter space of the two Higgs doublet model is significantly narrowed down by considering the synergy between direct searches for additional Higgs bosons at the LHC and its luminosity upgraded operation and precision measurements of the Higgs boson properties at future electron-positron colliders such as the International Linear Collider. We show that, in the case where the coupling constants of the discovered Higgs boson are slightly different from the predicted values in the standard model, most of the parameter space is explored by the direct searches of extra Higgs bosons, in particular for the decays of the extra Higgs bosons into the discovered Higgs boson, and also by the theoretical arguments such as perturbative unitarity and vacuum stability. This can be done because there appears an upper limit on the mass of the extra Higgs bosons as long as the deviation exists in the Higgs boson coupling. We also show that in the alignment limit where all the Higgs boson couplings take the standard model like values most of the parameter space cannot be excluded because most of the Higgs to Higgs decays are suppressed and also there is no upper limit on the masses from the theoretical arguments.

Searching for new light particles at the international linear collider main beam dump

We perform a feasibility study of a beam dump experiment at the International Linear Collider (ILC). To investigate the sensitivity to new light particles at the experiment, we consider models for axion-like particles (ALPs) and a light scalar particle coupled to charged leptons. For both models, we show that the detection sensitivity is almost an order of magnitude higher than other beam dump experiments in the small coupling region. For ALPs, it is shown that the ILC beam dump experiment is highly complementary to bounds from astrophysics. In addition, for the model of the scalar particle, the region favored by the muon $g-2$ experiment can be explored.

Tau g{-}2 at e^-e^+ colliders with momentum-dependent form factor

The deviation between the prediction based on the standard model and the measurement of the muon g{-}2 is currently at 3{-}4 sigma . If this discrepancy is attributable to new physics, it is expected that the new contributions to the tau g{-}2 even larger than those of muon due to its large mass. However, it is much more difficult to directly measure the tau g{-}2 because of its short lifetime. In this report, we consider the effect of the tau g{-}2 at e^-e^+ colliders using a model independent approach. Using the tau pair production channel at the Large Electron Position Collider (LEP), we have determined the allowed range for the new physics contribution of the tau g{-}2 assuming a q-square-dependence ansatz for the magnetic form factor. We also investigated the prospect at future e^+e^- colliders, such as International Linear Collider, the Compact Linear Collider, the Future Circular e^+e^- Collider, and Circular Electron Positron Collider, and determined the expected allowed range for the new physics contribution to the tau anomalous magnetic moment. The best limits are about 4{-}5 times more severe than the LEP one due to the beam polarization and the high luminosities at future colliders.

Dark matter nature at electron-positron colliders: scalar or fermionic?

In this work, we investigated the possibility of identifying DM nature, at the future electron-positron colliders such as the International Linear Collider (ILC) and Compact Linear Collider (CLIC), at and 1 TeV, using the final state . For this purpose, we consider two models in which DM could be either a real scalar or a heavy right-handed neutrino (RHN). So we considered two parameter values sets for both models, and we defined and investigated the different recent experimental constraints. After that we define a set of kinematical cuts that suppress the background, and generate different distributions that are useful in identifying the DM nature. The use of polarized beams P(e −, e +) = [+0.8,−0.3] at the ILC makes the signal detection easier and the DM identification more clear, where the statistical significance gets enhanced by twice (five times) for scalar (fermionic) DM.

Main parameters of SppC-based “linac-ring eA” and “ring-ring µA” colliders

Concerning future lepton-nucleus colliders, International Linear Collider (ILC) and Plasma Wake Field Accelerator-Linear Collider (PWFA-LC) electrons in the order of 0.5 TeV and 5 TeV, respectively, colliding with Super proton–proton Collider (SppC)’s lead ions, are considered as “linac-ring eA” options. In addition, the 1.5 TeV option of the Muon Collider (MC) vs. 208Pb82+ ions of the SppC is also taken into account as a “ring-ring µA” collider. Luminosity values of the SppC-based eA and µA colliders are estimated. 3075 TeV lead parameters for the 100 km-circumference option of the SppC are taken into account to optimize luminosities of electron–nucleus and muon–nucleus collisions, keeping beam–beam effects and disruption in mind. It is shown that luminosities around 1030 cm−2 s−1 and 1032 cm−2 s−1 for eA and µA colliders, respectively, can be achieved by moderate upgrades of lepton and nucleus beam parameters.

