[en] Using recent experimental measurements on Br(b→sγ) from CLEO, we study the constraints on the charged Higgs sector in various three-Higgs-doublet models. Some phenomenological implications in these models with emphasis on CP violation are presented. In particular, in some of these models, the CP violating muon polarization in K_μ3 can be detected using the current KEK experiment E246. (author)

[en] We discuss the parity-violating left-right asymmetries (LRAs) in Moeller scattering at the International Linear Collider (ILC) induced by doubly charged Higgs bosons in models with SU(2)_L triplet and singlet scalar bosons, which couple to the left- and right-handed charged leptons, respectively. These bosons are important in scenarios for the generation of the neutrino mass. We demonstrate that the contributions to the LRAs from the triplet and singlet bosons are opposite to each other. In particular, we show that the doubly charged Higgs boson from the singlet scalar can be tested at the ILC by using the resonance effect. (orig.)

[en] Despite the direct observation of top-like signals at the Tevatron and the CLEO measurement of (2.32±0.57±0.35)x10^-4 for inclusive b→sγ decay, we show that a light top quark (with m_t≤M_W) in supersymmetric-type two Higgs doublet models is not yet fully excluded. This is in large part due to theoretical uncertainties such as the scale dependence for QCD corrections, which cannot be resolved without complete knowledge of next-to-leading order effects. Combined with the direct search for t→bH⁺ via H⁺→τ⁺ν at the Tevatron, a window exists around tanβ∼1 for m_H⁺t< M_W. The decay t→bH⁺ should certainly be searched for at LEP-II. (author)

[en] We study the observational constraints on the exponential gravity model of f(R)=-βR_s(1-e^-R/R_s). We use the latest observational data including Supernova Cosmology Project Union2 compilation, Two-Degree Field Galaxy Redshift Survey, Sloan Digital Sky Survey Data Release 7, and Seven-Year Wilkinson Microwave Anisotropy Probe in our analysis. From these observations, we obtain a lower bound on the model parameter β at 1.27 (95% C.L.) but no appreciable upper bound. The constraint on the present matter density parameter is 0.245<Ω_m⁰<0.311 (95% C.L.). We also find out the best-fit value of model parameters on several cases.

[en] We investigate thermodynamics of the apparent horizon in f(R) gravity in the Palatini formalism with non-equilibrium and equilibrium descriptions. We demonstrate that it is more transparent to understand the horizon entropy in the equilibrium framework than that in the non-equilibrium one. Furthermore, we show that the second law of thermodynamics can be explicitly verified in both phantom and non-phantom phases for the same temperature of the universe outside and inside the apparent horizon

[en] We explore thermodynamics of the apparent horizon in f(T) gravity with both equilibrium and non-equilibrium descriptions. We find the same dual equilibrium/non-equilibrium formulation for f(T) as for f(R) gravity. In particular, we show that the second law of thermodynamics can be satisfied for the universe with the same temperature outside and inside the apparent horizon

[en] We analyze the stability of the Einstein static closed and open universe in two types of exponential f(T) gravity theory. We show that stable solutions exist in these two models. In particular, we find that large regions of parameter space in equation of state w=p/ρ for the stable universe are allowed in the f(T) theories. (orig.)

[en] We explore the cosmological evolution in the exponential gravity f(R) = R+c₁(1−e^−c₂R) (c_1,2 = constant). We summarize various viability conditions and explicitly demonstrate that the late-time cosmic acceleration following the matter-dominated stage can be realized. We also study the equation of state for dark energy and confirm that the crossing of the phantom divide from the phantom phase to the non-phantom (quintessence) one can occur. Furthermore, we illustrate that the cosmological horizon entropy globally increases with time

[en] We study gravitational waves in viable f(R) theories under a non-zero background curvature. In general, an f(R) theory contains an extra scalar degree of freedom corresponding to a massive scalar mode of gravitational wave. For viable f(R) models, since there always exits a de-Sitter point where the background curvature in vacuum is non-zero, the mass squared of the scalar mode of gravitational wave is about the de-Sitter point curvature R_d ∼ 10⁻⁶⁶eV². We illustrate our results in two types of viable f(R) models: the exponential gravity and Starobinsky models. In both cases, the mass will be in the order of 10⁻³³eV when it propagates in vacuum. However, in the presence of matter density in galaxy, the scalar mode can be heavy. Explicitly, in the exponential gravity model, the mass becomes almost infinity, implying the disappearance of the scalar mode of gravitational wave, while the Starobinsky model gives the lowest mass around 10⁻²⁴eV, corresponding to the lowest frequency of 10⁻⁹ Hz, which may be detected by the current and future gravitational wave probes, such as LISA and ASTROD-GW

[en] We investigate the scalar-torsion mode in a cosmological model of the Poincaré gauge theory of gravity. We treat the geometric effect of torsion as an effective quantity, which behaves like dark energy, and study the effective equation of state (EoS) of the model. We concentrate on two cases with the constant curvature solution and positive kinetic energy, respectively. In the former, we find that the torsion EoS has different values in the various stages of the universe. In particular, it behaves like the radiation (matter) EoS of w_r = 1/3 (w_m = 0) in the radiation (matter) dominant epoch, while in the late time the torsion density is supportive for the accelerating universe. In the latter, our numerical analysis shows that in general the EoS has an asymptotic behavior in the high redshift regime, while it could cross the phantom divide line in the low redshift regime