Higgs boson pair production in new physics models at hadron, lepton, and photon colliders

We discuss 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 vectorlike quark model. Cross sections for these processes can differ significantly from the standard model predictions, due to the nondecoupling effect in the triple Higgs boson coupling constant. For the one-loop induced processes such as gg → hh and γγ → hh, new physics particles can mediate in a loop diagram, which can affect cross sections. Measurements of the Higgs boson pair production processes can be useful not only to test the standard model but also to explore new physics models

Yukawa Matrix for the Neutrino and Lepton Flavour Violation

We estimate the magnitude of Lepton Flavour Violation (LFV) from the phase of the neutrino Yukawa matrix. In the minimal supersymmetric standard model with right-handed neutrinos, the LFV processes l_i \to l_j \gamma can appear through the slepton mixing, which comes from the renormalization group effect on the right-handed neutrino Yukawa interaction between the Grand Unified Theory scale and the heavy right-handed neutrino mass scale. Two types of phases exist in the neutrino Yukawa matrix. One is the Majorana phase, which can change the magnitude of the LFV branching ratios by a few factor. The other phases relate for the size of the Yukawa hierarchy and its phase effect can change the LFV branching ratios by several orders of magnitude.Comment: Talk given by K. Tsumura at NuFact04, Osaka, Japan, July 26 - August 1,2004 - 3 pages, 2 figure

Bi-maximal mixing at GUT, the low energy data and the leptogenesis

In the framework of the minimum supersymmetric model with right-handed neutrinos, we consider the Bi-maximal mixing which is realized at the GUT scale and discuss a question that this model can reproduce the low energy phenomena and the leptogenesis.Comment: Talk given by E. Takasugi at NuFact04, Osaka, Japan, July 26 - August 1,2004 - 3 pages, 4 figure