The $$R^2$$-Higgs inflation with two Higgs doublets

We study $$R^2$$ R 2 -Higgs inflation in a model with two Higgs doublets in which the Higgs sector of the Standard Model is extended by an additional Higgs doublet, thereby four scalar fields are involved in the inflationary evolutions. We first derive the set of equations required to follow the inflationary dynamics in this two Higgs doublet model, allowing a nonminimal coupling between the Higgs-squared and the Ricci scalar R, as well as the $$R^2$$ R 2 term in the covariant formalism. By numerically solving the system of equations, we find that, in parameter space where a successful $$R^2$$ R 2 -Higgs inflation are realized and consistent with low energy constraints, the inflationary dynamics can be effectively described by a single slow-roll formalism even though four fields are involved in the model. We also argue that the parameter space favored by $$R^2$$ R 2 -Higgs inflation requires nearly degenerate masses for $$m_{\mathsf {H}}$$ m H , $$m_A$$ m A and $$m_{{\mathsf {H}}^{\pm }}$$ m H ± , where $${\mathsf {H}}$$ H , A, and $${\mathsf {H}}^{\pm }$$ H ± are the extra CP even, CP odd, and charged Higgs bosons in the general two Higgs doublet model taking renormalization group evolutions of the parameters into account. Discovery of such heavy scalars at the Large Hadron Collider (LHC) are possible if they are in the sub-TeV mass range. Indirect evidences may also emerge at the LHCb and Belle-II experiments, however, to probe the quasi degenerate mass spectra one would likely require high luminosity LHC or future lepton colliders such as the International Linear Collider and the Future Circular Collider.

Higgs production in association with a dark-Z at future electron positron colliders

In recent years there have been many proposals for new electron-positron colliders, such as the Circular Electron-Positron Collider, the International Linear Collider, and the Future Circular Collider in electron-positron mode. Much of the motivation for these colliders is precision measurements of the Higgs boson and searches for new electroweak states. Hence, many of these studies are focused on energies above the h Z threshold. However, there are proposals to run these colliders at the lower WW threshold and Z-pole energies. In this paper, we study a new search for Higgs physics accessible at lower energies: e+e− → h Zd, where Zdis a new light gauge boson such as a dark photon or dark-Z. Such searches can be conducted at the WW threshold, i.e. energies below the h Z threshold where exotic Higgs decays can be searched for in earnest. Additionally, due to very good angular and energy resolution at future electron-positron colliders, these searches will be sensitive to Zd masses below 1 GeV, which is lower than the current direct LHC searches. We will show that at √s = 160 GeV with 10 ab−1, a search for e+e− → h Zd is sensitive to h −Z −Zd couplings of δ ∼ 9 × 10−3and cross sections of ∼ 2 − 3 ab for Zd masses below 1 GeV. The results are similar at √s = 240 GeV with 5 ab−1.

Next-to-leading-order corrections to the Higgs strahlung process from electron–positron collisions in extended Higgs models

We present the cross section for $$e^{+}e^{-}\rightarrow hZ$$ e + e - → h Z with arbitrary sets of electron and Z boson polarizations at the full next-to-leading order in various extended Higgs models, such as the Higgs singlet model (HSM), the inert doublet model (IDM) and the two Higgs doublet model (2HDM). We systematically perform complete one-loop calculations to the helicity amplitudes in the on-shell renormalization scheme, and present the full analytic results as well as numerical evaluations. The deviation $$\Delta R^{hZ}$$ Δ R hZ in the total cross section from its standard model (SM) prediction is comprehensively analyzed, and the differences among these models are discussed in details. We find that new physics effects appearing in the renormalized hZZ vertex almost govern the behavior of $$\Delta R^{hZ}$$ Δ R hZ , and it takes a negative value in most cases. The possible size of $$\Delta R^{hZ}$$ Δ R hZ reaches several percent under the theoretical and experimental bounds. We also analyze the deviation $$\Delta R^{hZ}_{XY}$$ Δ R XY hZ in the total cross section times decay branching ratios of the discovered Higgs boson by utilizing the program. It is found that the four types of 2HDMs can be discriminated by analyzing the correlation between $$\Delta R^{hZ}_{\tau \tau }$$ Δ R τ τ hZ and $$\Delta R^{hZ}_{bb}$$ Δ R bb hZ and those between $$\Delta R^{hZ}_{\tau \tau }$$ Δ R τ τ hZ and $$\Delta R^{hZ}_{cc}$$ Δ R cc hZ . Furthermore, the HSM and the IDM can be discriminated from the 2HDMs by measuring $$\Delta R^{hZ}_{WW}$$ Δ R WW hZ . These signatures can be tested by precision measurements at future Higgs factories such as the International Linear Collider.

Sensitivity of future linear $$\hbox {e}^+\hbox {e}^-$$ colliders to processes of dark matter production with light mediator exchange

As any $$e^+e^-$$ e + e - scattering process can be accompanied by a hard photon emission from the initial state radiation, the analysis of the energy spectrum and angular distributions of those photons can be used to search for hard processes with an invisible final state. Thus high energy $$e^+e^-$$ e + e - colliders offer a unique possibility for the most general search of dark matter (DM) based on the mono-photon signature. We consider production of DM particles at the International Linear Collider (ILC) and Compact Linear Collider (CLIC) experiments via a light mediator exchange. Detector effects are taken into account within the Delphes fast simulation framework. Limits on the light DM production in a simplified model are set as a function of the mediator mass and width based on the expected two-dimensional distributions of the reconstructed mono-photon events. The experimental sensitivity is extracted in terms of the DM production cross section. Limits on the mediator couplings are then presented for a wide range of mediator masses and widths. For light mediators, for masses up to the centre-of-mass energy of the collider, coupling limits derived from the mono-photon analysis are more stringent than those expected from direct resonance searches in decay channels to SM particles.

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.

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 $$g{-}2$$ g - 2 is currently at $$3{-}4 \sigma $$ 3 - 4 σ . If this discrepancy is attributable to new physics, it is expected that the new contributions to the tau $$g{-}2$$ g - 2 even larger than those of muon due to its large mass. However, it is much more difficult to directly measure the tau $$g{-}2$$ g - 2 because of its short lifetime. In this report, we consider the effect of the tau $$g{-}2$$ g - 2 at $$e^-e^+$$ 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 $$g{-}2$$ g - 2 assuming a q-square-dependence ansatz for the magnetic form factor. We also investigated the prospect at future $$e^+e^-$$ e + e - colliders, such as International Linear Collider, the Compact Linear Collider, the Future Circular $$e^+e^-$$ 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 $$4{-}5$$ 4 - 5 times more severe than the LEP one due to the beam polarization and the high luminosities at future colliders.

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 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, 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 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^-2s^-1 for eA and 10³² cm^-2s^-1 for µA colliders, can be achieved by moderate upgrades of lepton and nucleus beam parameters.