Prospects of measuring the branching fraction of the Higgs boson decaying into muon pairs at the International Linear Collider

The prospects for measuring the branching fraction of H rightarrow mu ^{+} mu ^{-} at the International Linear Collider (ILC) have been evaluated based on a full detector simulation of the International Large Detector (ILD) concept, considering centre-of-mass energies (sqrt{s}) of 250 GeV and 500 GeV with two different beam polarisation configurations of {mathcal {P}} (e^{-}, e^{+}) = (-,80{%}, +,30{%}) and (+,80{%}, -,30{%}). For both sqrt{s} cases, the two final states e^{+} e^{-} rightarrow qoverline{q}H and e^{+} e^{-} rightarrow nu overline{nu }H have been analyzed. For integrated luminosities of 2 hbox {ab}^{-1} at sqrt{s} =250 GeV and 4 hbox {ab}^{-1} at sqrt{s} =500 GeV, the combined precision on the branching fraction of H rightarrow mu ^{+} mu ^{-} is estimated to be 17%. The impact of the transverse momentum resolution for this analysis is also studied.

Upgrade of the ILC Cryomodule

Results of testing modified components for the cryomodule of the International Linear Collider (ILC) are summarized. To reduce the ILC project cost, it is proposed to replace titanium cryomodule components with stainless steel (SS) ones. New bimetallic transitions Ti_SS, Nb_SS have been produced by a unique method based on explosion welding. Successive upgrading of these components to the latest version of the Nb/Ti/SS transition element has led to improvement of the ILC cryomodule. This new component resolves problems of residual stress, and its specific design prevents the possibility of a shift due to the difference in the linear expansion coefficients of the constituent metals. Leak tests with the He gas revealed no leaks at the background rate of 0.2x10-10atmxcc-1s. The test results are very encouraging. The up-to-date design of trimetallic Nb_Ti_SS element promises technologically simpler and less expensive manufacture. Investigations have shown that explosion welding allows unique trimetallic components to be made not only for cryogenic units of accelerators but also for laboratory equipment and for general engineering applications.

HL-LHC and ILC sensitivities in the hunt for heavy Higgs bosons

The prediction of additional Higgs bosons is one of the key features of physics beyond the Standard Model (SM) that gives rise to an extended Higgs sector. We assess the sensitivity of the Large Hadron Collider (LHC) in the high luminosity (HL) run alone and in combination with a possible future International Linear Collider (ILC) to probe heavy neutral Higgs bosons. We employ the Minimal Supersymmetric Standard Model (MSSM) as a framework and assume the light mathcal {CP}-even MSSM Higgs boson to be the Higgs boson observed at 125,mathrm{GeV}. We discuss the constraints on the MSSM parameter space arising from the precision measurements of the rates of the detected signal at 125,mathrm{GeV} and from direct searches for new heavy Higgs bosons in the tau ^+tau ^-, bbar{b} and di-Higgs (hh) final states. A new benchmark scenario for heavy Higgs searches in the bbar{b} channel is proposed in this context. For the future Higgs rate measurements at the HL-LHC and ILC two different scenarios are investigated, namely the case where the future rate measurements agree with the SM prediction and the case where the rates agree with the predictions of possible realizations of the MSSM Higgs sector in nature.

NLO QCD corrections to exclusive quarkonium-pair production in photon\u2013photon collision

We calculate the next-to-leading order (NLO) quantum chromodynamics (QCD) corrections to the exclusive processes gamma +gamma rightarrow {mathcal {Q}}+{mathcal {Q}}, with {mathcal {Q}}=J/psi , eta _c, Upsilon , or eta _b, in the framework of non-relativistic QCD (NRQCD) factorization formalism. The cross sections at the SuperKEKB electron–positron collider, as well as at the future colliders, like the circular electron positron collider (CEPC) and the international linear collider (ILC), are evaluated. Numerical result indicates that the processes for J/psi -pair production and eta _c-pair production are hopefully observable at the Belle II detector within the next decade.

