Higgs boson masses in an extension of the MSSM with vector-like quarks

We study the Higgs sector of the minimal supersymmetric standard model extended with vector-like quarks, at the one-loop level. The radiative corrections to the tree-level masses of the scalar Higgs bosons are calculated by including the contributions from the loops of top quark, vector-like quarks, and their scalar superpartners, for a reasonable parameter region. We find that the mass of the lightest scalar Higgs boson at the one-loop level should be larger than 85 GeV, if we take into account the negative experimental result for the Higgs search at LEP2. As the radiative corrections are calculated in some detail, we also find that the mass of the lightest scalar Higgs boson at the one-loop level is bounded from above at 280 GeV, This upper bound is increased from a previous result. It may provide a wider possibility for the future collider experiments to discover the lightest scalar Higgs boson of this model.

Electroweak phase transition in an extension of the Standard Model with scalar color octet

In an extension of the Standard Model with a scalar color octet, the possibility of the strongly first-order electroweak phase transition is studied, by examining the finite-temperature effective Higgs potential at the one-loop level. It is found that there are wide regions in the parameter space that allow the strongly first-order electroweak phase transition, where the Higgs boson mass is larger than the experimental lower bound of 115 GeV, and the masses of the scalar color octet is around 200 GeV. The parameter regions may be explored at the LHC with respect to the electroweak phase transition.

Electroweak baryogenesis in the MSSM with vector-like quarks

In the minimal supersymmetric standard model (MSSM), a strongly first-order electroweak phase transition (EWPT) is only possible in a confined parameter region where one of the scalar top quarks is lighter than the top quark and the other one is as heavy as the SUSY breaking scale. If the MSSM is enlarged to accommodate vector-like quarks and their superpartners, we find that the strongly first-order EWPT is possible without requiring light scalar top quark at the one-loop level, in the limit where the lightest scalar Higgs boson of the MSSM behaves like the Higgs boson of the standard model and the other Higgs bosons are all as heavy as the SUSY breaking scale. The strength of the first-order EWPT is found to be dependent on the mass of the lightest neutral Higgs boson and the mixing effects of the vector-like scalar quarks.

Explicit CP violation in the Dine-Seiberg-Thomas model

The possibility of explicit CP violation is studied in a supersymmetric model proposed by Dine, Seiberg, and Thomas, with two effective dimension-five operators. The explicit CP violation may be triggered by complex phases in the coefficients for the dimension-five operators in the Higgs potential, and by a complex phase in the scalar top quark masses. Although the scenario of explicit CP violation is found to be inconsistent with the experimental data at LEP2 at the tree level, it may be possible at the one-loop level. For a reasonable parameter space, the masses of the neutral Higgs bosons and their couplings to a pair of $Z$ bosons are consistent with the LEP2 data, at the one-loop level.

Possibility of spontaneous CP violation in Higgs physics beyond the minimal supersymmetric standard model

The Dine-Seiberg-Thomas model (DSTM) is the simplest version of the new physics beyond the minimal supersymmetric standard model (MSSM), in the sense that its Higgs sector has just two dimension-five operators, which are obtained from the power series of the energy scale for the new physics in the effective action analysis. We study the possibility of spontaneous CP violation in the Higgs sector of the DSTM, which consists of two Higgs doublets. We find that the CP violation may be triggered spontaneously by a complex phase, obtained as the relative phase between the vacuum expectation values of the two Higgs doublets. At the tree level, for a reasonably established parameter region, the masses of the three neutral Higgs bosons and their corresponding coupling coefficients to a pair of $Z$ bosons in the DSTM are calculated such that the results are inconsistent with the experimental constraint by the LEP data. Thus, the LEP2 data exclude the possibility of spontaneous CP violation in the DSTM at the tree level. On the other hand, we find that, for a wide area in the parameter region, the CP symmetry may be broken spontaneously in the Higgs sector of the DSTM at the one-loop level, where top quark and scalar top quark loops are taken into account. The upper bound on the radiatively corrected mass of the lightest neutral Higgs boson of the DSTM is about 87 GeV, in the spontaneous CP violation scenario. We confirm that the LEP data does not exclude this numerical result.