We discuss complementarity of discovery reaches of heavier neutral Higgs bosons and charged Higgs bosons at the LHC and the International Linear Collider (ILC) in two Higgs doublet models (2HDMs). We perform a comprehensive analysis on their producti
on and decay processes for all types of Yukawa interaction under the softly-broken discrete symmetry which is introduced to avoid flavour changing neutral currents, and we investigate parameter spaces of discovering additional Higgs bosons at the ILC beyond the LHC reach. We find that the 500 GeV run of the ILC with the integrated luminosity of 500 fb^{-1} shows an advantage for discovering the additional Higgs bosons in the region where the LHC cannot discover them with the integrated luminosity of 300 fb^{-1}. For the 1 TeV run of the ILC with the integrated luminosity of 1 ab^{-1}, production processes of an additional Higgs boson associated with the top quark can be useful as discovery channels in some parameter spaces where the LHC with the integrated luminosity of 3000 fb^{-1} cannot reach. It is emphasized that the complementary study at the LHC and the ILC is useful not only to survey additional Higgs bosons at the TeV scale, but also to discriminate types of Yukawa interaction in the 2HDM.
In the two Higgs doublet model, $tanbeta$ is an important parameter, which is defined as the ratio of the vacuum expectation values of the doublets. We study how accurately $tanbeta$ can be determined at linear colliders via the precision measurement
of the decay branching fraction of the standard model (SM) like Higgs boson. Since the effective coupling constants of the Higgs boson with the weak gauge bosons are expected to be measured accurately, the branching ratios can be precisely determined. Consequently, $tanbeta$ can be determined with a certain amount of accuracy. Comparing the method to those using direct production of the additional Higgs bosons, we find that, depending on the type of Yukawa interactions, the precision measurement of the decay of the SM-like Higgs boson can be the best way to determine $tanbeta$, when the deviations in the coupling constants with the gauge boson from the SM prediction are observed at linear colliders.
We study the feasibility of the Type-X two Higgs doublet model (THDM-X) at collider experiments. In the THDM-X, new Higgs bosons mostly decay into tau leptons in the wide range of the parameter space. Such scalar bosons are less constrained by curren
t experimental data, because of the suppressed quark Yukawa interactions. We discuss a search strategy of the THDM-X with multi-tau lepton final states at International linear collider and Large Hadron Collider. By using the collinear approximation, we show that a four tau lepton signature (e^+e^- -> HA -> 4tau) can be a clean signal.
We study an upper bound on masses of additional scalar bosons from the electroweak precision data and theoretical constraints such as perturbative unitarity and vacuum stability in the two Higgs doublet model taking account of recent Higgs boson sear
ch results. If the mass of the Standard-Model-like Higgs boson is rather heavy and is outside the allowed region by the electroweak precision data, such a discrepancy should be compensated by contributions from the additional scalar bosons. We show the upper bound on masses of the additional scalar bosons to be about 2 $(1)$ TeV for the mass of the Standard-Model-like Higgs boson to be 240 $(500)$ GeV.
We study Higgs boson pair production processes at future hadron and lepton colliders including the photon collision option in several new physics models; i.e., the two-Higgs-doublet model, the scalar leptoquark model, the sequential fourth generation
fermion model and the vector-like quark model. Cross sections for these processes can deviate significantly from the standard model predictions due to the one-loop correction to the triple Higgs boson coupling constant. For the one-loop induced processes such as $gg to hh$ and $gammagammato hh$, where $h$ is the (lightest) Higgs boson and $g$ and $gamma$ respectively represent a gluon and a photon, the cross sections can also be affected by new physics particles via additional one-loop diagrams. In the two-Higgs-doublet model and scalar leptoquark models, cross sections of $e^+e^-to hhZ$ and $gammagammato hh$ can be enhanced due to the non-decoupling effect in the one-loop corrections to the triple Higgs boson coupling constant. In the sequential fourth generation fermion model, the cross section for $ggto hh$ becomes very large because of the loop effect of the fermions. In the vector-like quark model, effects are small because the theory has decoupling property. Measurements of the Higgs boson pair production processes can be useful to explore new physics through the determination of the Higgs potential.
We studied a feasibility of measuring Higgs boson pair production in a Photon Linear Collider. The optimum energy of gamma gamma collision was estimated with a realistic luminosity distribution. We also discussed simulation study for detecting the signal against W boson pair backgrounds.
We study the impact of dimension-six operators on single- and double-Higgs production rates via gluon fusion at the Large Hadron Collider (LHC). If the top-Yukawa coupling is modified by some new physics whose scale is of the TeV scale, its effect ch
anges the cross sections of single-Higgs production $ggto H$ and double-Higgs production $ggto HH$ through the top-loop diagram. In particular, double-Higgs production can receive significant enhancement from the effective top-Yukawa coupling and the new dimension-five coupling $t{bar t}HH$ which are induced by the dimension-six operator. Comparing these results to the forthcoming data at the LHC, one can extract information of the dimension-six operators relevant to the top quark and the Higgs boson.
We propose an extended version of the standard model, in which neutrino oscillation, dark matter, and baryon asymmetry of the Universe can be simultaneously explained by the TeV-scale physics without assuming unnatural hierarchy among the mass scales
. Tiny neutrino masses are generated at the three loop level due to the exact $Z_2$ symmetry, by which stability of the dark matter candidate is guaranteed. The extra Higgs doublet is required not only for the tiny neutrino masses but also for successful electroweak baryogenesis. The model provides discriminative predictions especially in Higgs phenomenology, so that it is testable at current and future collider experiments.
We calculate the cross section of Higgs boson pair production at a photon collider in the two Higgs doublet model. We focus on the scenario in which the lightest CP even Higgs boson ($h$) has the standard model like couplings to the gauge bosons. We
take into account the one-loop correction to the $hhh$ coupling as well as additional one-loop diagrams due to charged Higgs bosons to the $gammagammato hh$ helicity amplitudes. It is found that the full cross section can be enhanced by both these effects to a considerable level. We discuss the impact of these corrections on the $hhh$ coupling measurement at the photon collider.
We study bounds on Higgs boson masses from perturbative unitarity in the Georgi-Machacek model, whose Higgs sector is composed of a scalar isospin doublet, a real and a complex isospin triplet fields. This model can be compatible with the electroweak
precision data without fine tuning because of the imposed global SU(2)_R symmetry in the Higgs potential, by which the electroweak rho parameter is unity at the tree level. All possible two-body elastic-scattering channels are taken into account to evaluate the S-wave amplitude matrix, and then the condition of perturbative unitarity is imposed on the eigenvalues to obtain constraint on the Higgs parameters. Masses of all scalar bosons turn out to be bounded from above, some of which receive more strict upper bounds as compared to that in the standard model (712 GeV). In particular, the upper bound of the lightest scalar boson, whatever it would be, is about 270 GeV.