Echoes of 2HDM inflation at the collider experiments

We study the correlation between the constraints on general two Higgs doublet model from Higgs inflation and from collider experiments. The parameter space receives meaningful constraints from direct searches at the large hadron collider and from flavor physics if m_H, m_A, and m_{H^pm } are in the sub-TeV range, where H, A, and H^pm are the CP even, CP odd, and charged Higgs bosons, respectively. We find that in the parameter region favored by the Higgs inflation, H, A, and H^pm are nearly degenerate in mass. We show that such near degeneracy can be probed directly in the upcoming runs of the Large Hadron Collider, while the future lepton colliders such as the International Linear Collider and the future circular collider would provide complementary probes.

Development of a compact Local Power Distribution System for the ILC

The Local Power Distribution System (LPDS) of the International Linear Collider (ILC) transmits RF power from a 10 MW klystron to 39 cavities. A variable hybrid and variable phase shifter are adopted in the LPDS to have been adopted in the LPDS to achieve the efficient beam acceleration in the ILC. As it is difficult to construct the LPDS in an accelerator tunnel, the LPDS can be assembled on a cryomodule on the ground. Then, the cryomodule integrated with the LPDS can be installed in the tunnel. Furthermore, production cost can be decreased by utilizing identical variable hybrids and variable phase shifters. In this study, a compact LPDS was developed to satisfy these two requirements. A compact variable hybrid and variable phase shifter were designed for a compact LPDS. The power transmitted to a cavity was adjusted by the variable hybrid to achieve the maximal accelerating field, which exhibited a variation of ±20% depending on cavity performance. The variable phase shifter compensated the phase shift of the variable hybrid. Fixed phase shifters were designed to adjust the phase difference between adjacent cavities for beam acceleration. These RF components were manufactured, and the S parameters and power handling capability of the components were tested. An L-band resonant ring was constructed at the Super-conducting RF Test Facility (STF) at KEK. The power handling capability of these RF components were tested using a power of 5.2 MW with a pulse width of 200 μs and a power of 2.0 MW with a pulse width of 2 ms. The S parameters and power handling capability satisfied the requirements of the compact LPDS. The compact LPDS will be constructed using these RF components and demonstrated at the STF.

Development of monolithic SOI pixel sensors capable of fine measurements of space and time

Silicon-on-insulator (SOI) for fine measurement of space and time (SOFIST) is designed taking full advantage of advanced SOI monolithic pixel detector fabrication technology to record the charge and time information of hit pixels. The aim is to achieve 3 μm position resolution and ultimately identify the International Linear Collider beam bunches of 554 ns separation. SOFIST Ver. 3 includes full circuits to read out the charge and time for each pixel of size 30×30μm. Its performance is evaluated using a 120 GeV proton beam. The time resolution is evaluated to be 1.92 μs (including calibration error), with an intrinsic time resolution of 1.34–1.55 μs. Additionally, multi-memory readout functionality is successfully examined. With SOFIST Ver. 4, the same functionality is to be realized with a 20×20μm pixel size by 3D stacking. The first chip was successfully tested to image β rays. The connection yield of Au micro-bumps is greater than 99.9%.

New pixel detector concept DuTiP for Belle II upgrade and the ILC with an SOI technology

Belle II detector upgrade is being discussed aiming to collect five times larger integrated luminosity of 250 ab−1. The beam background level is expected five times higher than current design, thus a new pixel vertex detector with faster readout should be developed. We have invented a new pixel detector concept DuTiP for the Belle II upgrade which can be also used for the International Linear Collider (ILC) with small modifications. To realize the DuTiP concept, an SOI technology is chosen as a baseline with a pixel size of 35μm×35μm. The DuTiP concept and its application to a monolithic pixel detector in an SOI technology are explained.